Why Some Stars Are Visible Only During Some Seasons

13:48 Wed, 26 Jan 2011

Some stars and constellations are visible only during certain times of the year. Why is that? With diagrams!

Most constellations are visible only during certain times of the year. For example, Scorpio is visible only during Winter where I live. Why is that?

I have read some reasons and come away confused. Really, it is quite simple, but, like many things, a picture makes it much easier. So I cobbled together some diagrams that will make the explanation very easy to understand.

First, the basics. As we all know, the Earth rotates roughly every 24 hours, plus it circles around the Sun about every 365¼ days. So the Earth is circling the Sun plus rotating at the same time.

Here's a diagram looking "down" on the Earth and the Sun. It is very basic. It shows the Sun in the middle, the Earth's circle around the Sun, and a bunch of stars scattered about. I made up the stars, they aren't in actual positions. Also, I shrunk everything to fit on one diagram. In reality, the stars are billions of kilometres away.

Earth
and Sun

The Earth's spin is anti-clockwise, but it doesn't really matter for this. (To clarify, the Earth's axis is from you the viewer down into your screen, i.e. the z axis of the diagram.)

Notice that the Earth has a light side that is facing the Sun, that side is in daylight. Notice the other, dark side is facing away from the Sun and is in night.

I labelled three positions: A, B, and C. Throughout the year, the Earth moves anti-clockwise around the Sun, so it goes past position A, then B, then C. Each position is roughly a ¼ way around, so it takes the Earth about 3 months to go to the next position.

Look at the diagram again with the Earth at position A. Which stars can people see? You might be tempted to say, "All of them. The Earth is rotating on its axis so a person can see all the stars at some time during the 24 hours." No, because you can see stars only at night.

This is important and, yes, it is stating the obvious, but let's run through it anyway. As the Earth rotates, sure enough every star in the sky at some time will be above the horizon. However, we can only see stars when the sky is dark, at night time. So, the only stars we can see are those stars that happen to be above the horizon when the sky is dark.

Here's the same diagram with some colours added to the background. The orange-yellow is where the sky is daytime and blue is where the sky is night time from the surface of Earth.

Earth
and Sun at position A

This diagram shows that everyone on Earth has the same night sky even though the Earth rotates. Night is always at the right-hand side of the Earth when the Earth is at position A. Now you can see that the answer is that everyone can see only those stars that are in the blue background.

Just to reiterate: every time the Earth is at position A, people can see only those stars in the blue area. We give position A a name, something like summer, winter or autumn. So during, say, summer, we can see only those stars in the blue area.

Let's move on three months to where the Earth is at position B. Here's the same diagram with the Earth moved to B.

Earth
and Sun at position B

What has changed? We can see that the tight cluster of stars at the right-hand side of the diagram is now in daylight for everyone, so we can't see those. However, we can now see a bunch of stars at the top of the diagram that we could not see before.

Three months later, the Earth is at position C.

Earth
and Sun at position C
We can no longer see the stars at the top-right corner, and we can see a whole new set of stars at the left of the diagram that we could not see three months earlier.

Let's call position C winter and we are still calling position A summer. I can see some stars (those at the right side) only in summer, and other stars (those at the left side) only in winter.

This is why I can see some stars and constellations only during certain seasons.

Complication

I, in the southern hemisphere, can see Scorpio during winter. So how come my colleague in the northern hemisphere can see it during summer? It is because the two of us call position C by different names. For me, position C is winter and for him it is summer. It is due to the tilt of the Earth's axis, a completely different subject for another time.

More Technical

I fudged things a bit in the diagrams to avoid overly complex explanations. In fact, looking at position C, the boundary between the blue area and the yellow area would be in the middle of the diagram, not two-thirds to the left.

The reason is that the width of the solar system is very small compared to the distance to the stars. If you drew the diagram to scale, the Earth's position would be right in the middle of the diagram rather than two-thirds over. So the blue line would be in the middle of the diagram.

[I drew the diagrams in Google Docs. It was pretty straightforward, especially if you have used MS Office tools before.]

Categories: astronomy, science

Comments

  1. Lexi     216.56.xxx.xxx
    2013-03-01 06:00

    Thank you so much! This website and information really helped me with my astronomy project! :)

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    2013-05-08 11:57

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    2013-05-08 11:58

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