Monday, August 15, 2005

Why Do the Stars Change?

When you gaze up at the night sky on a clear night, you have probably been able to identify the North Star as well as certain constellations (recognizable groupings of stars) such as the Big Dipper and Orion.

Other famous constellations include the twelve zodiac signs, which are based on ancient mythology. The two zodiac constellations that are assigned to the month of August are Leo and Virgo. Oddly enough, these two constellations are not among the easiest to see this month.

The constellations most easily visible in August are: Sagittarius (The Archer), Telescopium (The Telescope), Lyra (The Lyre), Scutum (The Shield), and Corona Australis (The Southern Crown). For diagrams of these constellations, as well as information on their important features, visit

Stars don't actually travel across the sky, so then why does the night sky change according to the season? In other words, why can you only see certain constellations during certain months?

The answer is that the planets in our solar system revolve (travel in a circular path) around the Sun and rotate (spin). The Earth rotates towards its eastward direction, and each rotation represents one day.

The part of the Earth that faces the Sun experiences daytime, while the side facing away from the Sun experiences nighttime. Thus, the stars that we see on a given night are only those that face the nighttime side of the Earth on that particular night.

So, since we are moving but the stars are not, our position changes in relation to the constellations. To better visualize how the movements of the Earth affect our night sky, do the Little Lion Experiment as directed at the end of this article.

While this concept may seem strange at first, think about how the position of the Sun in our sky changes during the course of a day. It is not the Sun moving, but the Earth moving that causes the Sun to appear as if it were moving across the sky. Our view of other stars is like this except the movements are less obvious since they are so much further away from us than the Sun is.

The Earth rotates more or less in a sideways (versus upwards or downwards) direction. It is as if the Earth were spinning about a line running from the North Pole to the South Pole. This imaginary line is called the axis of rotation.

However, the actual axis is tilted a bit. This is similar to a top spinning when it is just starting to tip over. The tilted axis of Earth is believed to be a result of the Earth having been hit by a large object (like an asteroid) a long time ago.

Little Lion Experiment:

To make a model of our solar system, put a ball on the table to represent the Sun. Use an orange or plum to represent the Earth (which revolves in a counter-clockwise direction). Pick an object in the room (like a clock on the wall) to represent a specific constellation, while a spot on the ceiling directly above you can represent the North Star.

Put a small piece of tape on the fruit to represent where you are. Stick a straw into the fruit where the tape is, pointing diagonally upward. Imagine standing where the tape is on your model Earth. What you could see through the straw represents the part of the sky you see when you look outside.

By acting out how the Earth rotates and revolves around the Sun, you can see when you experience day and night, as well as how your view of the universe (represented by the room in which you are sitting) changes.

To compare your model to the actual changes in the night sky, find the North Star, as well as one or two constellations that are easy for you to spot. Then, track their positions in the night sky over the next few weeks (once per week is sufficient).

Using your model, can you see why the position of the North Star stays relatively constant, while the constellations seem to move across the sky?