Science 122 General Information

Laboratory 2

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Plotting the Graph

Plotting a graph in celestial coordinates is just like plotting any other graph. The points are at the intersection of lines drawn parallel to the coordinate grid.

On the example below the first two points of the Mars retrograde data are plotted as examples.

There are several things to note to help understand how the graph is plotted.

1. Right ascension is measureed in hours and minutes from right to left. This corredsponds with the rotation of the celestial sphere from east to west. In traditional maps the directions are as shown at right and the graph keeps this orientation.

2. The graph for this exercise shows only the very small portion of the celestial sphere necessary to plot the data. It does not start at the point (0,0) as you may have seen graphs in the past.

3. Their are six grid lines between the major (dark) grid lines. This is because there are 60 minutes in one hour and also sixty minutes in one degree. Don't be confused by the use of the word "minutes" in both measures. In both cases it means 'one-sixtieth'.

Now look at the sample graph below. You should be able to see how the points are plotted easily enought to plot the rest of them. It may take some time, but once you get the hang of it the rest will proceed quickly. Be sure to indicate the dates for the starred (*) data on the graph. This is so you will be able to connect the points in the proper time sequence. If you do not you may get confused when you try to connect the points.

Do not show the lines on the plot. They are shown here only to illustrate how to locate the points.

Click here to see the celestial sphere and here to return to lab 2.

Here are the first two dates from Table 1 in the lab exercise:

Table 1. Positions of Mars








11 Oct

8 5



21 Oct

8 26 20 54

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