Field Astronomy: Clock System and Angle Measurement
Why Learn All This?
When you are out with a large group of students in the field, pointing to a star is always a problem. You may be pointing your hand at a particular star and the students may see it pointing in some other direction. This happens because you and the students are standing at some distance from each other and therefore are looking at different views. This phenomenon is more commonly termed as parallax. To get a feel of it, you can try a simple activity:
Stretch you hand in front of your eyes horizontally and look at one of your fingers with your left eye closed. Notice the background in front of which you see your finger. Now, look at it with your right eye closed. You will notice that the background changes. Even with a slight change in your viewing position, the difference is observable. So, you need some simple tricks to point out things to a large group because high power laser pointers are not very cheap.
Zenith: It is the position in the sky vertically above your head.
This is a tool to point out stars using a reference star. Generally, the reference star is taken to be the brightest star in a certain region or the pole star as they are easy to point out. Now, let us assume that we have our reference star 'A' and we want to point out star 'B'. Imagine a 12-hour clock with its center at star 'A' and 12 O'Clock pointing to the zenith. Now, you can simply imagine the hour hand pointing to star 'B' and say what time it is.
For example, look at the figure on the right. Star 'A' is taken as the reference star. We imagine a clock with its center at 'A' and 12 O'Clock pointing to zenith. It can be clearly seen in the image that star 'B' is at approximately 1 O'Clock. Similarly, you can look at the other bright stars in the image. They can be seen at 3 O'Clock and 4 O'Clock from star 'A'.
Usually, distances between stars in the night sky are quoted in angles. But what angle are we really talking about? Suppose observer 'O' says that star 'B' is 'x' degrees away from star 'A'. This means that the angle BOA is equal to 'x' degrees. Since the stars are too far away, observer 'O' can be assumed to be at the center of the Earth.
Although the clock system is a very helpful tool to point out stars, it can only give the general direction of the star relative to the reference star. To exactly mention which star you are talking about, you need to somehow convey the angular distance approximately. Here, we will try to do exactly that.
Fully stretch you hand horizontally in front of your eyes. Now, close your fist. The angle between the two ends of your fist is approximately 10 degrees. This means that if you can see one star at one end of your fist and another at the other end, they are 10 degrees apart. Now, stick out your thumb (as in thumbs up). The angle between the tip of your thumb and the far end of your fist is approximately 15 degrees. Now, stretch your thumb and little finger as far apart as you can. The angle between the tip of your thumb and the tip of your little finger is roughly 20 degrees.
Note: Stretch your hand fully while doing these measurements.
Using these two tools, you can easily point out any star once you have a reference star. However, it is a good practice to choose a reference star as near as possible to the star you're attempting to point out .