Autumn brings not only a change in the weather, but also a major transition in your stargazing targets. Many of the stars in the summer sky are bright, such as Vega and Arcturus. And the constellations have many easily recognizable shapes, for example the Big Dipper and Scorpius. The brightest part of the Milky Way is high overhead, and the sky is filled with scores of globular clusters, hundreds of open clusters and numerous emission nebulae. The winter sky is dominated by Orion and the bright stars of the Winter Hexagon, which includes five of the 10 brightest stars in the sky.
But fall brings a break between the two seasons. None of the stars or constellations are as bright as those in the summer or winter. The zodiac is less spectacular than summer or winter, as Capricorn, Aquarius, Pisces and Aries are all relatively dim. But there are still plenty of things to see if you get out your binoculars or telescope.
Pegasus is one of these transition constellations. The Great Square of Pegasus is tilted at a 45 degree angle in the eastern sky right after sunset. The four stars making the fairly symmetrical square are all between second and third magnitude – easily visible but not too bright. The square is approximately 15 degrees, or 1½ fist-widths on a side. The interior of the square is remarkably empty of stars, with only one star being brighter than magnitude 4.5.
Pegasus is the winged horse, and I have seen illustrations that show the square as either the body or as the wings. The forelimbs and head stretch upward from the square. Alpheratz, the left-most star of the square, is now considered to be part of Andromeda. And the hind legs of the horse have been completely taken over by (or I should say reassigned to) the constellation Andromeda.
The magnitude 6 Pegasus globular cluster, M15, is above and to the right of the square. It will appear as a small, fuzzy ball in binoculars. The galaxy in Andromeda, M31, is halfway down Andromeda (the horse’s back legs), or almost halfway between the square of Pegasus and the “W” of Cassiopeia. M31, at 2.5 million light years distance, is usually considered the farthest thing that can be seen with the naked eye. Its apparent magnitude of 3.4 suggests it should be easier to see than it is, but the brightness is spread over an area about six times the size of a full moon, so the large spiral arms are very hard to see.
This monthSaturn is setting quickly in the western sky after sunset. Catch a glimpse before you have to wait until next summer for it to be in the evening sky again.
Uranus is at opposition on Oct. 19. That means it will be crossing the meridian around 1 a.m. that night when it is the closest it will get to Earth. Although not one of the historical visible “wanderers” in the sky, it is visible with the naked eye in dark locations such as the Four Corners. Unfortunately, there are no bright stars nearby to help you find it. It will be below the Pegasus square about another square width.
Comet C-2017/O1 (ASASSN1) was discovered this summer. It is moving quickly through the northeastern sky and might be visible in binoculars, but predicting the brightness of new comets is much less reliable than predicting the weather. Because it is moving from night to night, you will need a star chart to help locate it. The closest approach to Earth will be on Oct. 18.
The Orionid meteor shower peaks on Oct. 21 and 22. This is the shower associated with Haley’s comet and typically produces 10 to 20 meteors per hour. You need to wait for Orion to rise (after midnight) to see any of the meteors associated with this show.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at firstname.lastname@example.org.