The celestial coordinate that takes the place of longitude is called Right Ascension, but this is often shortened to RA. It can be measured in degrees. Just as longitude on the Earth starts with 0º at the Prime Meridian, RA starts with 0º at a specific line on the celestial sphere.
The celestial coordinate that takes the place of latitude is called Declination, often abbreviated Dec. It is measured in degrees north or south of the Celestial Equator. The Celestial Equator is at 0º dec; the North Celestial Pole is at +90º, and the South Celestial Pole is at -90º.
Together, the RA and Dec of a star make up an ordered pair that can be plotted on a graph. For example, the coordinates of seven well-known stars are given in the following table:
| Star Name | RA | Dec |
|---|---|---|
| Betelgeuse | 88.8 | 7.4 |
| Rigel | 78.6 | -8.2 |
| Bellatrix | 81.3 | 6.3 |
| Mintaka | 83.0 | 0.3 |
| Alnilam | 84.1 | -1.2 |
| Alnitak | 85.2 | -1.9 |
| Saiph | 86.9 | -9.7 |
Practice 1. What are the ordered pairs of (RA, Dec) for these stars?
Question 3. As you may know from math, when you have a list of ordered pairs, you can make a graph. If you made a graph with RA on the x-axis and Dec on the y-axis, what do you think you would see?
Explore 2. Make a graph of the stars above. You can do this in one of two ways: either on a sheet of graph paper, or with a spreadsheet program. Whichever method you use, make sure you make a map with the right scale: RA from 0 to 360, Dec from -90 to 90. Also, make sure the graph is twice as long as it is wide.
To use a spreadsheet program, download the data as a CSV (comma-separated value) file.
Or, if you are using Microsoft Excel, download this Excel spreadsheet template, which includes a graph ready for you to fill in with the data above.
If you have trouble seeing the stars plotted on your graph, try changing the scales to RA from 0 to 120, Dec from -30 to 30.
Question 4. What do you see in the graph? Do you recognize the shape of this group of stars? Is there anything about the way the stars appear that surprises you?
If you did this for all of the bright stars in the sky, you’d have a map of all the constellations. We won’t be using maps and coordinates to study stars, though. We’ll be studying objects in the Solar System, all of which are much closer to us.
Click Next to go on to Lesson 2, where you will see how you can use RA and Dec to map the Solar System.