Tonight’s Sky

The Greater Hazleton Area Astronomical Society

Star-hop to the Hunting Dogs

Posted by Bruce McClure in TONIGHT | 4 hours ago

Tonight, find the Hunting Dogs. The chart above looks directly overhead at nightfall or early evening in May, as seen from a mid-latitude in the Northern Hemisphere. It’s as if we’re viewing the sky from the comfort of a reclining lawn chair, with our feet pointing southward. The constellation Leo the Lion stands high in the southern sky, while the upside-down Big Dipper is high in the north. Notice the Big Dipper and Leo. You can use them to star-hop to to the constellation Canes Venatici, the Hunting Dogs.

Many people know how to find Polaris, the North Star, by drawing a line through the Big Dipper pointer stars, Dubhe and Merak. You can also find Leo by drawing a line through these same pointer stars, but in the opposite direction.

Extend a line from the star Alkaid in the Big Dipper to the star Denebola in Leo. One-third the way along this line, you’ll see Cor Caroli, Canes Venatici’s brightest star. A telescope reveals that Cor Caroli is a binary star: two stars orbiting a common center of mass.

The two component stars are an estimated 675 astronomical units (AU) apart with an orbital period of around 8,300 years. Given this information, astronomers can figure out the combined mass of Cor Caroli in solar masses with this equation: mass = a3/p2, whereby a = semi-major axis (mean distance) = 675 AU, and p = orbital period = 8,300 years. If you do the calculations, you’ll find that Cor Caroli has about 4.46 times the mass of our sun.

Read more: How astronomers determine the masses of binary starsMust Watch Sky Events in 2021

By the way, Cor Caroli (Latin for “Heart of Charles”) is named in honor of England’s King Charles I, who had his head cut off in 1649. The name first appeared on English star maps in the late 1600s as Cor Caroli Regis Martyris (“Heart of Charles the Martyr King”). King Charles II, the son of King Charles I, founded the Royal Observatory in Greenwich, England, in 1675.

If you’re familiar with the constellation Leo the Lion, you can star-hop to Cor Caroli by drawing an imaginary line from the star Alkaid of the Big Dipper to the Leo star Denebola. Image via Wikimedia Commons.

Bottom line: Star-hop to Canes Venatici, the Hunting Dogs, tonight! You can do it, if you can find the constellation Leo the Lion and the famous Big Dipper asterism.

Bright star Vega on May evenings

Posted by Deborah Byrd in TONIGHT | May 7, 2021

Vega is a lovely star to come to know. When I was first learning the night sky, 45+ years ago, I spent hours, days, weeks, months poring over charts and books. So I sometimes came to know the names and whereabouts of certain stars before seeing them in the night sky. One soft May evening, I happened to glance toward the northeast. I was thrilled at the sight of Vega – gleaming sapphire-blue – and surprisingly bright for being so low in the sky.

Look for this star tonight. It’s the 5th brightest star in our sky. If you’re in the Northern Hemisphere, you’ll find beautiful, bluish Vega easily, simply by looking northeastward at mid-evening in May. Vega is so bright that you can see it on a moonlit night.

From far south in the Southern Hemisphere, you can’t see Vega until late at night in May. That’s because Vega is located so far north on the sky’s dome. Vega will reach its high point for the night around three to four hours after midnight, at which time people in the Southern Hemisphere can see this star in their northern sky. As seen from mid-northern latitudes, the star shines high overhead at this early morning hour.

Because it’s the brightest star in the constellation Lyra the Harp, Vega is sometimes called the Harp Star. Like all stars, Vega rises some four minutes earlier each day as Earth moves around the sun. So Vega will ornament our evening sky throughout the summer and fall.

Although Vega is considered a late spring or summer star, it’s actually so far north on the sky’s dome that – from mid-latitudes in the Northern Hemisphere – you can find it at some time during the night, nearly every night of the year.Must Watch Sky Events in 2021

View at EarthSky Community Photos. | Here’s Vega as seen around 3 a.m. from Valencia, Philippines, on May 10, 2019, from our friend Dr Ski. See Vega’s beautiful blue color? Notice the star near Vega, marked the Greek letter Epsilon. This star is Epsilon Lyrae, a famous double-double star.

Bottom line: If you’re in the Northern Hemisphere, Vega is easy to identify in its constellation Lyra at this time of year. Just look northeast in the evening hours for a bright, bluish star above the northeastern horizon.

Arc to Arcturus, spike to Spica

Posted by Deborah Byrd in TONIGHT | May 8, 2021


Follow the arc to Arcturus, and drive a spike (or speed on) to Spica.

First locate the Big Dipper in the northeastern sky. Draw an imaginary line following the curve in the Dipper’s handle until you come to a bright orange star. This star is Arcturus in the constellation Boötes, known in skylore as the bear guard.

Arcturus is a giant star with an estimated distance of 37 light-years. It’s special because it’s not moving with the general stream of stars, in the flat disk of the Milky Way galaxy. Instead, Arcturus is cutting perpendicularly through the galaxy’s disk at a tremendous rate of speed … some 100 miles (150 km) per second. Millions of years from now this star will be lost from the view of any future inhabitants of Earth, or at least those who are earthbound and looking with the eye alone.

Now drive a spike or, as some say, speed on to Spica in the constellation Virgo.

Spica in the constellation Virgo looks like one star, but this single point of light is really a multiple star system – with two hot stars orbiting very close together – located an estimated distance of 262 light-years away from Earth.Must

Watch Sky Events in 2021Sky chart of constellation Corvus with line between two stars extending to Spica.

Here’s another way to verify that you’re looking at Spica, the brightest star in the constellation Virgo.

Bottom line: Follow the arc to Arcturus, and drive a spike to Spica.

Look for the beautiful Northern Crown

Posted by Deborah Byrd in TONIGHT | May 9, 2021

Photo at top: Corona Borealis, the Northern Crown, with its brightest star Alphecca, via Fred Espenak and AstroPixels. Used with permission.

Tonight, look for a constellation that’s easy to see on the sky’s dome, if your sky is dark enough. Corona Borealis – aka the Northern Crown – is exciting to find. It’s an almost-perfect semicircle of stars. You’ll find this beautiful pattern in the evening sky from now until October.

The constellation Corona Borealis is located more or less along a line between two bright stars, Arcturus in the constellation Boötes the Herdsman and Vega in the constellation Lyra the Harp.

At nightfall and early evening, you’ll see Arcturus fairly high in the east, noticeable for its brightness and yellow-orange color. Vega will be rather low in the northeast, bright and blue-white in color. The Northern Crown is found more or less between these 2 bright stars, though closer to Arcturus.

You’ll need a fairly dark sky to see Corona Borealis between Vega and Arcturus. It’s a semicircle of stars – very noticeable.

The brightest star in Corona Borealis is Alphecca, also known as Gemma, sometimes called the Pearl of the Crown. The name Alphecca originated with a description of Corona Borealis as the “broken one,” in reference to the fact that these stars appear in a semi-circle, rather than a full circle. Alphecca is a blue-white star, with an intrinsic luminosity some 60 times that of our sun. It’s located about 75 light-years from Earth.Must Watch Sky Events in 2021Star chart, constellations, stars black on white background.

The C-shaped constellation Corona Borealis shines between the constellations Boötes and Hercules. Constellation chart via IAU.

View at EarthSky Conmmunity Photos. | Dr Ski in Valencia, Philippines, caught this photo of Arcturus and its constellation Boötes, next to the Northern Crown, on May 24, 2019. Thanks, Dr Ski!

Bottom line: Look for Corona Borealis – the Northern Crown – between the brilliant stars Arcturus and Vega tonight! This constellation is very noticeable, if you have a dark sky.

Find the Keystone in Hercules

Posted by Bruce McClure in TONIGHT | May 10, 2021

Tonight, from mid-northern latitudes, you can easily find the brilliant star Vega in the eastern sky at dusk and nightfall. Vega acts as your guide star to the Keystone – a pattern of four stars in the constellation Hercules.

Look for the Keystone asterism – star pattern – to the upper right of the brilliant blue-white star Vega. Hold your fist at arm’s length. There is easily enough room between Vega and the Keystone for your fist to fit between the two.

You can also locate the Keystone by using Vega in conjunction with the brilliant yellow-orange star Arcturus. From mid-northern latitudes, Arcturus is found quite high in the eastern sky at nightfall and evening. By late evening, Arcturus will have moved over to the southern sky. The Keystone is found about one-third of the way from Vega to Arcturus, the two brightest stars to grace the Northern Hemisphere’s spring and summertime sky.

Before you can find M13, you need to find the Keystone in Hercules, a pattern of four stars. As darkness falls, look for the Keystone to the upper right of the brilliant star Vega. Image via Wikimedia Commons.

The Keystone, in turn, is your ticket to finding a famous globular star cluster in Hercules, otherwise known as the Great Cluster in Hercules, aka Messier 13 or M13.00:19/02:1510 Sec4.1M1.7KMust Watch Sky Events in 2021

Most likely, you’ll need binoculars to see the Hercules cluster, although sharp-eyed people can see it with the unaided eye in a dark, transparent sky. Through binoculars, this cluster looks like a dim and somewhat hazy star. But a telescope begins to resolve this faint fuzzy object into what it really is – a great big, globe-shaped stellar city populated with hundreds of thousands of stars!

The Keystone and the Hercules cluster swing high overhead after midnight, and are found in the western sky before dawn.

View larger. | Can you find the Keystone on this chart? See the compact grouping of four stars at the center of Hercules? That’s it. Note the whereabouts of Messier 13 within the Keystone pattern.

The Great Cluster in Hercules – aka M13 – as captured by our friend Scott MacNeill at Frosty Drew Observatory in Charlestown, Rhode Island. Thanks, Scott!

Bottom line: Let the bright star Vega guide you to a famous star pattern in Hercules – called the Keystone – and then to the Great Cluster in Hercules, aka M13, a famous globular star cluster.

Meet Corvus the Crow

Posted by Deborah Byrd in TONIGHT | May 11, 2021

Sky chart of Corvus the Crow and Spica, the constellation Virgo's brightest star.

A favorite constellation for many, little squarish Corvus the Crow can be found after sunset at this time of year. It’s not far from Spica, the only bright star in the constellation Virgo. Once you find Spica, you’ll recognize Corvus easily. It’s always near the star Spica on the sky’s dome, recognizable for its compact, boxy shape. Spica is supposed to represent an Ear of Wheat, held by Virgo the Maiden. With a good imagination and a dark-enough sky, you can almost see Corvus as a real crow, pecking toward Spica, trying to snatch the wheat.

And thus the stories of the heavens were born …

You don’t need Corvus to identify the bright star Spica. You can use the Big Dipper for that as shown on the diagram below:

Use the handle of the Big Dipper to locate the stars Arcturus and Spica. Just follow the arc in the Dipper’s handle. And remember the phrase “follow the arc to Arcturus, and drive a spike to Spica.”Sky with stars labeled Spica and Corvus.

View at EarthSky Community Photos. | Prateek Pandey in Bhopal, Madhya Pradesh, India, captured this photo of Corvus on March 3, 2021. He wrote: “Small constellation right next to the Spica.”

After you find Spica, Corvus is easy. It’s right next to the bright star, a small boxy pattern that’s noticeable to the eye. Because Corvus is such an easy and fun constellation to pick out in the sky, there are many legends in skylore about it. A lovely one comes from China, where this grouping of stars was seen as an imperial chariot, riding on the wind. In ancient Israel, and sometimes in Greek mythology, Corvus was said to be a raven, not a crow. The early Greeks saw Corvus as a cupbearer to Apollo, god of the sun. The website explains that Corvus was:

… Apollo’s sacred bird in Greek mythology. According to the myth, the raven originally had white feathers. In one story, Apollo told the bird to watch over Coronis, one of his lovers, who was pregnant at the time.

Coronis gradually lost interest in Apollo and fell in love with a mortal man, Ischys. When the raven reported the affair to Apollo, the god was so enraged that the bird did nothing to stop it that he flung a curse on it, scorching the raven’s feathers. That, the legend goes, is why all ravens are black.

Corvus is a friendly sight in the heavens. Along with all the stars, Corvus’s stars will be found a bit farther west at nightfall in the coming weeks and months as Earth moves around the sun. Check it out now and watch for it in the next few months.Must Watch Sky Events in 2021

The bright star Spica – near Corvus on our sky’s dome – will always be there to guide your eye.

View at EarthSky Community Photos. | Dr Ski in Valencia, Philippines, wrote: “Corvus (the Crow) and Crux (the Southern Cross) are at the same right ascension on the celestial sphere. Meaning, they transit the meridian at the same time (approximately 8:30 p.m. local time or 9:30 p.m. local daylight saving time). If you reside in mid-northern latitudes … look for the familiar polygon of Corvus at this time. Then extrapolate a line approximately 40° down to get an idea of how far below your horizon the Southern Cross is.” If you’re further south, of course, as Dr Ski is … just look!

Moon, 3 evening planets May 12-15

Posted by Bruce McClure in TONIGHT | May 12, 2021

Young moon goes by the evening planets, Venus, Mercury and Mars from May 12 to 15, 2021.

At dusk/nightfall these next several evenings – May 12-15, 2021 – watch for the young and slender waxing crescent moon to sweep by three bright evening planets: Venus, the brightest planet, on May 12, and then Mercury, the solar system’s innermost planet, the following day, and then the red planet Mars on or near May 15.

Although the chart is especially designed form mid-northern latitudes in North America, people from around the world can still use the moon and this chart as a guide to moon’s position relative to this line-up of planets, at dusk/nightfall.

As viewed from Europe and Africa, the moon is offset about one-fourth the way toward the previous date.

From western Asia, the moon is offset roughly halfway toward the previous date. On May 13, the places the moon about midway between Mercury and Venus.

From far-eastern Asia, Australia and New Zealand, the offset is around three-fourths the way to the previous date, so the moon and Venus are closest together at dusk on May 13.

No matter where you live, however, you’ll need an unobstructed horizon in the direction of sunset to increase your chances of catching the exceedingly thin young moon and Venus low in the sky, and near the horizon, after sunset May 12. Even though the moon and Venus rank as the second-brightest and third-brightest celestial bodies to light up the heavens, respectively, after the sun, the brilliant twosome must contend with the glow of evening twilight. The moon and Venus will follow the sun beneath the horizon before nightfall.

Here, in the mainland United States, the moon and Venus set roughly one hour after the sun on May 12 (given a level horizon). In a clear sky, you might catch the celestial couple with binoculars 40 to 60 minutes (or possibly earlier) after sundown.

To find out when the sun and moon set in your sky, click on this Sunrise Sunset Calendar, remembering to check the moonrise and moonset box.

Find out when Venus (and the other planets) set in your sky via Old Farmer’s Almanac (USA and Canada) or TimeandDate (worldwide)

If you miss the moon and/or Venus on May 12, try again on May 13, as a wider yet still slender lunar crescent shines higher up at sunset, and stays out longer after darkness falls. What’s more, the illuminated side of the moon points at Venus, and you’re more likely to catch the soft glow of earthshine adorning the dark (nighttime) side of the moon. Think photo opportunity!Must Watch Sky Events in 2021

In their order of brilliance, Venus is the brightest planet by far, followed by Mercury and then Mars. Yet, the fainter planets have the advantage of setting later, as Mercury sets after Venus, and Mars sets after Mercury. On May 12, Venus is some 36 times brighter than Mercury and 163 times brighter than Mars (making Mercury about 4.5 times brighter than Mars). Given clear skies, you should have a good chance of spotting Mercury and Mars with the eye alone.Inner solar system - Mercury, Venus, Earth and Mars - as positioned on May 12, 2021.

Bird’s-eye view of the north side of the inner solar system – Mercury, Venus, Earth and Mars – on May 12, 2021. The portion of each orbit above the plane of the ecliptic is drawn in blue, the portion below in green. Get a refresher of planetary symbols via NASA, and see the entire solar system via Solar System Live.

Mercury shines on par with the sky’s most brilliant stars, so you should be able to view this world with the eye alone an hour or so after sunset. But there’s nothing like binoculars to help out when seeing conditions are less than ideal, or to enable you to spot Mercury all the sooner after sundown.

Mars stays out for a few hours after nightfall. Although Mars is only modesty-bright, this world should be relatively easy to see in a dark sky.

In the mainland United States, Mercury sets almost 2 hours after sunset whereas Mars sets nearly 4 hours hours after sunset. To find out when they sets in your sky, check out a recommended almanac.

These three evening planets plus our planet Earth – Mercury, Venus, Earth and mars – are called the inner planets, to distinguish them from the four outer planets: Jupiter, Saturn, Uranus and Neptune. Whereas the inner solar system is composed of rather small terrestrial or rocky planets, the outer solar system are much larger, consisting of gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune).

Here is the inner part of our solar system, from the sun to the 5th planet, Jupiter. In this illustration, the asteroid belt is the white donut-shaped cloud. Image via Wikimedia Commons.

The inner planets orbit the sun inside the asteroid belt while the outer planets go around the sun outside the asteroid belt. By the way, the two bright outer planets that can be easily seen with the eye alone (Jupiter and Saturn) are now found in the morning sky. The moon will be sweeping by these gas giant worlds in late May and early June 2021.Row of planets and row of dwarf planets in solar system order.

Sizes of planets are to scale but the distances apart are not. Find out planetary distances via Heavens-Above.

In the meantime, use the young waxing crescent moon to find the planets of the inner solar system from May 12-15, 2021: Mercury, Venus, (Earth), and Mars!

Mercury at greatest elongation

Posted by Bruce McClure in TONIGHT | May 16, 2021

Moon and three planets - Mercury, Venus and Mars - after sunset May 16.

Want to nab Mercury, the innermost planet of the solar system? Now’s the time to catch this world, as Mercury the inferior planet swings to its greatest elongation (maximum angular separation) of 22 degrees) east of the setting sun. Look for this world to adorn the western sky, popping out above the blazing planet Venus, as evening dusk ebbs into night.Moo in front of Gemini on the evening of May 16 and 17, 2021.

After dark, when Mercury and Venus are no longer in the sky, look for the moon to the south of the Gemini stars, Castor and Pollux, and to the north of Procyon, the Little Dog star.

The trick is not to look too soon or too late. If you seek too soon, Mercury will still be bleached out by the glow of evening twilight; yet if you search is too late, Mercury will have followed the sun beneath the horizon. The “Goldilocks Zone” for teasing out this furtive world may be around one hour (60 minutes) to 1 1/2 hours (90 minutes) after sunset. Yet we must warn you that a decent viewing window really depends on a crystal-clear sky.Venus and Mercury over New York City.

A year ago, in May 2020, Alexander Krivenyshev of the website caught Venus and Mercury on May 21, 2020, shining over New York City. Alexander was in North Bergen, New Jersey when he captured this image. Thanks, Alexander! We’re wondering how many people will catch Venus and Mercury in May 2021.

Binoculars always help out with any Mercury quest. Binoculars can sometimes tease out Mercury whenever the murk near the horizon would otherwise obscure it from view. Moreover, even when excellent seeing conditions prevail, binoculars enable you to glimpse this stealthy world all the sooner after sunset.

You can also use binoculars in conjunction with the moon and Venus to find your elusive treasure. After all, the moon and Venus rank as the second-brightest and third-brightest celestial luminaries to light up the heavens, respectively, after the sun. These two brilliant beauties will probably be the first two orbs to punctuate the evening twilight, serving as your markers in the sky to track down the trickster planet.

Conveniently, the lit side of the waxing crescent moon points right at Mercury, though it’s a fairly long jump from the moon to Mercury. Undoubtedly, Venus – being some 50 times brighter than Mercury – will come out before Mercury. So if you see the moon and Venus, but not Mercury, seek for Mercury a good way below the moon yet a short hop above Venus. Keep in mind, though, that you might not see Mercury with the eye alone until after Venus sets.Must Watch Sky Events in 2021

We give you the approximate setting times for Venus and Mercury at various latitudes. These setting times presume an absolutely level horizon:

60 degrees north latitude
Venus sets about 1 1/2 hours (90 minutes) after sunset
Mercury sets about 2 5/6 hours (170 minutes) after sunset

40 degrees north latitude
Venus sets about 1 1/12 hours (65 minutes) after sunset
Mercury sets about 1 11/12 hours (115 minutes) after sunset

Equator (0 degrees latitude)
Venus sets about 11/12 hour (55 minutes) after sunset
Mercury sets about 1 1/2 hours (90 minutes) after sunset

35 degrees south latitude
Venus sets about 3/4 hour (45 minutes) after sunset
Mercury sets about 1 1/6 hours (70 minutes) after sunset

For more specific information, consult Old Farmer’s Almanac if you live in the USA or Canada, or TimeandDate for virtually anyplace worldwide.

Mercury initially entered the evening sky (at superior conjunction) on April 19, 2021, and will leave the evening sky (at inferior conjunction) on June 11, 2021. (See the diagram below for superior and inferior conjunction.) These next few days will showcase Mercury at and near its greatest eastern elongation, and, as a general rule, it’s around this time that Mercury is highest up in the western sky at sunset and stays out longest after sundown.

Not to scale. Mercury’s highly elliptical orbit places the innermost planet anywhere from 0.31 to 0.47 the Earth’s distance from the sun (0.31 to 0.47 of an astronomical unit). Mercury swung on the far side of the sun at superior conjunction on April 19, 2021. It’ll reach its greatest eastern (evening) elongation on May 17, 2021, and then sweep between the sun and Earth at inferior conjunction on June 11, 2021.

Once Venus and Mercury disappear below the horizon, and night has fallen, let the illuminated side of the lunar crescent show you the planet Mars, which may appear surprisingly underwhelming to you. Presently, Mars is only about 1/40th as bright as it was during Mars’ heyday in October 2020. Even so, Mars should still be relatively easy to see with the unaided eye in a dark sky.

Read more: What to expect from Mars in 2021

Try for all three bright evening planets – Mercury, Venus and Mars – after sunset. Seek for Venus and Mercury in the darkening evening twilight, and then for Mars after nightfall. With an unobstructed horizon in the direction of sunset, good chance that’ll you spot Mercury with the eye alone an hour or so after sunset.

First quarter moon, Regulus on May 19

Posted by Bruce McClure in TONIGHT | May 19, 2021

Moon and the star Regulus evening of May 19, 2021.

From around the world this evening – May 19, 2021 – you’ll see the moon at or close to its half-illuminated first quarter phase, and in the vicinity of Regulus, the brightest star in the constellation Leo the Lion. The first quarter moon comes to pass on May 19, at 19:12 UTC. At mainland United States’ time zones, that converts to 3:12 p.m. Eastern Time, 2:12 p.m. Central Time, 1:12 p.m. Mountain Time and 12:12 p.m. Pacific Time.

The moon will be somewhat past its first quarter phase as darkness falls on May 19 in the USA. Even so, the eastern half of the United States can actually view the first quarter moon in a daytime sky, because the moon rises over the eastern horizon before it assumes its first quarter phase.

Want to find out when the moon rises into your sky and reaches first quarter phase? Click on Sunrise Sunset Calendars, and remember to check the Moon phases and Moonrise and moonset boxes.

Regulus, a blue-white gem of a star, is the only 1st-magnitude star to sit almost squarely upon the ecliptic, the apparent path of the sun in front of the constellations of the zodiac. Whereas the sun annually has its conjunction with this star once a year (on or near August 23), the moon and Regulus have their conjunction every month (or rarely: twice in one calendar month).

A computer generated model of Regulus, created in 2005, by Georgia State University’s Center for High Angular Resolution Astronomy (CHARA). A model of the sun is shown next to it for scale. The high rotation rate of Regulus creates pronounced equatorial bulging such that its diameter across its equator is one-third longer than its north-south diameter. Image via Wenjin Huang / Georgia State University / NSF.

The moon reaches its first quarter phase and passes to the north of Regulus almost simultaneously at their conjunction on May 19, 2021. Next month, the moon will sweep north of Regulus for the following conjunction on June 15, 2021. Yet, the moon won’t return to its first quarter phase until a few days later, on June 18, 2021. That’s because the moon laps a given star of the zodiac in a mean period of 27.3217 days, yet returns to the same phase in a mean period of 29.5306 days.Must Watch Sky Events in 2021

Although the sun meets up with Regulus on or near the same date every year, the same can’t be said for the first quarter moon. One lunar year consists of 12 lunar months of about 354 days. Twelve lunar months (12 returns to first quarter moon) later – on May 9, 2022 – the May 2022 first quarter moon will recur noticeably west of Regulus. On the other hand, 13 lunar months later – on June 7, 2022 – will find the first quarter moon appearing considerably east of Regulus. There will be no similarly good coincidence of the first quarter moon with Regulus in the year 2022.

Lunar cycles are not necessarily easy to commensurate. For instance, the sidereal month – the duration of time between successive returns of the moon to the same star – represents a mean period of 27.3217 days. On the other hand, the lunar month – the span of time between successive lunar phases – depicts a mean period of 29.5306 days. (Incidentally, the lunar month is also called a lunation or the synodic month.)

Over the centuries, numerous attempts have been made to correlate sidereal and lunar months, with varying degrees of success. At some indefinite time in human history, it was discovered that 67 sidereal months are nearly equal to 62 lunar months:

27.3217 x 67 sidereal months = 1830.55 days

29.5306 x 62 lunar months = 1830.90 days

Moon’s orbit around Earth, seen from above. The moon’s dark side faces Earth at new moon, and its illuminated side faces Earth at full moon. At the quarter moons, we see one-half of the moon’s daytime side and one-half of its nighttime side. Read more: Understanding moon phases

This approximate 1830-day period is the equivalent of 5 calendar years plus a few to several days. On May 23, 2026, the moon makes its 67th return to Regulus slightly before the moon’s 62nd return to first quarter phase. However, the coincidence of first quarter moon with the moon-Regulus conjunction is actually a bit closer timewise on May 19, 2021, than it will be on May 23, 2026.

In other respects, though, the first quarter moon will align much more closely with Regulus on May 23, 2026. Depending on the year, the wayward moon can swing anywhere from 5 degrees north to 5 degrees south of the ecliptic whenever the moon and Regulus stage their monthly conjunction.

For instance, on May 19, 2021, the first quarter moon swings a whopping 5 degrees (10 moon-diameters) north of the ecliptic. Five degrees away from the ecliptic represents a northern extreme for the moon’s travels.

Some five years later on May 23, 2026 the moon will display its first quarter phase and – almost concurrently – will cross the ecliptic at its descending node). At its node, the moon coincides with the ecliptic.

Thus, the moon will be only a hair’s breadth shy of its first quarter phase as it occults – covers over – Regulus fon May 23, 2026. For a map of the occultation viewing area, click In-the-Sky and scroll to the bottom of the page.

This evening – May 19, 2021 – enjoy the moon as it showcases its first quarter phase and shines to the north of Regulus, the constellation Leo the Lion’s brightest star.

Jupiter at west quadrature on May 21

Posted by Bruce McClure in TONIGHT | May 20, 2021

Saturn in front of the constellation Capricornus and Jupiter in front of Aquarius in May 2021 morning sky.

If you could look down on the solar system plane from above on May 21, 2021, you’d see that the sun, Earth and Jupiter form a 90 degree angle in space, with our planet Earth at the vertex of this angle. Astronomers say that Jupiter is at west quadrature at this juncture. That is, it’s 90 degree west of the sun. By definition, a superior planet – such as Jupiter – is said to be at west quadrature whenever it resides 90 degrees west of the sun on the sky’s dome.

Incidentally, the moon reaches west quadrature (90 degrees west of the sun) at its last quarter phase. Earlier this month, the moon and Saturn both reached west quadrature on the same date: May 3, 2021.Solar system diagram showing positions of Earth, sun, and outer planet.

This diagram is approximately to scale for when Mars is at quadrature. But Jupiter’s mean distance from the sun is a little over 5 times the Earth-sun distance – otherwise known as the astronomical unit. Scroll to the bottom of this post for a diagram closer to scale.

When the moon is at west quadrature – or 90 degrees west of the sun – we say the moon is at its last quarter phase. At such times, the moon soars to its highest point in the sky at about 6 a.m. local time (7 a.m. daylight saving time). Whereas the moon sweeps to west quadrature every month, Jupiter and Saturn only do so once a year.

Because Jupiter now lies 90 degrees west of the sun, Jupiter reaches its highest point in the sky approximately 6 hours before the sun does likewise at solar noon (midway between sunrise and sunset). Click here and scroll to the bottom of the page to find out when Jupiter transits (climbs highest up for the day) in your sky.

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Some three months from now – on May 20, 2021 – Jupiter will reach opposition. The king planet will then be opposite the sun in Earth’s sky, or 180 degrees from the sun. If you could look down on the solar system plane at that time, you would see the sun, Earth and Jupiter making a straight line in space. At opposition, a heavenly body climbs highest up in the sky at midnight. (By midnight, we mean midway between sunset and sunrise.)

Opposition and quadrature can happen only to solar system bodies that orbit the sun outside of Earth’s orbit. Planets that orbit the sun inside of Earth’s orbit (Mercury and Venus) can never reach opposition or quadrature. Instead, they always remain near the sun as seen from Earth. So we see them either in the east before sunrise, or in the west after sunset.

Opposition and quadrature happen only to solar system bodies outside of Earth’s orbit, as shown on diagram

Jupiter’s quadratures and oppositions enabled the innovative astronomer Copernicus (1473-1543) to compute Jupiter’s distance from the sun. He did this by charting Jupiter’s (and the Earth’s) change of position from west quadrature to opposition, and from opposition to east quadrature (when the planet lies 90 degrees east of the sun). All the while, Copernicus presumed that Jupiter and Earth both orbit a central sun.

By using the astronomical unit – the Earth-sun distance – as his baseline, Copernicus relied upon the magic of geometry to figure out that Jupiter is over five times the Earth’s distance from the sun!

At west quadrature, Jupiter’s shadow points maximally westward of Jupiter in Earth’s sky. It’s at this time, as viewed through the telescope, that Jupiter’s four major moons – Io, Europa, Callisto and Ganymede – swing into Jupiter’s shadow to the west of Jupiter, and then pass behind the planet itself. Then, half an orbit later, the shadows of the moons transit (cross) Jupiter’s disk before the moons themselves do.

Find Jupiter’s Moons via Sky & Telescope

Bird’s-eye view of the north side of the solar system, whereby the planets revolve counterclockwise around the sun, and Jupiter’s moons revolve counterclockwise around Jupiter. Distances approximately to scale but the sizes of the sun, Earth and Jupiter are not.

Some three months from now – at Jupiter’s opposition on August 20, 2021 – Jupiter’s shadow will point straight outward from Jupiter, neither slanting westward nor eastward of the king planet. Then, on November 15, 2021, Jupiter will swing to east quadrature – 90 degrees east of the sun – at which time Jupiter’s shadow will point maximally eastward of Jupiter. Jupiter’s moons will then travel through Jupiter’s shadow to the east of Jupiter.

Bottom line: Look for Jupiter – the brightest star-like object in the morning sky, at west quadrature, or 90 degrees west of the sun, on May 21, 2021. At eastern quadrature, Jupiter climbs highest in the sky around 6 a.m. (7 a.m. daylight saving time).

Moon and Spica on May 22 and 23

Posted by Bruce McClure in TONIGHT | May 22, 2021

Star-hopping from the constellation Corvus to Spica, the constellation Virgo's 1st-magnitude star.

On May 22 and 23, 2021, use the waxing gibbous moon to find Spica, the brightest star in the constellation Virgo the Maiden. In fact, Spica is Virgo’s one and only 1st-magnitude star. Although the bright moon will wipe out a number of fainter stars from the canopy of night tonight, bright Spica should withstand the moonlit glare. If you have trouble seeing Spica, place your finger over the moon and look for a nearby bright star.

We in the Northern Hemisphere associate the star Spica with the spring and summer seasons. That’s because Spica first lights up the early evening sky in late March or early April, and then disappears from the evening sky around the September equinox.

The constellation Virgo stands as a memorial to that old legend of Hades, god of the underworld, who was said to have abducted Persephone, daughter of Demeter, goddess of the harvest. According to the legend, Hades took Persephone to his underground hideaway. Demeter’s grief was so great that she abandoned her role in insuring fruitfulness and fertility. In some parts of the globe, it’s said, winter cold came out of season and turned the once-verdant Earth in to a frigid wasteland. Elsewhere, summer heat was said to scorch the Earth and give rise to pestilence and disease. According to the myth, Earth would not bear fruit again until Demeter was reunited with her daughter.

Zeus, the king of the gods, intervened, insisting that Persephone be returned to her mother. However, Persephone was instructed to abstain from food until the reunion with her mother was a done deal. Alas, Hades purposely gave Persephone a pomegranate to take along, knowing she would eat a few seeds on her way home. Because of Persephone’s slip-up, Persephone has to return to the underworld for a number of months each year. When she does so, Demeter grieves, and winter reigns.

The constellation Virgo is linked to Demeter (and also Ishtar of Babylonian mythology, Isis of Egyptian mythology and Ceres of Roman mythology). Virgo is seen as a Maiden, associated with the harvest and fertility. The Latin word spicum refers to the ear of wheat Virgo holds in her left hand. The star Spica takes its name from this ear of wheat. Each evening, if you watch at the same time, you’ll see Spica slowly shift westward, toward the sunset direction. Eventually, Spica will get so close to the sunset that it’ll fade into the glare of evening twilight. Once Spica disappears from the evening sky, we at northerly latitudes must harvest our crops and put away firewood, because the cold winter season is on its way.Must Watch Sky Events in 2021Diagram of solar system with figures of zodiac (Virgo, Scorpio, Aquarius, etc.) around the edge.

We are surrounded by stars. Because Earth orbits in a flat plane around the sun, we see the sun against the same stars again and again throughout the year. Those constellations, which have been special to people throughout the ages, are the constellations of the zodiac. Image via Professor Marcia Rieke.

The constellations of the zodiac – like Virgo – define the sun’s path across our sky. Putting it another way, each year, the sun passes in front of all the constellations of the zodiac. This year, 2021, the sun leaves the constellation Leo to enter the constellation Virgo on September 16, 2021. Then the sun leaves the constellation Virgo to enter the constellation Libra on October 31, 2021 (Halloween).

Three other 1st-magnitude zodiacal stars join up with Spica to help sky gazers envision the ecliptic – the sun’s annual path in front of the backdrop stars: Aldebaran, Regulus and Antares. Every year, the sun has its annual conjunction with Aldebaran on or near June 1, Regulus on or near August 23, Spica around mid-October, and Antares on or near December 1.

Of course, all these stars are invisible on their conjunction dates with the sun because they are totally lost in the sun’s glare at that time. However, six months before or after these stars’ conjunction dates, these stars are out all night long. Six months one way or the other of their conjunction, these stars reside opposite the sun in the sky and therefore stay out all night (Regulus around February 23, Spica around mid-April, Antares around June 1 and Aldebaran around December 1).Sky chart of the constellation Virgo with latitude lines and blue line of ecliptic.

The ecliptic – Earth’s orbital plane projected onto the constellations of the zodiac – crosses the celestial equator (declination of O degrees) in the constellation Virgo. Because Spica resides so close to the ecliptic, it is considered a major star of the zodiac. Virgo constellation chart via the International Astronomical Union

Watch for the legendary green flash

Posted by Deborah Byrd in TONIGHT | May 24, 2021

The green flash image at the top of this post was taken by Jim Grant, an EarthSky friend on Facebook. He captured it off the coast of Ocean Beach, California, and identified it as a mock mirage green flash.

It’s not hard to see a green flash with the eye alone, when sky conditions are right, and when you’re looking toward a very clear and very distant horizon. That’s why those who live near an ocean tend to report green flashes most often. A sea horizon is the best place to see them.

The video below, posted to EarthSky by Vladek in 2016, is an excellent example of the experience of seeing a green flash:

Most people see green flashes just at sunset, at the last moment before the sun disappears below the horizon. Be careful and don’t look too soon. If you do look too soon, the light of the sunset will dazzle (or damage) your eyes, and you’ll miss your green flash chance that day.Must Watch Sky Events in 2021

But if you wait – looking away until just the thinnest rim of the sun appears above the horizon – that day’s green flash could be yours.

Read more: Is Zubeneschamali a green star?

Of course, the green flash can be seen before sunrise, too, although it’s harder at that time of day to know precisely when to look.

Mock mirage and green flash seen from San Francisco in 2006. Image via Brocken Inaglory/ Wikimedia Commons.

There are many different types of green flash. Some describe a streak or ray of the color green … like a green flame shooting up from the sunrise or sunset horizon.

The most common green flash, though – the one most people describe – is a flash of the color green seen when the sun is nearly entirely below the horizon.

Again … you need a distant horizon to see any of these phenomena, and you need a distinct edge to the horizon. That’s why these green flashes, streaks, and rays are most often seen over the ocean. But you can see them over land, too, if your horizon is far enough away.

Pollution or haze on the horizon will hide this instantaneous flash of the color green.

Jim Grant photographed this green flash on April 27, 2012, off the coast of San Diego.

If you’re interested in green flashes, Andrew Young’s green flash page is great. He also has a page of links to pictures of green flashes taken by people from around the globe.

And, of course, Les Cowley at the great website Atmospheric Optics devotes many pages to the green flash phenomenon. Notice the menu bar at the left side of the page; it’ll let you explore many different types of green flashes.

Green flash atop sun pyramid via astrophotographer Colin Legg in Australia.

Bottom line: The green flash is legendary, and some people have told us they thought it was a myth, like a unicorn or a pot of gold at the end of a rainbow. But green flashes are very real. You need a distant and exceedingly clear horizon to see them at the last moment before the sun disappears below the horizon at sunset.

Total lunar eclipse May 26, 2021

Posted by Bruce McClure in TONIGHT | May 25, 2021

Lunar craters and "seas" on the moon's surface, visible during the eclipse.

Above photo: The last time we had a total eclipse of the moon was nearly 2 1/2 years ago, on January 21, 2019. Tom Wildoner captured this image of this total lunar eclipse from Weatherly, Pennsylvania. Thank you Tom!

On May 26, 2021, the full moon sweeps through the Earth’s dark umbral shadow to stage a short-lived total eclipse of the moon. Although totality lasts for less than 15 minutes, a partial umbral eclipse precedes and then follows totality by nearly 1 1/2 hours each time. So, from start to finish, the moon takes a little over three hours to cross the Earth’s dark shadow.

This May full moon counts as the closest (and therefore the biggest) full moon of the year. Some people may call it a supermoon. A total eclipse of the year’s closest full moon last occurred on September 28, 2015.

This total lunar eclipse is visible from western North America, southern and far-western South America, the Pacific Ocean, New Zealand, Australia and southeast Asia. From the Americas, this eclipse takes place in the early morning hours before sunrise May 26; whereas from the world’s Eastern Hemisphere (New Zealand, Australia and southeast Asia), the eclipse occurs in the evening hours after sunset May 26. The worldwide map below helps to explain:Worldwide map of day and night sides of Earth at mid-eclipse.

The day and night sides of Earth at mid-eclipse or greatest eclipse (May 26 at 11:19 UTC). The shadow line at left (running through North and South America) depicts sunrise (moonset) May 26, whereas the shadow at right (running across Asia), represents sunset (moonrise) May 26. You have to be on the nighttime side of Earth to see the moon at mid-eclipse (greatest eclipse).

A swath of the Americas to the right (east) of the sunrise line on the worldwide map will miss the total eclipse but can watch a partial umbral eclipse of the moon; whereas a section of Asia to the left (west) of the sunset line will miss the total eclipse but can watch a partial umbral eclipse. The arrows in the worldwide map below show where the umbral eclipse begins in the Americas and where the umbral eclipse ends in Asia. The regions marked U4 to P4 in Asia, and U1 to P1 in the Americas, sit outside the umbral eclipse viewing area. So this outlier region must be content with a faint penumbral eclipse.Must Watch Sky Events in 2021worldwide map of day and night at partial eclipse.

One arrow shows where the umbral eclipse begins in the Americas, and the other arrow points to where the umbral eclipse ends in Asia.

Find out the eclipse times for your part of the world via TimeandDate, remembering to place your city in the lookup box.

We give the eclipse times first in UTC (Coordinated Universal Time) and then in local time for U.S. time zones:

Eclipse times in UTC (May 26, 2021):

Partial umbral eclipse begins: 9:45 UTC
Total eclipse begins: 11:11 UTC
Greatest eclipse: 11:19 UTC
Total eclipse ends: 11:26 UTC
Partial umbral eclipse ends: 12:52 UTC

Eclipse times for North American time zones:

Eastern Daylight Time (May 26, 2021)
Partial umbral eclipse begins: 5:45 a.m. EDT
Total eclipse begins: 7:11 a.m. EDT
Greatest eclipse: 7:19 a.m. EDT
Total eclipse ends: 7:26 a.m. EDT
Partial umbral eclipse ends: 8:52 a.m. EDT

Central Daylight Time (May 26, 2021)
Partial umbral eclipse begins: 4:45 a.m. CDT
Total eclipse begins: 6:11 a.m. CDT
Greatest eclipse: 6:19 a.m. CDT
Total eclipse ends: 6:26 a.m. CDT
Partial umbral eclipse ends: 7:52 a.m. CDT

Mountain Daylight Time (May 26, 2021)
Partial umbral eclipse begins: 3:45 a.m. MDT
Total eclipse begins: 5:11 a.m. MDT
Greatest eclipse: 5:19 a.m. MDT
Total eclipse ends: 5:26 a.m. MDT
Partial umbral eclipse ends: 6:52 a.m. MDT

Pacific Daylight Time (May 26, 2021)
Partial umbral eclipse begins: 2:45 a.m. PDT
Total eclipse begins: 4:11 a.m. PDT
Greatest eclipse: 4:19 a.m. PDT
Total eclipse ends: 4:26 a.m. PDT
Partial umbral eclipse ends: 5:52 a.m. PDT

Alaskan Daylight Time (May 26, 2021)
Partial umbral eclipse begins: 1:45 a.m. AKDT
Total eclipse begins: 3:11 a.m. AKDT
Greatest eclipse: 3:19 a.m. AKDT
Total eclipse ends: 3:26 a.m. AKDT
Partial umbral eclipse ends: 4:52 a.m. AKDT

Hawaiian Standard Time (May 25-26, 2021)
Partial umbral eclipse begins: 11:45 p.m. HST (May 25, 2021)
Total eclipse begins: 1:11 a.m. HST (May 26, 2021)
Greatest eclipse: 1:19 a.m. HST
Total eclipse ends: 1:26 a.m. HST
Partial umbral eclipse ends: 2:52 a.m. HSTMoon's path through Earth's shadow on May 26, 2021.

The moon travels from west-to-east across the Earth’s shadow. Because the moon doesn’t cross the Earth’s shadow dead center, the northern part of the moon will appear brighter than the southern part, which is more deeply submerged in shadow.

Why such a short total lunar eclipse?

A total lunar eclipse can only happen at full moon, or when the moon is opposite the sun in Earth’s sky. More often than not, however, the full moon passes to the north or to the south of Earth’s dark shadow, and therefore avoids being eclipsed. For instance, last month – on April 27 – the full moon swept to the north of the Earth’s shadow; whereas next month – on June 24 – the full moon will swing to the south of Earth’s shadow.

In fact, the full moons of July, August, September and October 2021 will all travel south of the Earth’s shadow. Finally, the November 2021 full moon will meet up with the Earth’s dark shadow on November 19, 2021. It won’t be a perfect alignment, however, and the November full moon will just miss being totally eclipsed.

Yet, the total lunar eclipse on May 26, 2021, doesn’t make a perfect alignment, either. In fact, this May 2021 full moon doesn’t even cross the center of the Earth’s shadow. This full moon is about as far as it can get from the center of the Earth’s shadow, and still be totally eclipsed. That’s why this total eclipse lasts for short period of time, or less than 15 minutes.

The more closely that the full moon’s center aligns with the center of the Earth’s shadow, the deeper and longer the total eclipse. On July 27, 2018, the alignment between full moon and Earth’s shadow was almost perfect, to produce the longest total lunar eclipse of the 21st century (2001 to 2100). This total eclipse lasted for 1 hour and 43 minutes, or nearly 1 1/2 hours longer than the May 16, 2021 eclipse.

Read more: Century’s longest eclipse July 27, 2018

The moon crosses the Earth’s shadow from west to east. The full moon on July 27, 2018, crossed the Earth’s shadow pretty much dead center, to produce the longest total solar eclipse of the 21st century (2001 to 2100).

Although the total lunar eclipse on May 26, 2021, is a rather shallow one, it’s the only total lunar eclipse to occur in the year 2021, and the first to take place since January 21, 2019. Enjoy the grand natural attraction while the time is at hand!

Closest Mercury-Venus conjunction until 2033

Posted by Bruce McClure in TONIGHT | May 28, 2021

Conjunction of Mercury and Venus low in the west after sunset.

Late May 2021 presents the closest visible conjunction of the planets Mercury and Venus until November 5, 2033. No matter where you live worldwide, it’s to your advantage to find an unobstructed horizon in the direction of sunset. Seek for the planetary pair low in your western sky as evening dusk is ebbing toward night. Look for Venus, the brighter of the twosome, to be visible to the eye alone some 30 to 40 minutes (if not sooner) after sunset.

Most likely, you’ll need an optical aid to view Mercury next to Venus; and even at that, Mercury may be hard to catch. Because Mercury is exhibiting a thin waning crescent phase right now (though you need a telescope to discern its phase), Mercury appears much dimmer now than at the beginning of the month. In early May, Mercury’s disk was over 80% illuminated by sunshine. By the month’s end, the illuminated portion will shrink to 10%, making Mercury around 40 times fainter now than in early May.Thin crescent moon with tiny dots labeled as Mercury and Venus.

View larger. Riste Spiroski of Ohrid, North Macedonia captured the moon, Mercury and Venus at dusk on May 25, 2020, at around 8:30 p.m. local time. Last year, in 2020, that year’s sole Mercury-Venus conjunction occurred a few days before this photo was taken: May 22, 2020, at 7 hours UTC.

In short, don’t be be too surprised if you spot Venus with relative ease, but Mercury not at all. Venus ranks as the 3rd-brightest celestial body to light up the heavens, after the sun and moon, respectively. Presently, Venus outshines Mercury by a few hundredfold.

Depending on where you live on the Earth’s globe, Mercury and Venus will come closest together at dusk on May 28 or 29, 2021. Both dates, however, will present Mercury and Venus plenty close together on the sky’s dome. If the weather cooperates, try both dates, and notice the slight change of position between the two worlds from one day to the next. At conjunction, Venus will pass a scant 0.4 degrees north of Mercury on May 29, at 6 hours UTC. (For reference, the moon’s angular diameter spans about 0.5 degree.)

After their conjunction, Venus will continue to climb upward, away from the setting sun, whereas Mercury will sink sunward, eventually to meet up with the setting sun. Venus will remain in the evening sky for the rest of 2021, whereas Mercury will leave the evening sky to enter the morning sky on June 11, 2021. Mercury probably won’t be visible to the eye as a morning “star” until late June or July 2021.Must Watch Sky Events in 2021

Mercury and Venus stay out longer after sunset at more northerly latitudes, yet set sooner after sundown at more more southerly latitudes. We give you the approximate setting time for Venus, the brighter of these two planets, for the next few days at varying latitudes:

60 degrees north latitude
Venus sets about 1 3/4 hours (105 minutes) after sunset

40 degrees north latitude
Venus sets about 1 1/3 hours (80 minutes) after sunset

Equator (0 degrees latitude)
Venus sets about 1 1/6 hours (70 minutes) after sunset

35 degrees south latitude
Venus sets about one hour (60 minutes) after sunset

For more specific information, consult Old Farmer’s Almanac if you live in the USA or Canada, or TimeandDate for virtually anyplace worldwide.

Map of the inner solar sytem - Mercury, Venus, Earth and Mars - as the planets Mercury and Venus are in conjunction on May 29, 2021.

Bird’s-eye view of the inner solar system – Mercury, Venus, Earth and Mars – as Mercury and Venus reach their conjunction, as seen from Earth, on May 29, 2021, at 6 hours UTC. At this juncture, Venus lodges one astronomical unit (AU) farther from Earth than Mercury does. Planetary symbols available at NASA/Lunar and Planetary Institute. Map via Solar System Live.

These two worlds – Mercury and Venus – nearly coincide on the same line of sight, but are not truly close together in space. Astronomers often give the distances of solar system planets in terms of the astronomical unit (AU): sun-Earth distance of about 93 million miles or 150 million km. At conjunction, Mercury resides 0.63 AU from Earth, whereas Venus lodges one astronomical unit beyond Mercury, at 1.63 AU.

Late May 2021 showcases the closest coupling of Mercury and Venus for years to come. Brilliant Venus boldly punctuates the evening twilight, but demure Mercury lurks at the threshold of visibility. You may well need binoculars for any chance of spotting Mercury, the innermost planet of the solar system. Good Luck!

Moon swings by Saturn and Jupiter

Posted by Bruce McClure in TONIGHT | May 29, 2021

Waning moon swings by four morning planets in late May and early June 2021.

These next several mornings – May 30 to June 2, 2021 – watch for the waning moon to sweep by the two gas giant planets, Saturn and Jupiter. Unless you’re a night owl, you probably won’t see the moon, Saturn and Jupiter rising into your sky before your bedtime. Look for the threesome – the moon, Saturn and Jupiter – to grace the predawn/dawn sky.

View larger at EarthSky Community Photos. Cattleya Flores-Viray in La Jolla, California took this photo at 5:45 pm on January 9, 2021. In early January 2021, Jupiter and Saturn were hard to see in the glow of evening twilight. Then in late January, February and March 2021, these worlds were difficult to view in the glow of morning dawn. Now’s the time to enjoy an eyeful of these morning planets, as they adorn the predawn sky throughout May and June 2021.

It is pretty easy to distinguish Jupiter from Saturn, because Jupiter is the much brighter of the two. Although Saturn shines as brilliantly as a 1st-magnitude star, Saturn pales next to king planet Jupiter, which outshines this golden-colored world by some 16 times. Jupiter ranks as the 4th-brightest celestial body to light up the heavens, after the sun, moon and the planet Venus, respectively. Fortunately, there’s no way to mistake Jupiter for Venus, or vice versa, because Venus only appears the evening sky.

Have a telescope? Saturn’s glorious rings and Jupiter’s four major moons – Io, Europa, Ganymede and Callisto – are easy to view through the telescope (even with a small, backyard variety). Saturn’s rings are inclined at 17 degrees toward Earth, making the rings readily observable through a telescope throughout 2021. Some people can even see Jupiter’s 4 major moons – sometimes called the Galilean moons – with binoculars. In short, if you can view Saturn and Jupiter with the unaided eye, you should be able to spot Saturn’s rings and Jupiter’s four major moons with an optical aid.

Although you need a telescope to see Saturn’s rings, a favorable tilt of Saturn’s rings toward Earth helps to make Saturn all the brighter to the eye alone. Saturn’s rotational axis is tilted at 27 degrees out of vertical, relative to the plane of Saturn’s orbit around the sun. So whenever it’s solstice time on Saturn, Saturn’s rings are inclined at a maximum of 27 degrees toward Earth. Saturn’s most recent solstice occurred in May 2017, and will next place in April 2032.

Image via Wikimedia Commons. The tilt of Saturn’s rings has a great impact on its overall brightness. In years when Saturn’s rings are edge-on as seen from Earth (2009 and 2025), Saturn appears considerably dimmer than in years when Saturn’s rings are maximally titled toward Earth (2017 and 2032). This year, in 2021, Saturn’s rings are still highly inclined toward Earth.

The rings circle Saturn above Saturn’s equatorial plane. So when Saturn is at an equinox, we then see the ring’s edge-on from Earth. Saturn’s next equinox will take place on May 6, 2025. At and around that time, Saturn’s rings will appear as a thin line to the eye, or possibly even disappear from view.Must Watch Sky Events in 2021

By the way, Jupiter had its most recent equinox on May 2, 2021. That means we are viewing Jupiter’s equator pretty much edge-on this year. Because the orbits of the Galilean moons coincide with Jupiter’s equatorial plane, the moons of Jupiter tend to eclipse one another during a Jovian equinox year. Jupiter will swing to its next equinox on December 16, 2026.

Expect to see Jupiter’s moons lining up on or near the same plane all year long. Now and again, a moon or two could be “missing” as these moons flit in front of or behind Jupiter. This year, in 2021, it’s even possible for one moon to hide another moon.JUpiter's four major moons: Io, Europa, Ganymede and Callisto.

From left to right, the four major moons of Jupiter: Io, Europa, Ganymede and Callisto. Earth’s moon is larger than Europa, and nearly the same size as Io. Ganymede, the largest moon in the solar system, is even larger than the planet Mercury. Callisto is the third-largest moon in the solar system, a touch smaller than Saturn’s Titan. Photo via JPL/NASA.

To find out the position of Jupiter’s moons for right now or some chosen time, click on this interactive tool via Sky & Telescope.

The outermost Galilean moon – Callisto – is the only one of Jupiter’s four major moons not to swing in front of and in back of Jupiter during each orbit around the king planet. At a Jovian solstice, Jupiter’s rotational axis is most inclined toward Earth. Centered on a solstice, Callisto swings to the north and south of Jupiter, as seen from Earth, for a few years. Jupiter’s last solstice occurred on April 27, 2018, and will next happen on January 20, 2024.

These next several mornings – May 30 to June 2, 2021 – use the moon to guide you to the bright planets, Saturn and Jupiter.