## Using a Star Chart

A star chart is a map of the night sky. It generally shows the relative positions of the stars and their brightness. However, because at a given time we can only see a portion of the celestial sphere, we cannot see all the stars at the same time. Thus we must first pick the chart appropriate for the season we are observing in.

FINDING THE PROPER CHART

In the back of the text book and on the inside front cover, you will find four charts, one for each season of the year. (These seasons are labeled for the northern hemisphere). Thus, if you want to learn the winter constellations, pick the winter chart, which is sketched in very rough outline below. Because the stars move across the sky, rising in the east and setting in the west, you need to consider the time of night. The charts we will discuss below show the sky as it appears about 8 PM local time.

Notice that the chart has labels north, south, etc. If you hold the chart so that north is at the bottom, the lower half of the chart shows the sky approximately as you would see it looking north,. The center of the chart is approximately overhead. for a viewer at middle latitudes. Referring to the chart sketched above, it therefore shows the sky as you would see it looking south at about 8 PM in December in New York or Los Angeles. (Keep in mind that this is only a sketch. Most of the stars are omitted.)

## Exercise #1

Use the sketch of the chart to find the constellations Orion and Taurus on the actual chart in the book.

## Exercise #2

What direction should you look to see the constellation Orion? What direction should you look to see the constellation Cassiopeia?

LEARNING THE CONSTELLATIONS

To learn the constellations you need to learn not only their shapes but also their relative sizes. Let's start with shapes. Find Cassiopeia on the Winter Chart and notice its shape.... roughly a "W" (You can remember this if you think of the "W" as the chair or throne on which Cassiopeia, the Queen, sits).

Exercise #3.

Find Orion the Hunter. He is usually depicted as a standing figure with raised arms, one arm holding his shield, the other his club to defend himself against the bull, Taurus. Orion thus has a roughly "H" shape.

Exercise #4.

Find Taurus on the Winter chart. Taurus the Bull is easy to find if you think of his bright red eye (the star Aldebaran) and his horns that form a "V."

With the shape of these three constellations in mind (W, H, and V), now notice their very different size. Notice that Orion is much larger than Cassiopeia. But how much of the real sky does Orion cover? For example, could you cover it with your thumb at arms length?

DESCRIBING SIZES OF CONSTELLATIONS

Size is important in identifying constellations, so we need some way to measure, at least roughly, their sizes. We'll use a measure that you always have with you...your hand. To see how to use your hand to measure size (and distances on the sky) we need to first describe briefly how astronomers make such measurements.

Astronomers traditionally use angles to measure sizes and distances on the sky. To understand how, imagine two sticks joined at one end with a nut and bolt so they can be spread apart, as shown in the sketch below. If you hold these sticks up to your eye and sight along one to a given star and along the other to another star and then measure with a protractor the angle between the sticks, you will have found the angular distance between the two stars.

The figure below shows how that angle can be found for the size of the "bowl" in the star grouping we call the big dipper. The figure shows that this distance is about 15 degrees.

In learning the constellations you could use such a pair of sticks and protractor, but a much easier way is to use your hand at arms length. For most people, the angle from tip of thumb to tip of little finger when your arm is extended is about 20 degrees (see sketch below). Thus, your spread hand will span slightly more of the sky than the width of the big dipper's bowl. Similarly, if you make a fist, then the angle between the knuckles at arm's length is about 5 degrees. These are of course only approximate, but will help you estimate relative size and distance in the sky.

Copyright T. Arny, UMass, Amherst, MA, USA 01003