Saturday, August 15, 2009

What Can I See Tonight?

This summer I was at a star party, showing the sky to interested non-astronomers, as I do many nights each year. One participant came armed with the knowledge of a number of the finest objects to view in the sky. They asked me to show one, then another. They and the others there all enjoyed seeing each of these objects as I put them in the telescope's view.

Then they said "How about the Orion Nebula? I hear that's really beautiful!"

It is, but I couldn't show it to them. I had to apologize, then explain that we couldn't see Orion that night.


The Sun and the  Ecliptic, showing the Earth's orbit and position relative to the Sun.
The Sun blocks part of the sky from the Earth's view. Here, the area near Pisces is behind the Sun. Image by Tau'olunga.

As the Earth travels around the Sun during the year, there is part of the sky behind the Sun that is not visible. The part of the sky that is behind the Sun from our point of view changes depending upon where we are in our orbit. In the image above, Pisces lays on the far side of the Sun from the Earth. The other constellations nearby will also be hidden since they are only visible during the daytime. Pisces would be high in the sky at noon, but you can't see stars at that time because the atmosphere will be lit by the Sun.

Even if you were outside the Earth's atmosphere, there will still be stars hidden behind the Sun. But you would be able to see more of those close to the Sun since the Sun's glare wouldn't be spread out the way it is when you're looking from within the atmosphere.

Six months later, the Earth is on the other side of the Sun. At that time Pisces will be high in the sky at midnight. But the stars in Virgo and the area around it will now be hidden by the Sun.

M-42, the Orion Nebula.
The Orion Nebula. Impressive, but don't expect to see it in summer. Image by Filip Lolić.

Orion is called a "Winter" constellation because it is highest in the sky at midnight during winter. It's close enough to the ecliptic that it's hidden by the Sun during summer. That means the nebulas in it, including M-42, the Great Orion Nebula, will not be visible in summer. They are visible at other times of the year. Though only in winter are they visible at relatively convenient times of the night, near sunset. At other times you'll need to either get up early or stay up late.

There's a myth about Orion and Scorpius. It's said that they were mortal enemies and would always fight. In the sky this is reflected by the fact that they are at opposite sides of the sky, and are never in the sky at the same time. So, while Orion cannot be seen in summer, the wonderful sights of Scorpius are not visible in the Winter. So if Scorpius is up, Orion isn't, and vice versa.

The M-4 Globular Cluster
The M-4 Globular Star Cluster, in Scorpius. See it in the summer, you won't be able to in the winter. Image by Ole Nielsen

How much does the sky change each night? A reasonably accurate rule of thumb is that the stars rise three minutes earlier each night. Note that this doesn't apply to the Moon, it moves in an entirely different way, coming up about 40-some-odd minutes later each night. Another rule of thumb is that the sky changes by about 1 degree each night. This is because the Earth moves about 1 degree in its orbit about the Sun each day. This means that something that is up tonight by about 10 degrees above the western hemisphere will disappear in about a week and a half. So see it while you can!

Likewise, if you see the Pleiades just above the horizon at sunset, you have less than an hour to wait before the rest of Taurus, including Aldebaran (the Eye of the Bull) and the Hyades (another nice star cluster in Taurus) are visible.