>> >My understanding is that the sextant measures the angle between the Moon or >Sun and the horizon. If you're careful reading the solar or lunor tables >it gives you the latitude where you are. Longitude is something else. I >know it requires a good timepiece, but how does one convert the time to >longitude?
Not quite right. Measuring the distance between the moon and another body on the ecliptic and then the height of these two bodies above the horizon and then doing some very complicated spherical geometry actually allows you to calculate longitude. This is called doing a Lunar, and was the competing method to using a seagoing chronometer during the time of the Royal Astronomical Society's competition. It works fine and has an accuracy of about a quarter degree of longitude. The reason that this works is because the moon moves across the background of the stars, and can be thought of as the moving hand of a clock against this background. Therefore you can use the moon as your chronometer. The reason that this method lost to Harrison's seagoing chronometer is that fewer sights needed to be taken, the calculations were much simpler, less of the sky had to be visible to take sights, and the chronometer results were more accurate if the chronometer had a steady rate. It is interesting to note that doing a Lunar was Nevil Maskelyne's (the Astronomer Royal ate the time) preferred method of calculating longitude, and that he opposed the seagoing chronometer idea. -- Eric Haberfellner [ http://EricHaberfellner.com ]http://EricHaberfellner.com
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