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Finding the position of the sun at sunrise (sunset)

Finding the position of the sun at sunrise is almost identical to finding the position of a star when it first rises. The only difference is that you first have to locate the sun in the sky. To do this, use the back of the astrolabe as you did in the previous problem. Unlike stars, the sun's rising position depends on the date. Once you have located the sun on the ecliptic, simply align the sun with the eastern horizon and read its position using the almucantars, which are drawn every ten degrees. To find the position of the sun at sunset rotate the rete until the sun is on the western horizon on the right side of the astrolabe. Notice that the number of degrees is the same for sunrise and sunset-- the only change is substituting west for east.

Example: Where on the horizon will the sun rise on the 29th of January? Using the back of the astrolabe, locate the sun on the ecliptic. In this case, it will be in the 10th degree of Aquarius. Now using the front of the astrolabe, place the 10th degree of Aquarius on the eastern horizon (see below). The sun appears to be between the 20 and 30 degree azimuths, slightly closer to the latter. A good estimate of the position of sunrise might be 27 degrees south of east. After making this measurement, you can easily infer that the sun will set 27 degrees south of west on that data. Check and see for yourself.