Tonight’s Sky April 2021

The Greater Hazleton Area Astronomical Society

Moon, Jupiter, Saturn before sunup on April 5-8

Posted by Bruce McClure in TONIGHT | 1 day ago

Chart: 4 positions of waning moon near 2 planets along slanted ecliptic line.

On April 5, 6, 7 and 8, 2021, let the waning crescent moon help guide your eye to the largest and second-largest planets of the solar system: Jupiter and Saturn, respectively. Because day by day the moon always travels eastward (in the direction of sunrise) relative to the backdrop stars (and planets) of the zodiac, watch for the moon to sweep past Saturn and then Jupiter in the days ahead.

Find out which constellations of the zodiac appear behind the moon at Heavens-Above.

At present, the king planet Jupiter and the ringed planet Saturn both reside in front of the constellation Capricornus the Seagoat. Jupiter and Saturn, the fifth and sixth planets from the sun, respectively, also go eastward in front of the background stars of the zodiac, but much more slowly than the moon does. Whereas the moon takes some 27 1/3 days to go full circle in front of the constellations of the zodiac, it takes Jupiter nearly 12 years and Saturn nearly 30 years.Star chart with stars in black on white.

View a larger chart. | Have a birthday sometime between January 20 and February 16? If so, the sun was probably in front of the constellation Capricornus on your birthday. This year, 2021, you can use the planet Saturn to locate Capricornus all year long!

The moon will pass out of the constellation Capricornus and into the constellation Aquarius after several days. But Jupiter won’t pass into the constellation Aquarius until April 25, 2021. Saturn, the most distant world that you can easily see with the eye alone, will remain in front of the constellation Capricornus for the rest of the year. In short, Jupiter stays in front of a constellation for about one year, while Saturn does so for about 2 1/2 years.

Find out which constellation of the zodiac lies behind Jupiter and Saturn at Heavens-Above.10 SecMust Watch Sky Events in 2021

In one year, Jupiter travels around 30 degrees eastward yearly in front of the backdrop stars of the zodiac, while Saturn travels about 12 degrees eastward yearly. Ancient astronomers correctly figured that Saturn must lie farther away from Earth than Jupiter does because Saturn moves more slowly in front of the constellations of the zodiac. In this respect, ancient astronomers were correct.

Because the ancients were such acute observers, they observed the retrograde (westward) motions of the superior planets: planets that orbit the sun outside Earth’s orbit (Mars, Jupiter and Saturn). However, the ancients had great difficulty coming up with a model that could adequately explain these mysterious retrogrades. That’s because the ancients, for the most part, were led astray by the presumption that the planets orbit the Earth, instead of the sun.2 diagrams with planets, sun and moon lined up on circular orbits in different orders.

Contrasting Ptolemy’s Earth-centered model with Copernicus’s sun-centered model. In the Earth-centered model (left), Mercury and Venus orbit the sun inside the sun’s orbit around Earth. In the sun-centered model (right), Mercury and Venus orbit the sun inside Earth’s orbit. Graphic via Amazing Space.

The moon always travels eastward in front of the constellations of the zodiac, yet never in retrograde. On the other hand, the superior planets – Mars, Jupiter and Saturn – go eastward overall, yet display mysterious retrograde motions from time to time. Moreover, the superior planet always shines most brilliantly in Earth’s sky at the middle of a retrograde, at which juncture that planet is at opposition or opposite the sun in Earth’s sky.

The more distant the planet, the longer the retrograde. Because the distances of these worlds vary somewhat as they orbit the sun, so do the lengths of their retrogrades, depending on the year. The approximate retrograde periods: Mars (two months), Jupiter (4 months) and Saturn (4 1/2 months). We give you Jupiter and Saturn’s retrogrades and oppositions for 2021:

Jupiter
Retrograde begins: June 21, 2021
Opposition: August 20, 2021
Retrograde ends: October 18, 2021

Saturn:
Retrograde begins: May 23, 2021
Opposition: August 2, 2021
Retrograde ends: October 11, 2021

Note: Mars last reached opposition on October 13, 2020, and will next reach opposition on December 8, 2022

The innovative astronomer Copernicus (1473-1543) figured the moon never displays retrograde motion because the moon revolves around the Earth, not the sun. On the other hand, the planets show retrograde motion because all the planets (including Earth) revolve around the sun. When Earth in its smaller and faster orbit swings by a slower-moving superior planet – Mars, Jupiter or Saturn – that planet appears to go backwards for the same reason that a car appears to go backwards (relative to the distant background) when you pass that slower-moving car on the highway. The diagram below helps to explain.

Inferior planets – planets that orbit the sun inside of Earth’s orbit – show retrograde motions as well, but for the opposite reason. When a faster-moving inferior planet – such as Mercury or Venus – sweeps by the slower-moving Earth, Earth appears to retrograde in its sky, and that inferior planet appears to retrograde in ours.Diagram with lines from orbit of inner planet to orbit of outer planet.

The planets move in a counterclockwise direction as viewed from the north side of the solar system. Relative to the backdrop stars, a faster-moving inner planet sees a slower-moving outer planet moving in retrograde from A2 to A4, and at opposition at A3. Diagram via Wikipedia.

Bottom line: Are you an early riser? Be sure to get an eyeful of the picturesque morning tableau as the waning crescent moon sweeps by the giant planets Jupiter and Saturn from April 5-8, 2021.

Use the Big Dipper to find the Little Dipper

Posted by EarthSky in TONIGHT | April 8, 2021

Use Big Dipper to find North Star.

So you say you can find the Big Dipper, but not the Little Dipper? This post is for you. Here’s the view northward on April evenings. At present the Big Dipper is high in the north during the evening hours. Notice the two outer stars in the bowl of the Big Dipper. These two stars – called Duhbe and Merak – always point to Polaris, the North Star. Find Polaris, and you can find the Little Dipper.

Polaris is special because Earth’s northern axis nearly points to its location in the sky. It’s the star around which the entire northern sky appears to turn.

Polaris is also fun to locate for another reason. It’s part of a famous – though elusive – star pattern, known as the Little Dipper.

So here it is! The Little Dipper! The North Star, Polaris, marks the end of its handle.

View larger. | No matter where you see the Big Dipper, the two outer stars in its bowl point to Polaris. In this shot, Tom Wildoner caught the Big Dipper and Polaris at around 3:30 a.m. in July 2013. Thanks, Tom!

By the way, Polaris is less than a degree away from the true north celestial pole on the sky’s dome now. It’ll be closest to true north – less than half a degree away – in the year 2102. The change is due to a motion of Earth called “precession,” which causes Earth’s axis to trace out a circle among the stars every 26,000 years.10 SecMust Watch Sky Events in 2021

By the way, thousands of years ago, Polaris was an ordinary star in the northern sky, known to the Greeks by the name Phoenice.

Other ordinary stars in the northern sky now – Kochab and Pherkad, the two outermost stars in the bowl of the Little Dipper (see chart below) – have had the honor of being pole stars.

Kochab and Pherkad served as twin pole stars from about 1500 B.C. to about 500 B.C.

They’re still sometimes called the Guardians of the Pole.

Kochab is located about 126 light-years away. Pherkad is more distant, at about 480 light-years by some estimates. Meanwhile, Polaris is a bit more than 400 light-years away.

Kochab and Pherkad are the 2 outermost stars in the bowl of the Little Dipper.

Bottom line: The Big Dipper is usually pretty easy to find, but the Little Dipper is less easy. This post tells you how to use the Big Dipper to find Polaris and the Little Dipper, plus how to recognize the stars Kochab and Pherkad.

Star-hop to the Coma star cluster

Posted by Bruce McClure in TONIGHT | April 9, 2021

Our chart at the top of this post shows the constellation Leo the Lion highest up for the night at roughly 9 to 10 p.m. local time (10 to 11 p.m. local daylight saving time). That’s the time on your clock no matter where you are on the globe.

Long ago, the Coma star cluster represented the Lion’s tufted tail. It is a beautiful cluster, well worth taking the time to pick out in the night sky. You can see Leo from the suburbs, but you’ll need a dark sky to find the cluster. In mid-evening now, as seen from mid-northern latitudes in the Northern Hemisphere, the constellation of the Lion will be high in the southern sky. You’ll see the Lion at nightfall, too, though more in the south-southeastern portion of the sky.Diagram of Big Dipper with arrows to Polaris and Leo.

An imaginary line drawn between the pointer stars in the Big Dipper – the two outer stars in the Dipper’s bowl – points in one direction toward Polaris, the North Star, and in the opposite direction toward Leo.

We’ll talk more about the star cluster shortly, but first here’s how to spot Leo. Notice the chart above. A line between the two outer stars in the Bowl of the Big Dipper always points toward Leo. Two distinctive star patterns make the Lion fairly easy to identify. Leo’s brightest star – the sparkling blue-white gem Regulus – marks the bottom of a backward question mark of stars known as The Sickle. A triangle of stars highlights the Lion’s hindquarters and tail. If you see a Lion in this pattern of stars, the Sickle outlines the Lion’s mane. The triangle, meanwhile, marks the back side of Leo. Denebola, the name of the outermost star in this rear triangle of Leo, is an Arabic term meaning the Lion’s Tail.

Leo? Here’s your constellation

EarthSky Facebook friend Zhean Peter Nacionales in the Philippines captured this view of the Coma star cluster in April 2013. He wrote, “I learned at the EarthSky page to star-hop from Leo to the Coma star cluster, so I packed up my camera and tripod. It was worth it. I definitely saw this star cluster.” Thank you, Zhean!

Now let’s try star-hopping from Leo the Lion to the Coma star cluster. Nowadays, this part of the sky belongs to another constellation, Coma Berenices or Berenice’s Hair. As shown on the chart at the top of this post, or in the photo above from Zhean Peter Nacionales, you can draw a line from the star Regulus through the top star of the triangle (Zosma), and go about twice this distance to locate the cluster.10 SecMust Watch Sky Events in 2021

The Coma star cluster is also called Melotte 111. It’s visible to the unaided eye in a dark country sky. You might need binoculars to see this loose tangle of stars if your skies are beset by light pollution. It is a beautiful sight in a dark sky. Find a dark sky location near you here.

This is an open star cluster. That means its stars were probably born together from a single cloud of gas and dust in space, and they are still loosely bound by gravity. There are about 100 stars in the Coma star cluster, which lies some 288 light-years distant. In other words, this star cluster lies within our Milky Way galaxy.Star field with several dozen scattered bright stars relatively close together.

Coma star cluster in the direction of our constellation Coma Berenices. It is an open star cluster, whose member stars are thought to be loosely bound by gravity. Image via NASA/ Expedition 6.Large pink and white spiral galaxy with smaller fuzzy galaxies behind it.

A majestic face-on spiral galaxy located deep within the Coma Cluster of galaxies. Image via NASA.

But wait. There’s more in this direction of space: a vast collection of galaxies external to our Milky Way and the Virgo supercluster of galaxies.

The Coma galaxy cluster, also in the direction of the constellation Coma Berenices, is made up not of individual stars, but of whole galaxies of stars.

It is invisible to the unaided eye (or even in a small telescope), but astronomers with large telescopes study this region of space to learn more about the cosmos.

Coma Cluster: Galaxies in Coma Berenices

Bottom line: On springtime evenings, star-hop from the constellation Leo to the Coma star cluster in the constellation Coma Berenices. Plus – although you cannot see it with your eye or with binoculars – a huge cluster of external galaxies lies in the direction of Coma Berenices.

Leo loses his tail. We gain a constellation

Posted by Larry Sessions in TONIGHT | April 10, 2021

  • Apr 05 – Apr 11

Tonight’s chart shows the sky in April high to the south around mid-evening. To the upper left of the constellation Leo the Lion are dozens of very faint stars. They make up the constellation Coma Berenices, otherwise known as Berenice’s Hair. You need a dark sky to appreciate the constellation Coma Berenices. If you have one … it’s very beautiful.

The Greek-Egyptian astronomer Ptolemy and others considered these stars the tuft at the end of Leo the Lion’s tail. Coma Berenices remained part of Leo until a few hundred years ago, when it was first listed as a separate constellation.

The story goes that an ancient Egyptian queen, Berenice, feared for her husband’s life as he went into battle. She prayed to Aphrodite, promising to cut off her long, luxurious curls if the king returned safely. He did, and Berenice kept her promise and cut off her hair, placing it as a sacrifice on Aphrodite’s altar.

But the next day the hair was gone!

The constellation Coma Berenices. Click here for a larger sky chart.

The king was enraged that the temple priests had not protected the precious locks. A quick-thinking astronomer saved the day, or rather night, by pointing to the cascading stars at the end of Leo’s tail. He told the king that these were the queen’s tresses placed in the sky by Aphrodite for all to see.Must Watch Sky Events in 2021

The king and queen were appeased, and no priests were beheaded.Large spiral galaxy with smaller fuzzy oblong galaxies behind it.

There’s a vast cluster of galaxies located in the direction of the constellation Coma Berenices. Here is a majestic face-on spiral galaxy located deep within the Coma Cluster. Read more about the Coma galaxy cluster. Image via NASA.

Bottom line: The constellation Leo once had a tail, a clump of faint stars. Now these same stars are known as Coma Berenices, the hair of a queen.

Star-hop from Leo to the Coma star cluster

How do you star hop?

Posted by Deborah Byrd in TONIGHT | April 11, 2021

Rebecca wrote:

What is star hopping? What does that mean?

Amateur astronomers use star hopping to go from stars and constellations they know … to ones they don’t know yet. First, look for noticeable patterns on the sky’s dome. One very easy pattern to find at this time of year is the constellation Orion the Hunter. You’ll find it descending in the west after sunset. Orion is easy to find because it contains a very noticeable pattern of three medium-bright stars in a short straight row. These stars represent Orion’s Belt.

If you can find Orion, you can use it to star hop to Sirius, the sky’s brightest star, in the constellation Canis Major. Orion and Sirius are dropping into the sun’s glare at this time of year, so be sure to look for them soon after the sun goes down.

And that’s how you come to know the constellations. You use what you’ve already learned to build outward to find new patterns.Star field above building roof with long red arrow from pointer stars to Polaris.

View larger. | Another example of star hopping. The two outer stars in the bowl of the Big Dipper always point to Polaris, the North Star. Photo by EarthSky Facebook friend Abhijit Juvekar. Thank you, Abhijit!

Bottom line: Find new stars and constellations by star hopping from ones you already know.

Look for the legendary green flash

Posted by Deborah Byrd in TONIGHT | April 12, 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. 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.Skip Ad

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.

Enjoying EarthSky so far? Sign up for our free daily newsletter today!Old moon and morning planets on April 17, 2020.

Are you an early riser? Then catch the old moon beneath the line-up of three bright morning planets: Jupiter, Saturn and Mars.

See young moon, Mars from April 13-17

Posted by Bruce McClure in TONIGHT | April 13, 2021

Chart: 4 positions of crescent moon along steep ecliptic line with Mars, Pleiades, Aldebaran labeled.

Watch for the young moon to return to the evening sky on April 13 or 14, sitting low in the west as evening dusk ebbs toward nightfall. If you miss this delicately slender young moon on April 13, try again on April 14. Each day will present as a wider – yet still slender – waxing crescent moon higher in the west at sunset, to stay our longer after dark. To increase your chances of catching this slender beauty on April 13, find an unobstructed horizon in the direction of sunset, and bring along binoculars.

Find out when the moon will set in your sky from Old Farmer’s Almanac (USA and Canada), or the Sunrise Sunset Calendar (worldwide), remembering to check the moonrise and moonset box.

Day by day, watch for the widening moon to travel upward, to sweep in between the constellation Taurus the Bull’s two most prominent features: Aldebaran, the Bull’s brightest star, and the Pleiades star cluster.

After that, the moon will meet up with the red planet Mars, which is only modesty-bright right now, not much more than half as bright as Aldebaran. In fact, if you’re at the right spot worldwide (Southeast Asia), you can watch the moon occult (cover over) Mars on the evening of April 17, 2021. More about this occultation below.

These next few weeks will present your last chance to view the constellation Taurus in the evening sky. That’s because the sun will be entering this constellation on May 14, 2021, and will remain in front of Taurus until June 21, 2021. So use the young moon to find the star Aldebaran and the Pleiades cluster now, before the Bull succumbs to the glow of evening twilight by late April 2021.Must Watch Sky Events in 2021

On or near March 13, the red planet Mars resides right between the two stars depicting the tips of the Bull’s horns: Elnath and Zeta Tauri. Notice that on or near this date, Mars is found about midway between two brilliant stars, Capella and Betelgeuse. When the moon or Mars are no longer in this region of the sky to guide you, locate the horn stars (Elnath and Zeta Tauri) midway between Capella and Betelgeuse.

On or near March 13, the red planet Mars resides right between the two stars depicting the tips of the Bull’s horns: Elnath and Zeta Tauri. Notice that on or near this date, Mars is found about midway between two brilliant stars, Capella and Betelgeuse. When the moon or Mars are no longer in this region of the sky to guide you, locate the horn stars (Elnath and Zeta Tauri) midway between Capella and Betelgeuse.

Incidentally, once you become familiar Elnath and Zeta Tauri, you’ve pretty much stumbled upon Messier 1 (Crab Nebula), a remnant of supernova explosion which was visible in the year 1054. Better seen with the telescope, some people have claimed to see the Crab nebula as tiny, faint smudge with 7 x 50 binoculars.Chart showing constellations Orion and Taurus.

The 3 stars at the midsection of Orion – called the Belt of Orion – always point to the bright red star Aldebaran, in the V-shaped head of Taurus the Bull. Note M1 (Crab Nebula) between the star Elnath (Beta Tauri) and Zeta Taurui. Chart via the IAU (International Astronomical Union).

Who will see the lunar occultation of Mars on April 17, 2021?

Take a look at the worldwide map below via IOTA. The swath between the solid white lines shows where the moon occults Mars in the April 17 evening sky. The small section between the blue lines depicts where the occultation is visible at dusk April 17, and the area in between the dotted red lines shows where the occultaion takes place during the daytime hours on April 17.Worldwide map of the lunar occultation of Mars on April 17, 2021

Worldwide map of the lunar occultation of Mars via IOTA (international Occultation Timing Association).

Mars disappears behind the moon’s dark side and then reappears from behind the moon’s illuminated side. The looped section at the right has Mars disappearing behind the moon in a nighttime sky, but the moon will set before the occultation ends. So the reappearance of Mars won’t be visible from this looped area.

Meanwhile, the looped section on the left (over the Atlantic Ocean) may watch Mars’ reappearance (with an optical aid in a daytime sky), but the moon rises after the occultation has already begun. Therefore, the initial disappearance of Mars won’t be visible from this part of the world.

Click here to find out the occultation times in Universal Time. You must convert Universal Time to your local time. Here’s how.

We give an example of how to figure out the local times of the occultation for Hanoi, Vietnam. We click here to find that Mars disappears on April 17, at 13:34:44 UTC, and then reappears on April 17, at 14:01:51 UTC. To convert UTC to Vietnam’s local time zone, we need to add 7 hours to UTC.

Indochina Time (ICT) = UTC + 7 hours

For Hanoi, Vietnam
Mars disappears: 13:34:44 + 7 = 20:34:44 (8:34:44 p.m.) on April 17
Mars reappears: 14:01:51 + 7 = 21:01:51 (9:01:51 p.m.) on April 17

View full size. | World map of time zones. Image via TimeZonesBoy/ CIA/ Wikimedia.

Although only a small portion of the word can view the lunar occultation of Mars in a nighttime sky, most everyone can enjoy watching the young moon sweeping through the constellation Taurus the Bull over the several few days, and in close vicinity of Mars on April 16 and 17.

Moon, Gemini stars on April 18 and 19

Posted by Larry Sessions in TONIGHT | April 18, 2021

Moon passing through the constellation Gemini, highlighting Gemini's brightest stars, Castor and Pollux

On April 18 and, 19, 2021, you’ll find the moon displaying a wide waxing crescent phase and in the vicinity of Castor and Pollux, the two brightest stars in the constellation Gemini the Twins.

And there’s another bright star on the other side of the moon these next few nights. It’s Procyon, the brightest in the constellation Canis Minor the Lesser Dog. Procyon would be hard to pick out without the help of tonight’s moon, or a good constellation chart. On the other hand, Castor and Pollux are extremely noticeable in the night sky. No other two such bright stars appear so close together.

Regardless of the seeming connection between these two stars, Castor and Pollux aren’t close together in space. They just happen to reside along the same line of sight from Earth.

Both Castor and Pollux are bright stars, and they’ve been known as Twins for centuries at least. But they don’t really look alike. Pollux is golden in color, and Castor is pure white. If you have binoculars, they’ll help you to more easily distinguish the color contrast between Castor and Pollux.

Also, Castor and Pollux are different kinds of stars. Castor is a hot, white-colored star that is well known for being a multiple star system. It consists of three pairs of binary stars, that is, six stars bound together in an intricate gravitational dance. Pollux is a cool and bloated orange-colored star, said to be the closest giant star to Earth. A star with the mass of our sun swells up into a giant in its old age. But astronomers assure us that our sun won’t become a giant for another 5 billion years or so.Must Watch Sky Events in 2021

On April 16, 2021, the moon crossed the ecliptic – Earth’s orbital plane and the annual pathway of the sun across our sky – at its ascending node. That happened at 5:53 UTC. Then about 4 days later, the moon will reach its first quarter phase on April 20 at 6:59 UTC.

The moon will remain north of the ecliptic (Earth’s orbital plane) until the moon crosses its descending node on April 29, 2021, at 9:18 UTC. Then the moon will stay south of the ecliptic until it crosses the ecliptic at its ascending node on May 13, 2021, at 10:29 UTC.

You can see the comparative size of the star Pollux and our sun in this image, as well as some other stars. Pollux is a giant star by virtue of its age. Many stars – including our sun – will swell to the giant stage as they age.

In many cultures, Castor and Pollux were seen as twin stars, usually as heroes. Many old sky myths invoke the idea of twins to explain their proximity on the sky’s dome.

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If you have a dark sky, notice that 2 nearly parallel streams of stars extend from Castor and Pollux. These stars likely reinforced the idea of twins in various cultures around the world. Every December, the Geminid meteor shower radiates from near star Castor in Gemini.

In Greek and Roman mythology, Castor and Pollux were the twin sons of Jupiter and Leda and brothers of Helen of Troy. They sailed with Jason as two of his Argonauts.

Pollux, represented by the brighter star, was immortal, but his brother Castor was not. When Castor was killed in a fight, Jupiter wanted the two to remain together, so he decreed that they each should spend some time in the underworld and some time in the heavens. This is a fanciful way of explaining why the constellation is above the horizon for part of each day and below the horizon for the rest. Castor and Pollux are sometimes said to represent brotherly love.

Meanwhile, in China, these two stars were associated with water, as part of constellations representing rivers. They were sometimes also seen as the complementary elements of yin and yang.

Castor and Pollux mark the starry eyes of the Gemini Twins. Image via Wikipedia.

Because Gemini is a constellation of the zodiac, the sun passes in front of this constellation for about a month each year, from about June 21 to July 20.

Bottom line: The moon is near the stars Castor and Pollux in the constellation Gemini the Twins on April 18 and 19, 2021. These stars represent twins in many cultures.

Find Hercules between 2 bright stars

Posted by Deborah Byrd in TONIGHT | April 22, 2021

Sky chart of constellation Hercules

Tonight, try locating one of the coolest constellations up there. The constellation Hercules the Kneeling Giant can be seen ascending in the east-northeast on these Northern Hemisphere spring evenings. You can find Hercules between two brilliant stars: Arcturus and Vega. The chart at the top of this post shows the sky for around 8 to 9 p.m. local time (9 to 10 p.m. daylight saving time), when the constellation Hercules, and the two stars so essential for finding it, are well up in the northeastern to eastern sky.

Arcturus is in the constellation Boötes, and Vega is in the constellation Lyra. At nightfall, Vega may still be below in your horizon. If so, wait awhile … it’ll rise soon.

A line between Arcturus and Vega passes through what is known as the Keystone – an asterism, or noticeable pattern – in Hercules. The Keystone is a squarish figure in the center of Hercules. See it on the charts above and below?

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The constellation Hercules, with its prominent Keystone asterism marked. Image via Wikimedia Commons.

The Keystone is a helpful pattern for more reasons than one. First, it’s noticeable on the sky’s dome, so it can lead your eye to Hercules.Must Watch Sky Events in 2021

Also, the Keystone in Hercules can help you find the most fascinating telescopic object within the boundaries of this constellation. This object is a globular star cluster known to stargazers as M13 or the Great Cluster in Hercules. M13 is barely visible to the eye alone in the darkest of skies, but binoculars show it as a nebulous starlike patch of light. And telescopes show stars both on the periphery of the cluster and toward its center.Star chart, black stars on white background.

The chart shows M13 (the great Hercules cluster) in the Keystone. This chart of the constellation Hercules is via the IAU.

This beautiful object is one of the galaxy’s oldest inhabitants. It’s a tightly packed spherical collection of about one million stars.

Read more: M13 or the Great Cluster in Hercules

M13, aka the Great Cluster in Hercules. This object is a globular star cluster, one of our galaxy’s oldest inhabitants. Photo via ESA/ Hubble/ NASA.

Bottom line: Use the brilliant stars Arcturus and Vega to find the constellation Hercules tonight!

Mercury, Venus pair up dusk April 24-26

Posted by Bruce McClure in TONIGHT | April 24, 2021

Constellation Orion, slanted ecliptic line, 2 planets close together just over horizon.

Up for a big observing challenge? Try to spot the planets Mercury and Venus coupling up on the sky’s dome at dusk on April 24, 25 and 26, 2021. We give you fair warning. The quest won’t be easy, even with binoculars. You’ll need an unobstructed horizon in the direction of sunset and crystal-clear skies to view these close-knit worlds in the glow of evening twilight. The real sky will appear brighter than it does on our sky chart, and you probably won’t see the constellation Orion until after Mercury and Venus have already set.Map of inner solar system - Mercury, Venus, Earth and Mars - showing Mercury and Venus in conjunction as viwed from Earth.

Map of the inner solar system – Mercury, Venus, Earth and Mars – on April 25, 2021, via Solar System Live. Although Mercury and Venus appear on nearly the same line of sight as seen from Earth, Mercury and Venus are not close together in space. From Earth, Mercury and Venus reside 1.28 and 1.71 astronomical units (AU) away from us, respectively. From the sun, Mercury and Venus lie 0.31 and 0.72 AU distant. Solar system symbols via NASA/Lunar and Planetary Institute

The Northern Hemisphere has the advantage because these two worlds stay out longer after sunset at more northerly latitudes, At southerly latitudes, the planetary twosome sets sooner after the sun. We give you the approximate setting times for the next few days at various latitudes:

60 degrees north latitude: Mercury-Venus set about 50 minutes after the sun

40 degrees north latitude: Mercury-Venus set about 35 minutes after the sun

Equator (0 degrees latitude): Mercury-Venus set about 30 minutes after the sun

35 degrees south latitude: Mercury-Venus set about 20 minutes after the sun

Want more specific information? Try Old Farmer’s Almanac or TimeandDate

The good news is that both of these planets are brilliant. Venus ranks as the third-brightest celestial object to light up the heavens, after the sun and moon, respectively. Although Mercury is some 8 times dimmer than Venus, Mercury at present shines as brilliantly as Sirius, the brightest star in the nighttime sky. Unlike Sirius, though, Mercury and Venus will set before nightfall, and Sirius probably won’t pop out until after Mercury and Venus set.

Don’t worry if you miss these hard-to-find worlds over the next several days. Day by day, both Mercury and Venus will climb up higher in the sky at sunset and will stay out longer after sundown. Mercury, the faster-moving planet, will soar above Venus in the evening twilight for the next month or so, or until Mercury reaches its greatest elongation – maximum angular distance from the setting sun – on May 17, 2021. After that, Mercury will descend downward, toward the sunset, as Venus continues its climb upward, away from the setting sun.

Mercury and Venus will meet up again for their final conjunction for the year on May 29, 2021 (from North America: evening of May 28). This time around, their rendezvous will find them higher up in the sky at sunset, to say out longer after sundown.Must Watch Sky Events in 2021

Shortly after sunset – April 24, 25 and 26, 2021 – try to spot the furtive rendezvous of the two inferior planets, Mercury and Venus, at evening dusk.

Moon near star Spica on April 25

Posted by Bruce McClure in TONIGHT | April 25, 2021

Moon between the springtime stars, Arcturus and Spica.

At nightfall – April, 25, 2021 – look eastward to view the bright waxing gibbous moon and the star Spica over the horizon. First look for the moon, and that nearby bright star will be Spica, the brightest star in the constellation Virgo the Maiden.

Spica serves as a prime example of a 1st-magnitude star; in other words, it’s one of the brightest stars in our sky. You should have no trouble picking it out, even in the glare of the waning gibbous moon.

Spica is a blue-white gem of a star, and, for stars, color indicates the star’s surface temperature. Spica’s blue-white complexion reveals that its surface temperature is extremely high (39,860 degrees Fahrenheit, or 22,127 degrees Celsius). In contrast, our yellow-colored sun has a much cooler surface (only 9,980 degrees F, or 5,527 degrees C). The surface temperature of an orange star, such as Antares, is even cooler (7,300 degrees F, or 4,038 degrees C).Color-coded table of star temperatures ranging from hot blue to cool red.

Color/temperature chart via Wikipedia.

Spica lies nearly on the ecliptic, the annual pathway of the sun in front of the background stars. If you could see stars during the day, you’d see the sun in front of Virgo from approximately September 16 to October 31 each year.

The moon (more or less) follows the ecliptic as well, and thus – as the moon makes its monthly rounds in front of the constellations of the zodiac – it spends several days each month in front of Virgo, routinely passing near Spica.Must Watch Sky Events in 2021Star chart with stars in black on white and line of ecliptic going through Virgo.

Virgo constellation chart via the International Astronomical Union (IAU).

But the moon’s motion along the ecliptic is somewhat wayward. The moon undergoes an 18.6-year cycle, whereby the moon – as it passes Spica – can swing anywhere from 5 degrees (10 moon-diameters) to the north of the ecliptic to 5 degrees (10 moon-diameters) south of the ecliptic.

Since Spica lies 2 degrees (4 moon-diameters) south of the ecliptic, that means the moon has periods when it occults (passes in front of) this star. The next occultation series of Spica will start on June 16, 2024, and will conclude on November 17, 2025, featuring a total of 20 occultations.

Although yellow-orange Arcturus appears brighter than blue-white Spica to our eye, that’s only because Spica is so much farther away from us. Arcturus resides about 37 light-years away, whereas Spica lies some seven times farther off than that, at some 260 light-years distant. If Arcturus stood at the sun’s distance from us, it’d shine over 100 times more brightly than the sun. But Spica at the sun’s distance away would shine with the firepower of over 2,000 suns!

Bottom line: From around the world on the night of April 25, 2021, use the waxing gibbous moon to locate Spica, the brightest star in the constellation Virgo the Maiden.

April full moon year’s 1st supermoon

Posted by Bruce McClure in TONIGHT | April 26, 2021

Two grey full moons on top of each other, one a full supermoon (full moon at perigee) overlaid with a micro-moon (full moon at apogee).

Above: Contrasting a full supermoon (full moon at perigee) with a micro-moon (full moon at apogee). Image credit: Stefano Sciarpetti

Around the world, the moon will look full to the eye on the nights of April 26 and 27, 2021. However, the crest of the full moon will fall on April 27 at 3:31 UTC. At United States’ times zones, that translates to April 26, at 11:31 p.m Eastern Time, 10:31 p.m. Central Time, 9:31 p.m. Mountain Time and 8:31 p.m. Pacific Time. In North America, we call the April full moon the Pink Moon. This April full moon presents the first in a “season” of three straight full moon supermoons. Overall, this April full moon gives us second-closest full moon of the year.

Here are the distances (as measured between the centers of the moon and Earth) for the three upcoming full moon supermoons:

Full moon supermoons:

April 27, 2021: 222,212 miles (357,615 km)

May 26, 2021: 222,117 miles (357,462 km)

June 24, 2021: 224,662 miles (361,558 km)

In contrast, the most distant and smallest full moon of the year will fall on December 19, 2021. Sometimes called a micromoon, it’ll be 252,235 miles (405,932 km) away. That’s a whopping 30,118 miles (48,470 km) farther away than the year’s closest and biggest full moon on May 26, 2021.Must Watch Sky Events in 2021Two photos of half the moon facing each other. The one on the left is bigger.

Eliot Herman in Tucson also compared the June full moon with the November 2016 supermoon. He wrote: “A comparison of the closest supermoon since 1948 on November 14, 2016, with farthest mini-moon on June 9, 2017. Both images captured with a Questar telescope and a Nikon D800 camera. The images were combined with Photoshop.”

Visit Sunrise Sunset Calendars to find out when the moon turns full in your time zone. Be sure to check the moon phase box.

Some people prefer to call a full supermoon a perigean full moon. That’s when the full moon and lunar perigee – the moon’s closest point to Earth in its monthly orbit – closely coincide for a few months. This April full moon occurs about 12 hours before the moon sweeps to lunar perigee. Next month, the May full moon will turn full some 9 hours after the moon swings to perigee. April and May feature the only two months in 2021 whereby full moon and perigee occur less than 24 hours apart.

What’s more, Earth’s oceans feel the extra pull of these supermoons (full moons near perigee). All full moons (and new moons) combine with the sun to create larger-than-usual tides, called spring tides. But closer-than-average full moons (or closer-than-average new moons) – that is, supermoons – elevate the tides even more. These extra-high spring tides are wide ranging. High tides climb up especially high, and, on the same day, low tides plunge especially low. Experts call these perigean spring tides, in honor of the moon’s nearness. If you live along an ocean coastline, watch for them! They typically follow the supermoon by a day or two.

Do extra-high supermoon tides cause flooding? Maybe yes, and maybe no. Flooding typically occurs when a strong weather system accompanies an especially high spring tide.

The recurring cycle of supermoons

The full moon supermoon series of 2021 will recur after 14 lunar months (14 returns to full moon). That’s because 14 returns to full moon almost exactly equal 15 returns to perigee, a period of about one year, one month, and 18 days.

The mean lunar month (full moon to full moon, or new moon to new moon) = 29.53059 days, whereas the mean anomalistic month (perigee to perigee, or apogee to apogee) = 27.55455 days. Hence:

14 lunar months (14 returns to full moon) x 29.53059 days = 413.428 days
15 anomalistic months (15 returns to lunar perigee) x 27.55455 days = 413.318 days

Given that supermoons recur in cycles of 413 days (about one year, one month and 18 days), we can can expect the full moon supermoons to come about one month and 18 days later next year, in 2022.

Full moon distance (June 14, 2022): 222,212 miles or 357,658 km
Full moon distance (July 13, 2022): 222,117 miles or 357,418 km
Full moon distance (August 12, 2022): 224,662 miles or 361,409 km

Bottom line: The whole globe enjoys a full-looking moon on April 26 and 27, 2021. The exact time of full moon falls on the evening of April 26 for the clocks in the mainland United States. It’s the first supermoon of 2021 and second-closest full moon this year. No matter where you live worldwide, watch for the full moon to shine from dusk until dawn.

Watch for a daytime morning moon

Posted by Bruce McClure in TONIGHT | April 28, 2021

Daytime moon.

Image at top via Buddy Puckhaper of Charleston, South Carolina.

Full moon happened on the night of April 26-27, 2021, and by April 29 the moon is in a waning gibbous phase, rising later and later at night. For the mainland United States, the April 28 moon rises in the southeast, roughly two to three hours after sunset. The days following full moon present a good time time to catch a nearly-full daytime moon over your western horizon after sunrise. Watch for it!Chart: moon and the bright star Antares with slanted green line of ecliptic.

Look for the moon near the bright star Antares on the evening of April 28. Then, after sunrise on April 29, look for the moon close to the horizon.

On the evening of April 28, the moon, and the nearby bright star Antares, rise at mid-to-late evening at mid-northern latitudes. From the Southern hemisphere, the moon and Antares rise at early-to-mid evening. Whereas Antares will rise about 4 minutes earlier each evening, the moon will be rising in the east later and later each evening.

But if you’re not one to stay up late, then look for the daytime moon low in your western sky right after sunrise. Day by day, the lighted portion of the waning gibbous moon will shrink. The half-lit last quarter moon will come on May 3, 2021.Daytime moon in the southwest sky on late April/early May 2021.

The moon in the daytime sky after sunrise in late April and early May 2021, at mid-northern North American latitudes. In the Southern Hemisphere, the tilt of the ecliptic (the blue line on the chart) is much steeper than at northerly latitudes. From more southerly latitudes, the daytime moon will climb more upward than sideways with each passing day.

The moon is up in the daytime much of the time. But, because it’s pale against the blue sky, it’s not as noticeable during the day as at night. However, there are certain times of the month when the daytime moon is more noticeable. Late April/early May 2021 presents one of those windows for catching a daytime moon.Must Watch Sky Events in 2021

You’ll often miss the moon during the day because it’s so pale against the blue daytime sky. Look closely this weekend, especially in the hours after sunrise. Look west! You’ll see it. Our friend Jenney Disimon in Sabah, North Borneo, caught this daytime moon on January 4, 2018.

Why is the daytime moon most noticeable now? The moon is up during the day half the time. It must be, since it orbits around the whole Earth once a month. A crescent moon is hard to see, though, because it’s so near the sun in the sky. At the vicinity of last quarter moon about a week from now, you might have to crane your neck, looking up, to notice it after sunrise.

The moon in late April/early May is noticeable simply because the moon is still showing us most of its lighted face; it appears large in our sky. Also, in the hours after sunrise, the moon is fairly near the western horizon, so people out and about might catch sight of it.

At mid-northern latitudes in North America, the moon will set about two hours after sunrise on April 29. It’ll set roughly one hour later after sunrise each day thereafter.

These recommended almanacs can help you find the moon’s setting time in your sky

Daytime moon seen on December 18, 2010. Image by Brian Pate. Used with permission.

Bottom line: The moon is now in a waning gibbous phase. Beginning around April 29 – shortly after sunrise – you’ll see it floating pale and beautiful against a blue sky. Look west!