Fall Equinox is Here.
Today is the autumnal equinox, marking the astronomical end of summer and the beginning of autumn. The autumn sky promises to bring exciting new spectacles including fall constellations and a range of planets. Additionally, the sun continues to set earlier each night, giving us more time to appreciate the summer constellations.
What is the Equinox?
To understand the equinox, consider the Earth’s relationship to the sun. Imagine a thin sheet spread out across the solar system. The Sun is at the center and all the planets lie upon it circling the sun at their various distances, with inner planets circling faster and outer planets circling slower. The Earth’s equator is tipped with respect to this sheet, so for half of the year the northern hemisphere receives more direct sunlight, and for the other half of the year, the southern hemisphere does. The two points during the year where sunlight falls equally on both the northern and southern hemispheres are called the equinoxes.
Earth’s tilt also affects where planets are in the sky. Let’s imagine our “celestial sheet” from Earth’s point of view. From here in Michigan in the northern hemisphere, the path of the planets always remain to the south, but as the Earth turns, the angle it makes with the horizon bobs up and down with the day. In autumn, the angle is shallow in the evening and steep by morning. This means that any planets in the early morning hours will be well above the horizon, which makes for clearer viewing.
What Planets Can I See?
Currently, Jupiter and Saturn are particularly bright and easy to spot in the south all evening. Mars rises during twilight, and Venus rises in the morning. Mars is the particular feature of the season as the Earth makes a close pass of it during early October. It will briefly become brighter than Jupiter, and its surface features will be clearer in a telescope than they have been since 2003. The Moon is currently waxing in the evening sky toward the full moon on October 1, and will wane again into the morning through the first half of the month, which bodes well for dark skies in mid-October.
What Stars Can I See?
In summer we watched the planets Jupiter and Saturn and the Summer Triangle progress westward with each passing evening. The Summer Triangle remains prominent even now as chill comes into the air, but it is joined to the east by the Fall Square. The stars of the Fall Square are not as bright as those of the Summer Triangle, but the easy-to-spot square shape makes it unmistakable.
The left corner of the Fall Square is the bright blue star Alpheratz, a curiously-named star derived from the Arabic for “the horse’s belly button,” — named because it was traditionally part of the constellation Pegasus.
To the north, the Big Dipper is visible, but low enough to be blocked by trees. But, when the Dipper is low, the “W” of Cassiopeia swings high. Cassiopeia and the Fall Square are the two main star patterns to orient by in the autumn.
Challenge Yourself!
Pick a clear night and a dark observing location to try to see autumn’s most treasured deep sky object — the Andromeda galaxy. Orient yourself so that you can see both the“W” of Cassiopeia and Alpheratz in the Fall Square. Scan from Cassiopeia to Alpheratz with naked eye or binoculars. Look for a diffuse glow about halfway between them. It can be tricky, but be patient and try a darker night (or location) if it’s not apparent. If you succeed, you will be looking at the most distant object human eyes can see without optical aid.
Join experienced astronomers of the Grand Rapids Ameateur Astronomical Association (GRAAA) for Public Nights at the Veen Observatory to view the night sky. Visit graaa.org for more details.
By: John Foerch, Planetarium Production Programmer for the GRPM’s Roger B. Chaffee Planetarium.
Image credits: Credits: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler