Make your own Astrolabe

by Dominic Ford

6. The Equal Hours

The front side of the mother of the model astrolabe

The front side of the mother of the model astrolabe. Click to enlarge.

It is also possible to align the astrolabe to show the current sky using modern times in the 24-hour system.

To do so, the sequence of twenty-four symbols around the edge of the mother is used. Each letter signifies an hour of the day, with the cross marking noon and the letter 'M' midnight. The procedure is much simpler than that used to align to the unequal hours: the rule should be rotated to point to the desired hour around the outer scale. The rete should then be rotated beneath it until the Sun lies on the edge of the rule (the procedure for locating the Sun along the path of the ecliptic was described in the previous section). The astrolabe is then correctly configured to show the sky at the desired time.

One technicality is worthy of note here: the twenty-four symbols refer to the hours of "local apparent solar time", which is defined such that midday always occurs on any given day when the Sun is at its highest altitude in the sky. This may be offset relative to civil time for two reasons.

First, the observer will, in general, be some distance east or west of the meridian for which their civil timezone is defined. Secondly, the speed of the Sun's motion in right ascension varies over the course of the year such that days in June and December are a few seconds longer than those in March and September. Civil time is a mean time, in which this variation is averaged over the year, and as these seconds accumulate day-by-day, apparent noon can drift up to 16 minutes either side of midday depending on the time of year. This offset is given by the equation of time.


Like everything else on this website, these astrolabe kits are © Dominic Ford. However, they are provided for the benefit of amateur astronomers worldwide, and you are welcome to modify and/or redistribute any of the material on this website, under the following conditions: (1) Any item that has an associated copyright text must include that unmodified text in your redistributed version, (2) You must credit me, Dominic Ford, as the original author and copyright holder, (3) You may not derive any profit from your reproduction of material on this website, unless you are a registered charity whose express aim is the advancement of astronomical science, or you have the written permission of the author.





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