The fall evening sky is often thought of as the empty gap between the rich summer Milky Way sky and the bright winter constellations surrounding Orion. In the middle of this large vacant expanse of sky is Pegasus.
Pegasus is the winged horse of mythology. If you use your imagination, you might be able to make out such a pattern in the fall stars. By far the most prominent part of Pegasus is the asterism called the Great Square. The square is almost 15 degrees on each side, or about 1½ fist-widths. The four corner stars are all between magnitude 2 and 3 and can easily be seen with the naked eye. Other stars inside the square are much dimmer and might not be visible at all in a light-polluted area. I consider the square of Pegasus to be an anchor asterism. By that I mean that I use it to find not only the rest of Pegasus but neighboring constellations, including Andromeda and Pisces.
The northernmost and brightest star of the square is Alpheratz, but this isn’t officially even part of Pegasus. Historically, it was shared with the constellation Andromeda. For me to see a horse associated with the square, Andromeda must be the hind legs. Until the early 20th century, Alpheratz had the dual designation of alpha Andromeda and delta Pegasus, but now, it is associated exclusively with Andromeda.
I often use part of Pegasus to determine my limiting visual magnitude. In other words, what is the dimmest star I can see on a given night? As a reminder, the higher the magnitude number, the dimmer the star. Using Alpheratz and the two easternmost stars of the square to form a triangle, you can count all the stars you can see in that triangle. Then look up this number in a table on The Nine Planets website to find what your limiting magnitude is.
For example, if you can only see the three corner stars and one other star inside the triangle, then your limiting magnitude is 4.7. I hope you can find a darker place to observe than that, or else let your eyes get more adapted to the dark. If you count eight stars, your limiting magnitude is 5.9, and if you get up to 17 stars, your limiting magnitude is 6.5, which means you have a really dark site. Numerous regions in the sky have such information tabulated, including one region in neighboring Andromeda.
There are a couple of interesting deep-sky objects in Pegasus that require a telescope to see. M15 is a fairly bright globular cluster, and NGC7331 is a classic-looking, often-photographed spiral galaxy. There are also several smaller companion galaxies nearby, which might not be visible with smaller telescopes.
This monthRight after dusk, the Great Square of Pegasus can be found in the east about halfway between the horizon and zenith. The square is tilted about 30 degrees clockwise. A bit later in the evening, the square will be very close to zenith, so the orientation depends on which way you are facing. Although these stars aren’t bright, they are the brightest things around.
There is a transit of Mercury across the sun this month. It will already be in progress at dawn on the morning of Nov. 11 and be finished just after 11 a.m. Because of the tilt of Mercury’s orbit, this is a relatively rare event. The next Mercury transit is in 2032, and the transit in 2039 won’t be visible in this hemisphere. You would definitely need an appropriate solar-viewing aid to watch the transit – some of you might still have them from the August 2017 solar eclipse. Because Mercury is so small, a solar telescope would help you see the tiny dot crossing the sun.
Jupiter and Saturn are setting in the west soon after sunset. This month, they are joined by Venus, which will be the evening star for a while. During November, Venus will get higher and higher in the sky at dusk. There will be a conjunction between Venus and Jupiter on the evenings of Nov. 23 and 24, when the two planets will be just over a degree apart.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at firstname.lastname@example.org.