September is a great time of year to stargaze. I enjoy the crisp weather and the fact that it is getting dark a bit earlier than it was only a few weeks ago. And there is usually a smaller chance that you will need to dodge lightning from the afternoon thunderstorms. September is also the month when I typically get to give an introduction to the night sky to my Fort Lewis College astronomy class.
There isn’t space in this column to cover everything I like to show, but I can at least describe what we can see at dusk. I like to go out after sunset, when most people would say it is still daylight. Tonight (Saturday, Sept. 8) that will be about 7:45 p.m. This gives us the chance to pick out the very brightest things before the sky gets crowded with points of light that make the task of identification seem needlessly daunting. I always start with the planets, and then move to the bright stars and constellations once they become visible. Once it is completely dark, we can begin to see the Milky Way and, finally, star clusters, galaxies and nebulae.
The first question I will ask the class is what is the brightest thing you can see in the sky? As long as the moon isn’t out (and it won’t be tonight), the answer is invariably one of the planets. Some students will be able to recognize (or guess) the evening star near the western horizon as being Venus, even if they have never tried to identify it before. Your fist held at arm’s length spans about 10 degrees, so I have students measure with their extended fists how far Venus is off the horizon. By the time it is fully dark, Venus will have set.
The other planets to note are Jupiter, Mars and Saturn. Jupiter is less than 20 degrees (two fist-widths) up and to the left of Venus. Mars is very prominent in the southeastern sky and is easy to identify because of its color. Equally spaced between Mars and Jupiter are Saturn and the reddish star Antares.
After wrapping up the planets, we next turn to the bright stars. Two stars are tied for the brightest visible at our latitude at this time of year – Vega and Arcturus. Vega is almost directly overhead, and Arcturus is about three fist-widths above and to the right of Venus. (Sirius is brighter but is a winter star.) These two stars have noticeably different colors. Vega is blue-white and Arcturus is orange-white, which indicates that Vega is much hotter than Arcturus.
Vega is the brightest star in the small constellation Lyra but is also part of the much larger asterism called the summer triangle. The other two stars making the triangle are Altair and Deneb. If there is still a bit of daylight, those might be the only stars in that part of the sky. Altair is just over three fist-widths to the south and is part of the constellation Aquila, the eagle. Deneb is just over two fist-widths to the east and is the tail in the constellation Cygnus, the swan.
As dusk deepens into true night, more and more stars fill in the gaps between what we have identified. Although Polaris is very important for navigation, it is not very bright so probably won’t be visible until we have been out for a while. The Big Dipper and Cassiopeia are to the west and east respectively of Polaris and will most likely be seen before the pole star.
If the night is clear, the Milky Way will appear before it is truly dark. By 9 p.m., Venus will be gone, and we can start finding clusters and galaxies in binoculars or telescopes.
Everyone should spend the time to watch the sky get dark at least once. Your eyes will adjust as the evening progresses, so as soon as it is dark enough to see the faintest objects, your eyes will be ready. I hope you enjoy it.
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.