Use Big Dipper To Find Polaris In September
Posted byLarry SessionsSeptember 2, 2021
Use Big Dipper to find Polaris
The northern sky is like a large celestial clock, with Polaris at its center. In other words, the entire northern sky wheels in a great circle throughout the night (although it’s wheeling in a counter-clockwise direction). But the northern star Polaris stays still (or nearly so). That’s because Earth’s northern axis nearly points to it. And so Polaris is the famous North Star, used by sea navigators and scouts to find the direction north. Want to find it? You can use the famous Big Dipper asterism to locate Polaris, the North Star.
Notice that a line from the 2 outermost stars in the bowl of the Big Dipper points to Polaris. And notice that Polaris marks the tip of the handle of the Little Dipper. Just notice all this soon, because, in September, the Big Dipper is headed for its least noticeable time of year. The reason is that the Big Dipper swings full circle – 360 degrees – around Polaris in about 23 hours and 56 minutes. In 24 hours, the Big Dipper actually swings more than a full circle, or 361 degrees. Does that make a difference? Yes! It means that – if you look at the same time each evening – the Big Dipper will appear just a little bit lower in the northwestern evening sky.
In other words, the Dipper is descending in the northwestern evening sky, from one night to the next. And that means that, a month from now at mid-evening (say around early October) the Big Dipper will be noticeably lower in the northwest. For some months in autumn, the Big Dipper is beneath the horizon in the evening, as seen from southernmost latitudes in the United States. That might be why, if you’re just learning the sky, you sometimes look for the Dipper and can’t find it.
Meanwhile, the Big Dipper is circumpolar, or always above the northern horizon, from more northerly latitudes. You’ll find it in your sky throughout the year as seen from the northern U.S., Canada and similar latitudes.
The constant motion from night to night of these stars circling Polaris is a bit like a bear circling its prey, looking for a way to attack. Several ancient cultures from the Greeks and Romans to the Mi’kmaq Indians likened these stars to a bear.
In Greek mythology, the Big Dipper asterism represents the hindquarters and tail of the constellation Ursa Major, the Great Bear. The Mi’kmaq saw the three stars of the Big Dipper handle as hunters chasing the bear.
Watch the Big and Little Dippers circle around Polaris tonight!
Bottom line: To locate Polaris, the North Star, just draw a line between the two outer stars in the bowl of the Big Dipper.
Close-Up On Cassiopeia The Queen
Posted byEarthSkySeptember 3, 2021
Tonight – or any autumn evening – Cassiopeia the Queen can be found in the northeast after sunset. The shape of this constellation makes Cassiopeia’s stars very noticeable. It has the distinctive shape of a W, or M, depending on the time of night you see it.
Look for the Queen starting at nightfall or early evening. Depending on how far north you are, Cassiopeia may set as night passes, then rise again in the hours before sunrise. For those in the northern U.S. and Canada, Cassiopeia is circumpolar, above the horizon all night long.
Cassiopeia represents an ancient queen of Ethiopia. The entire constellation is sometimes also called Cassiopeia’s Chair, and some old star maps depict the queen sitting on the chair, marked by the five brightest stars of this constellation. These stars are Schedar, Caph, Gamma Cassiopeiae, Ruchbah and Segin.
00:04/02:1510 Sec4M2KMust Watch Sky Events in 2021
If you have a dark sky, you can look below Cassiopeia in the northeast on these autumn evenings for a famous binocular object. This object is called the Double Cluster in Perseus. These are open star clusters, each of which consists of young stars still moving together from the primordial cloud of gas and dust that gave birth to the cluster’s stars. These clusters are familiarly known to stargazers as H and Chi Persei.
Stargazers smile when they peer at them through their binoculars, not only because they are beautiful, but also because of their names. They are named from two different alphabets, the Greek and the Roman. Stars have Greek letter names, but most star clusters don’t. Johann Bayer (1572-1625) gave Chi Persei – the cluster on the top – its Greek letter name. Then, it’s said, he ran out of Greek letters. That’s when he used a Roman letter – the letter H – to name the other cluster.
After midnight, Cassiopeia swings above Polaris, the North Star. Before dawn, she is found in the northwest. But during the evening hours, Queen Cassiopeia lights up the northeast sky.
Bottom line: The constellation Cassiopeia the Queen has the distinct shape of a W or M. Find her in the north-northeast sky on September and October evenings.
Venus-Spica Conjunction Sept 5
Posted byBruce McClureSeptember 4, 2021
The Venus-Spica conjunction takes place on September 5 at 6 UTC. Depending on where you live worldwide, the planet Venus and the star Spica may appear closest together on the sky’s dome after sunset September 4 or 5. Try September 6, too. Look first for Venus, the brightest planet and brighter of the these two starlike points in the western twilight. Venus blazes away in your western sky some 30 to 45 minutes (or sooner) after sunset. Then, as dusk deepens into nightfall, watch for Spica, the constellation Virgo’s brightest star, to pop out close to Venus.
From most of the world, the twosome should become visible to the eye alone around 60 to 75 minutes after the sunset. We except far-northern latitudes, where Venus and Spica will follow the sun below the horizon shortly after sundown.
Practiced sky watchers may well catch Venus as little as 10 to 15 minutes after sundown. After all, Venus ranks as the 3rd-brightest celestial object to light up the heavens, after the sun and moon, respectively. Spica is an almost-perfect example of a 1st-magnitude star. Even so, Spica pales next to dazzling Venus, which shines a good hundred times more brilliantly than this blue-white gem of a star.
Venus setting times
The farther south you live on the globe, the longer that Venus and Spica stay out after dark. We give the approximate setting time for Venus, the much brighter of the two, for various latitudes:
60 degrees north latitude:
Venus sets 25 minutes after the sun
40 degrees north latitude:
Venus sets 1 1/2 hours (90 minutes) after sunset
Equator (0 degrees latitude):
Venus sets 2 1/2 hours (150 minutes) after sunset
35 degrees south latitude:
Venus sets 3 1/4 hours (195 minutes) after sunset
Tilt of the ecliptic favors Southern Hemisphere
At sunset/dusk/nightfall on September evenings, the ecliptic – pathway of the sun, moon and planets – arcs highest overhead for the year in the Southern Hemisphere.
In the Northern Hemisphere, it’s the exact opposite. The ecliptic sinks lowest down for the year on September evenings.
That’s why Venus and Spica stay out so much longer after sunset from southerly latitudes, yet so much sooner after sundown at northerly latitudes.
Still, no matter where you live worldwide, it’ll be to your advantage to find an unobstructed horizon in the direction of sunset. At far-northerly latitudes, you may need binoculars to see Venus and/or Spica taking stage in a single binocular field.
Bottom line: Enjoy the grand pairing of the sky’s brightest planet, Venus, and the constellation Virgo’s brightest star, Spica, around September 4, 5 and 6, 2021.
Zodiacal Light: All You Need To Know
False dawn, or false dusk
The zodiacal light is a cone of eerie light above the sunrise or sunset point on the horizon, before morning dawn breaks or after evening twilight ends. No matter where you are on Earth, your best chance to see it in the east before dawn is late summer or early autumn (false dawn). Your best chance to see it in the west at dusk is late winter or early spring (false dusk).
The light looks like a hazy pyramid. It appears in the sky just before true dawn lights the sky. It’s comparable in brightness to the Milky Way, but even milkier in appearance.
Maybe you’ve seen the zodiacal light in the sky already and not realized it. Maybe you glimpsed it while driving on a highway or country road. This strange light is a seasonal phenomenon. Springtime and autumn are best for seeing it, no matter where you live on Earth.
How can I see the zodiacal light?
Suppose you’re driving toward the east – in the dark hour before dawn – in late summer or early autumn. You might sight what you think is the light of a nearby town, just over the horizon. But it may not be a town, but the zodiacal light. The light extends up from the eastern horizon, shortly before morning twilight begins. The zodiacal light can be extremely bright and easy to see from latitudes like those in the southern U.S.
We also sometimes hear from skywatchers in the northern U.S. or Canada who’ve captured images of the zodiacal light.
You’ll need a dark sky location to see the zodiacal light, someplace where city lights aren’t obscuring the natural lights in the sky.
Most visible around the equinoxes
The zodiacal light is often visible from the tropics. That’s because the ecliptic – pathway of the sun and moon – hits the horizon at a steep angle from this part of the world all year long.
Outside the tropics, the zodiacal light is most likely to be visible before dawn in late summer/early autumn. That’s because the ecliptic – or path of the sun and moon – intersects the horizon most steeply for the year at sunrise on the autumn equinox.
On the other hand, the zodiacal light is most prevalent after dusk in late winter/early spring. That’s because then the ecliptic is most perpendicular to the western horizon at sunset on the spring equinox. This holds true for either the Northern or Southern Hemisphere.
Ecliptic steepest at autumn sunrises, or spring sunsets
In late summer/early autumn, the zodiacal light can be seen in the hour before true dawn begins. Or, in late winter/early spring, it can be seen for up to an hour after all traces of evening twilight leave the sky. Unlike true dawn or dusk, though, there’s no rosy color to the zodiacal light. The reddish skies at dawn and dusk are caused by Earth’s atmosphere, while the zodiacal light originates far outside our atmosphere, as explained below.
The darker your sky, the better your chances of seeing it. Your best bet is to pick a night when the moon is out of the sky, although it’s definitely possible, and very lovely, to see a slim crescent moon in the midst of this strange milky pyramid of light.
If you see it, let us know! If you catch a photo, submit it here.
Springtime? Autumn? When should I look?
Is there a Northern/Southern Hemisphere difference between the best time of year to view the zodiacal light? Yes and no. For both hemispheres, springtime is the best time to see the zodiacal light in the evening. Autumn is the best time to see it before dawn.
No matter where you live on Earth, look for the zodiacal light in the east before dawn around the time of your autumn equinox. Look for it in the west after sunset around the time of your spring equinox.
Of course, spring and autumn fall in different months for Earth’s Northern and Southern Hemispheres.
So if you’re in the Northern Hemisphere look for the zodiacal light before dawn from about late August through early November.
In those same months, if you’re in the Southern Hemisphere, look for the light in the evening.
Likewise, if you’re in the Northern Hemisphere, look for the evening zodiacal light from late February through early May. During those months, from the Southern Hemisphere, look for the light in the morning.
What is zodiacal light?
People used to think zodiacal light originated somehow from phenomena in Earth’s upper atmosphere, but today we understand it as sunlight reflecting off dust grains that circle the sun in the inner solar system. These grains are thought to be left over from the process that created our Earth and the other planets of our solar system 4.5 billion years ago.
These dust grains in space spread out from the sun in the same flat disc of space inhabited by Mercury, Venus, Earth, Mars and the other planets in our sun’s family. This flat space around the sun – the plane of our solar system – translates on our sky to a narrow pathway called the ecliptic. This is the same pathway traveled by the sun and moon as they journey across our sky.
The pathway of the sun and moon was called the zodiac or Pathway of Animals by our ancestors in honor of the constellations seen beyond it. The word zodiacal stems from the word zodiac.
Zodiacal light is a solar system phenomenon
The grains of dust that create the zodiacal light are like tiny worlds. They range from meter-sized to micron-sized. They’re densest around the immediate vicinity of the sun and extend outward beyond the orbit of Mars. Sunlight shines on these grains of dust to create the light we see. Since they lie in the flat sheet of space around the sun, we could, in theory, see them as a band of dust across our entire sky. If we could see them, they’d mark the same path that the sun follows during the day. And indeed there are sky phenomena associated with this band of dust, such as the gegenschein.
But seeing such elusive sky phenomena as the gegenschein is difficult. Most of us see only the more obvious part of this dust band – the zodiacal light – in either spring or fall. It’s time to watch for this elusive light! See it, and enjoy it.
Bottom line: The zodiacal light – aka false dawn or dusk – is a hazy pyramid of light, really sunlight reflecting off dust grains in the plane of our solar system. You need a dark sky to see it. Northern Hemisphere dwellers look east before dawn from about late August through early November. Southern Hemisphere dwellers look in the evening in those same months. Or, from the Northern Hemisphere, look for the evening zodiacal light from late February through early May. From the Southern Hemisphere, look for the light in the morning in those same months.