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How Many Stars Can You Fit Into a Square?

by Geoff Chester, USNO Public Affairs | 10 November 2020

by Geoff Chester, USNO Public Affairs | 10 November 2020

The Andromeda Galaxy, imaged 2018 October 17
Messier 31, the Andromeda Galaxy, imaged 2018 October 7 from Mollusk, Virginia
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor and a Canon EOS Rebel T2i DSLR

The Moon may be found in her waning crescent phases before dawn as the week opens.  New Moon occurs on the 15th at 12:07 am Eastern Standard Time.  Look for Luna’s crescent about six degrees above dazzling Venus in the morning twilight of the 12th.  On the following morning the Moon is about halfway between Venus and her inner solar system companion Mercury.

Crisp late autumn nights with no Moon in the sky mean that it’s time for the November observing campaign for the citizen-science program
Globe at Night.  This month’s target constellation is Pegasus, the mythical Flying Horse.  Its “landmark” feature is an almost perfect square made up of second-magnitude stars that transits the meridian at around 8:30 pm.  Pegasus is high in the sky for mid-northern observers, and I have always used it as a test for dark skies.  Within the square are a number of faint stars to help you judge the quality of your sky.  Urban and near-suburban skywatchers probably won’t see any stars within the bounds of the square, but as you venture further out into the country faint stars begin to fill it in.  If you can spot three or four stars in the square you are in a pretty dark locale, but observers in truly dark sites can spot nearly a dozen!  You can contribute your star count with the Globe at Night Web App, which gives you a very simple interface to report your findings.  So far this year the program has gathered over 26,000 reports, and the program’s goal is 28,000 by the end of the year.  The program aims to chart light pollution and its effects on both professional and amateur astronomers.

If you are in a place where you can see some of the faint stars in the square take a few moments to track down something that’s almost mind-boggling.  The upper left corner of the square is marked by the star Alpheratz, which is shared by Pegasus and the constellation of Andromeda.  If you allow your gaze to wander toward the “W” of Cassiopeia you will notice two diverging “chains” of stars that lead from Alpheratz.  Follow the lower, brighter chain to the second star, then draw an imaginary line to the second star in the upper, fainter chain.  Extend your gaze in the same direction for the same distance and you will notice a fuzzy patch of light that looks like a tiny detached portion of the Milky Way.  It is, in fact, another galaxy like our own, its faint light glimmering across 2.5 million light-years of intergalactic space.  Known as the Andromeda Galaxy, this is the most distant object that you can see with the unaided eye.  Its elongated shape becomes apparent in binoculars, and it stubbornly refuses to resolve into stars in virtually any amateur telescope.  Its light was described very aptly by the German astronomer Simon Marius in 1612 who remarked that it resembled “the light of a candle shining through horn”.  That light is the combined light of several hundred billion stars!

The Andromeda Galaxy and the Milky Way are the two largest members of the “Local Group” of galaxies, a loose grouping of a few dozen small systems under the sway of that larger ones.  The Andromeda Galaxy is moving toward us, and the mutual gravity of the two systems will bring them together in a slow-motion collision in about 4.5 billion years.

Back in our solar system, Jupiter and Saturn are struggling to stay ahead of the relentless Sun.  The two giant planets are still easy to spot in the southwest after sunset, but by the time that twilight ends they are dipping toward the horizon.  Jupiter will ultimately win the race, passing Saturn in a spectacular conjunction on the winter solstice.

Mars holds court in the evening sky, especially as Jupiter sinks into the trees.  The red planet is very hard to miss.  Not only is he bright, he sports a decidedly ruddy tint and has no competition from nearby bright objects.  However, his diminutive size means that as we earthlings recede from him he gets a bit fainter each passing night.  His apparent diameter is quickly shrinking as well.  That said, his disc won’t approach this size again until the year 2033.  

Venus continues to gradually inch toward the Sun in the pre-dawn sky.  She is still easy to see over the eastern horizon as twilight begins to gather.  The Moon will be in her vicinity on the mornings of the 12th and 13th.

Elusive Mercury also puts in an appearance just before sunrise.  He reaches greatest elongation from the Sun on the 10th, and you should be able to spot him about 10 degrees below Venus.  The best time to look for him will be the morning of the 13th, when the Moon is halfway between the two planets.


Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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