Right ascension (RA) and declination (DEC) are like latitude and longitude on
the celestial sphere. Right ascension is like longitude on the earth (measures horizontally)
and declination is like latitude (measures vertically). We can plot them on a graph
like a flat world map, as the x (RA) and y (DEC) axes on a graph.
RA is measured in hours, minutes and seconds like the clock.
DEC is measured in degrees, minutes and seconds like angles.
Click on the picture to see a larger jpeg.
Click on the 'play' button (the left sideof the bar below the movie) to start/stop.
The data on the graph for lab 2 are the actual observed locations of Mars in
the sky (on the grid of the celestial sphere) on the dates indicated.
Now look at the graph and the first three dates.
Table 1. Positions of Mars
Date
R.A
DEC.
h
m
d
m
11 Oct
8
5
21
49
21 Oct
8
26
20
54
31 Oct*
8
46
19
58
So plotting them is like plotting numbers on a graph. In the RA column are hours
(h) and minutes (m). In the DEC column are degrees (d) and minutes (m).
The vertical (y) axis is declination, marked in degrees. The horizontal (x) axis
is right ascension, marked in hours. You will notice that each is divided into six
subdivisions. That's because there are 60 minutes in an hour and 60 minutes in a
degree. So each subdivision equals 10 minutes.
It should be easy to plot the points if you round the minutes to the nearest 5 minutes,
or if you prefer, to the nearest 10 minutes.
When you are done plotting the points, connect the dots with a smooth line. You
will need to know the dates (those in red marked with asterisk (*) because you mus
connect the points chronologically to see the retrograde path represented correctly
on the celestial grid.