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Finding the time of sunrise

Problems involving time can be summarized with two basic principles

• The sun is the keeper of time (i.e. the position of the sun in the sky determines what time it is)
• If you place the edge of the rule against the sun, it will point to the time of day on the mater

Now, if we want to know when sunrise occurs, we have to set up the astrolabe so that the sun is on the eastern horizon. After locating the sun in the zodiac using the back of the astrolabe, place the sun on the eastern horizon. We have already done this, but this time we're going to line up the rule so that its inner edge rests against the sun. Now we can simply read off the time of day using the Roman numerals on the mater of the astrolabe. Notice that each hour is divided into fourths, or intervals of fifteen minutes.

Example: What time does the sun rise on the 29th of January? Again, the sun is in the 10th degree of Aquarius on January 29th. Using the front of the astrolabe, we place the 10th degree of Aquarius on the eastern horizon (see below). This time, however, we are also going to set the rule so that its inner edge rests against the sun. Looking at the Roman numerals on the outer edge of the astrolabe, we see that the rule is about 1/4th of the way past VII. In other words, the sun rises at 7:15 AM.