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 or hours. We will use decimal degrees, since that is what the computer will use. (When hours, minutes and seconds are given, it must be converted to decimal 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:
Using Journal 1: RA and Dec, answer the questions:
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, BUT, look at the numbers to adjust your scale to reflect your data. Also, make sure the graph is twice as long as it is wide.
To use a spreadsheet program, you can use the link below to download the data. First, use an excell spreadsheet, or a google sheet. If you use a google sheet, you can use something called "zoo tools". These are available free as an "add-on" from the Google Play store. Import the CSV file from your desktop to the spreadsheet. You can make a graph using the zoo tools under add-ons.
Download the data as a CSV (comma-separated value) file.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.
Go on to Lesson 2, where you will see how you can use RA and Dec to map the Solar System.