Back to Astrolabes

A Note on Time

Time measured directly from the position of sun is called local apparent time. In local apparent time, noon always occurs when the sun is directly on the meridian. An astrolabe calculates local apparent time. Unfortunately, modern conventions of time are a little more complicated. Modern days are exactly twenty four hours long. Because the sun moves a little more quickly across the sky in winter than in summer, noon as calculated by the astrolabe will have an inherent error of plus or minus fifteen minutes. There is also the error introduced by the fact that most people are not living at the very center of a time zone. Ignoring the inconstant motion of the sun for a moment, one should expect the sun to be on the meridian at the center of each time zone right at noon. Fortunately, this particular error can be easily corrected for. Find the longitude of the center of your time zone on a map (it will always be a multiple of fifteen degrees), then compare it to your actual longitude. For every degree you find yourself west of the center, subtract four minutes. For every degree east of the center add four minutes. For example, Stevens Point, Wisconsin is only 0.5 degrees east of the center of the Central Time Zone. Thus I would add 0.5*4 = 2 minutes to the time calculated by the astrolabe. In practice, the two minutes is inconsequential relative to the plus or minus fifteen minutes added by the inconstant motion of the sun. One final note: astrolabes don't adjust for Daylight Savings Time (DST). During DST you're going to have to add an hour to the time indicated by your astrolabe.