Different constellations, different times.
Published on Jan 23, 2007 at 5:18 pm.
14 Comments.
Filed under skywatching.
Why is it best to look for the constellation Orion in the winter? A reader asked this question in a comment on my posting about the constellation Orion. The answer has to do with the motion of the Earth. But, I thought that I’d extend the answer a bit.
There are several things that determine what constellations you see. One is simply your location on Earth. From some places, some constellations are never above the horizon and some never set. Another thing that determines what you see is the motion of the Earth itself. For now, I just want to discuss two types of motion: orbital motion and rotation.
Stars, constellations, and galaxies fill the universe. They are in all directions from Earth. At any time of the day, there are stars straight over your head. But, stars are generally pretty dim because they are a long way away from us. The nearest star (other than the Sun) is so far away that it takes light over 4.2 years to reach us. Now, the Sun is bright, and when it is up, its light scatters in the atmosphere, and washes out the dimmer objects. The Moon is quite bright, too, and you can see it in the daytime, but even it is a bit washed out compared to how it appears at night. Only the brightest stars and planets can be seen in the daytime, and they are very difficult to spot. It normally takes very good sky conditions and some degree of skill to spot them. It is hopeless to see the dimmer stars when the Sun is up, even though they are there. But at night, there is no Sun, so the stars are easier to see. The Moon also scatters light in the atmosphere, so when the Moon is up, the dimmest stars become hard to see. But, when neither the Moon nor the Sun is up, you can see far more stars. (Note: all this assumes that you are not looking from a bright city, where the light pollution also washes out the dimmer stars). In my diagram here, I show Earth at some spot on its orbit about the Sun. The side of the Earth towards the Sun is the daytime side, and the side on the Earth away from the Sun is the nighttime side.
But, the Earth orbits the Sun. Each night it moves a bit less than one degree around its orbit, so you see a slightly different sky. Months later, the Earth is significantly around its orbit, so you see different stars at night. If you look at the drawing here, you’ll see that in order to see the stars that are indicated in the first drawing, you’d have to be on the side of the Earth lit by the Sun. Obviously, the sunlight would interfere. So, you see different stars and constellations at different times of the year.
Half a year later, the Earth is on the far side of its orbit from where it was when you first looked. Then, you’d see an entirely different sky. Now, the stars and constellations that were visible at night six months earlier are still in the sky, but you would not be able to see them from the night side of the Earth. You’d have to be standing on the daylight side of Earth in order for them to be up, as shown in the following diagram. The only problem, though, is that the Sun is so bright, and that makes the sky so bright, that you wouldn’t be able to see the stars unless you could somehow turn down the Sun’s brightness. Now, sometimes during a deep solar eclipse, you can see some of the stars up in the daytime, but that’s about the only way from the surface of the Earth (other than a handful of the brightest stars and planets, and only then if conditions are just right, and you are really good at spotting them).
So, perhaps that answers my reader’s question. That is why the constellation Orion is best seen in the Winter. Yes, it is up in the summer, but only during the daytime, and the Sun makes the sky to bright to see it then.
On a related note, the rotation of the Earth also has an effect on what stars you see and when. For example, consider the following diagram of an observer on Earth right after sunset.
Some stars are overhead. At right angles to the stars overhead are stars rising and setting due East and West. As the Earth rotates, at some time later you will see other stars overhead. In this diagram, the Earth rotates counterclockwise, simulating a view looking down on the Earth’s north pole. My next diagram shows the Earth at the same location in its orbit, but after the rotation of the Earth had carried the observer from just past sunset to near midnight.
Notice that the rotation of the Earth is doing something similar to the orbital motion. You see different stars and constellations overhead. After the Earth has rotated six hours, stars that were rising due East are now the highest that they will be in the sky (Note: Unless you are on the equator, stars rising due East won’t be directly overhead, but they will still be as high as they are going to get after six hours). This is the same as if you were observing the sky at the same time of night three months later. In other words, you’d see the same sky after waiting six hours or observing at the same time three months later.
All this comes from the combination of two type of Earth’s motion: rotation and orbital motion around the Sun.
-Astroprof
Ed Minchau on January 24, 2007 at 12:15 am: 1
“… after six hours). This is the same as if you were observing the sky at the same time of night four months later”
I feel like Mister Nitpickey sometimes when I comment here… shouldn’t this be three months? ie 1/4 of the day rotation would be equivalent to 1/4 of the year revolution…
Astroprof on January 24, 2007 at 8:20 am: 2
Not nitpicking at all! Somebody needs to watch me for errors! I corrected what I said above. I don’t know how I got “four” unless I was thinking “1/4 year”. I do know that I was really rushed to finish that post before class. I had written it all before, but then I stopped to answer a couple questions before class, and then I posted, only to find that my session had timed out, and I had lost the whole thing! I rewrote frantically, and I guess that I didn’t proofread. So, thanks for catching that!
daniel on June 8, 2007 at 9:45 am: 3
Thanks for this explanation! I have a beginner’s guide to the night sky that provides monthly maps of the constellations, as they look at 10pm mid-month. When it said to flip to the next month’s map to see how the sky looks at 12am on the same night, I searched the internet for why this happens. This explanation really helped a lot. I’ve been since looking through the site and am finding it really informative. Thanks.
Joanna on April 14, 2009 at 11:18 pm: 4
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machan on August 7, 2009 at 11:02 pm: 5
thanks for helping me to understand further about my report..
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AlvinDala on November 15, 2009 at 4:33 pm: 6
hey, thanks for helping me with my science project. i never knew that this was so boring. lets hope i get excellence though :):):P
Jacqueline Hall on January 8, 2010 at 4:01 pm: 7
My son has a question on his homework regarding what constellation would he see if it was winter and noon, assuming he would be able to see the constellation, knowing full well that he wouldn’t be able to because of the sun.
honey mae manzano on February 25, 2010 at 6:57 pm: 8
what are the different constellation?
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