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Finding the Seasonal Hour using the Front of the Astrolabe

On the front of the astrolabe, you will notice that the sky is divided into twelve equal hours below the horizon. These lines are used to calculate the seasonal hour, which derives from the Medieval idea of dividing the day into twelve equal hours no matter how long the day actually lasts. Thus in the summer a seasonal hours will be longer than a standard hour and conversely in the winter it will be shorter. The trick to using the seasonal hour lines is to find the declination of the sun using the scale on the rule.

Find the sun in the zodiac and use the rule to determine its declination. Once you have done this, turn the rule 180 degrees so that the unlabeled side of the rule is next to the sun. By locating the declination of the sun on the part of the rule that is below the horizon, you can estimate the seasonal hour, noting its position with respect to the seasonal hour lines.

Example: What is the seasonal hour if the sun is 10 degrees above the eastern horizon on January 29th? Place the sun (i.e. the 10th degree of Aquarius) ten degrees above the eastern horizon. Measure its declination using the rule. You should obtain a declination of -19 degrees. Now turn the rule 180 degrees around so that the labeled half is below the horizon and the unlabeled half still rests against the sun. By observing where -19 degrees declination falls between the seasonal lines, we can see that it is just past the third seasonal hour.