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September 2016 Night Sky at a Glance

by Pouria Nazemi

Well, the lovely days of summer are numbered, and the countdown for the fall has already begun.

If you love the night sky, I am sure that you will find some motivation to go out even in the coldest nights of the winter, but you have to admit it is not an easy task, and even if you are out there, it is no guarantee that the weather will collaborate with you. So enjoy the last days of the summer, and take a look at the magnificent night sky before the first signs of the winter show up.

September is the transition month between summer and fall, and it is time for transition in the night sky as well.

The famous Summer Triangle started reducing its distance to the western horizon. This triangle actually is a staple of the summer sky. The three vertices of this triangle are Altair (the brightest star of Aquila, with an apparent magnitude of 0.75), Deneb (the brightest star of Cygnus, with an apparent magnitude of 1.25), and Vega (the brightest star of Lyra, with an apparent magnitude of 0). These three bright stars are key to finding your way in the summer night sky. But now, it is time for a new group of constellations to take over, and the most famous constellation of the fall is the great square of Pegasus, the mythological divine winged horse.

Pegasus and its surroundings
Pegasus and its surroundings. Click to enlarge.
Image courtesy of Starry Night, a Simulation Curriculum company.

All four stars that shape this giant celestial square are second-magnitude. However, one of them is not part of Pegasus; the fourth star of this group is called Alpheratz (magnitude 2) and is the leader of a V-shaped and very famous constellation: Andromeda.

By finding this square in the sky, you can find Andromeda, and this is the first step to look for a very unique deep sky object: Messier 31, the Andromeda Galaxy. If you are away from light pollution, you can see this giant spiral galaxy with the naked eye. This little patch of foggy light is actually a galaxy with around 1 trillion stars, and at a distance of 2.5 million light-years from us. So—thanks to Albert Einstein and the fact that the speed of light is constant—when you look at it, you are actually looking at this galaxy in the state that it was 2.5 million years ago… The night sky is the only available time machine that we have!

There are many more deep sky objects in the September and fall night sky, but we’ll talk about them in the coming months; for now, let’s take a look at a few special events of September 2016…

September 2016 eclipse map
© NASA / Fred Espenak

September will begin with a New Moon and an annular solar eclipse. This eclipse is not observable from Canada (see visibility map, right). The best place to watch it is Africa. An annular eclipse happens when the apparent angular size of the Moon is less than the apparent angular size of the disc of the Sun. So when the Moon covers the face of the Sun, you still can see a ring of sunlight around the Moon, which is usually called the “ring of fire.” In this kind of solar eclipse, you need to keep safety filters on your eyes all the time. And although it is a great opportunity to capture magnificent photos for amateur astronomers and eclipse chasers, there is very little scientific value in observing an annular eclipse.

Moon and bright planets
The Moon and bright planets Venus and Jupiter. Click to enlarge.
Image courtesy of Starry Night, a Simulation Curriculum company.

In the evening of September 2 and 3, try to find a place with an open western horizon. The Sun will set around 19:30 EST, and you will have 40 minutes (on Sept. 2) to around an hour (Sep. 3) to catch the crescent Moon with bright planets Venus and Jupiter just beside it.

Venus will shine with a magnitude of −3.9, and Jupiter at −1.6. If you are interested in landscape astrophotography, this is a great chance: Just find a good location with an open western horizon, locate the position of the Moon and Venus, choose a good background, and take your shot! Don’t forget to use long exposures, and try a few different settings.

If you want to check some samples for this kind of photography, take a look at the TWAN (The World At Night) Project.

Starting the month with an annular solar eclipse means—as you may anticipate—we will have a lunar eclipse at the middle of the month. This is going to be a penumbral lunar eclipse, which will happen on Sept. 16, but it will not be visible from any place in Canada.

And finally, on September 22, summer will officially end. This is the Autumn or Fall Equinox (for the northern hemisphere, and of course the Spring Equinox for people who live in the southern hemisphere). On that day, the Sun will cross (on its apparent path, the ecliptic) the celestial equator from North to South, so the duration of the night will be equal to that of the day—after that date, nights become longer than days until the December solstice.

So welcome to the fall, but before I say goodbye to summer, there is one more event in September.

Between all of the planets that you could see with the naked eye, Mercury is the most difficult one to catch. The reason is not that Mercury isn’t bright enough; it is. But at the same time, it is too close to the Sun; most of the time, it is hidden in the brightness of the Sun, and there are few days that you actually have a good chance to see it. It is odd, but the truth is that many amateur astronomers have never seen it!

The best time for observing Mercury is when it reaches a point called “greatest elongation.” This is the time when Mercury has the largest angular separation from the Sun, which means you have a chance to see it before sunrise or after sunset.

Mercury between the Moon and the Sun
Mercury between the Moon and the Sun, with some lens flares. Click to enlarge.
Image courtesy of Starry Night, a Simulation Curriculum company.

On September 28, Mercury will be at its greatest western elongation. It will thus be to the West of the Sun, and will rise around one and a half hour before it. If you want to catch it, you have to wake up a little early; Mercury will rise at 5:15 EDT, and if you have a clear sky and open sight to the East, then you can catch it before sunrise. Good luck!

I hope you will enjoy your adventure under the starry night of September. Feel free to tell us about it!


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