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Imagine you live in a desert country, where the skies are usually clear. You don’t have any clocks available, but you meet a friend for lunch at noon. You agree to meet at the same time the next day for lunch. How
would you define “the same time tomorrow?”
The obvious way is to use the Sun. It’s easy to measure when the Sun is at its highest point in the sky - the time we call “noon”. A stick in the ground will cast a shadow - and when that shadow is at its shortest,
that’s noon. You can do it visually, too - the Sun never goes higher than the roof of that house over there - when it’s just above the roof, it’s noon.
That, in fact, is exactly how we define “a day” - it’s the period from one noon to the next. This period we divide into the familiar 24 hours. We’ve been doing it for thousands of years.
Now suppose you meet your friend for dinner, when it’s dark. You agree to meet the same time the next day. How do you define “same time tomorrow” now? An obvious solution is to use a bright star. You may notice that
a bright or easily identifiable star - the leftmost star of Orion’s belt, for instance - is just over a particular rooftop. You decide to meet again when that same star is in the same place, just over that rooftop.
Now, the Earth spins around once a day, but it also moves around the Sun a little bit - by about a degree a day. For this reason, our two definitions of “same time tomorrow” - one using the Sun, the other using a
star - don’t quite match. There is about a four minute difference between the two times.
So, if I have a watch that measures days based on one noon to the next, and you have a watch that measures days based on a star re-appearing at the same point, when your watch claims that 24 hours have passed, my
watch claims that only 23 hours 56 minutes have passed. My watch measures “Sun time” - your watch measures “Star time”, which is also called sidereal time.
By convention, astrologers synchronise their watches - the sidereal watch and the solar watch - at the Autumn Equinox. The sidereal “watch” then gains four minutes a day, until by the next Autumn Equinox it’s gained
exactly 24 hours - and the two watches coincide again.
One final point to note - in Europe, we always use the 24 hour clock. In the USA, the military use this system (they call it “military time”). I find it far, far easier to get used to using the 24 hour system as
astrologers - it’s totally unambiguous. As the clock strikes midnight, and a new day starts, the time is 00:00. By 1am, it’s 01:00. At noon, it’s 12:00. 1pm is 13:00. The very last minute of the day, at 11.59pm, is
23:59, and then we start a new day at 00:00 the following day. I’ve used this convention throughout this article, and all ephemerides (including the most widely used one, The American Ephemeris, uses this convention).
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