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Finding the length of a day

As most people know, days are longer in the summer and shorter in the winter. The astrolabe can be used to determine exactly how long daylight will last on a given day of the year. We will see that this is similar to finding the length of time a star will be above the horizon. First, find the sun using the back of the astrolabe and set it on the eastern horizon. Align the rule with the Roman numeral XII at the top of the astrolabe. Now, using your thumb to hold the rule tight against the rete, rotate both the rule and the rete until the sun is on the western horizon. The rule should now point to the number of hours that have passed between sunrise and sunset. Again, keep in mind that once you go past XII again you have to add twelve to the number indicated by the rule.

You can also find the time of sunrise and sunset, then do a quick calculation in your head. The above solution is much more elegant. While you're trying this example, compare it with the problem of determining the length of time a star will be above the horizon.

Example: How long does daylight last on January 29th? 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. Point the rule to XII at the top of the astrolabe (see the sun rise example). Now carefully rotate both the rule and the rete, using your thumb to hold them together, until the sun is on the western horizon. The rule indicates that about 9.5 hours have passed between sunrise and sunset. Note that if you're clever you can use this same technique to calculate the length of time you will be able to observe galaxies with your telescope (i.e. the length of time between one astronomical twilight and the next.)