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How would we know when the winter comes, if it weren't for the seasons on Earth? One might say, well if there aren't any seasons, why would we even want to know such information? Let's say its kind of a mental exercise. The best way of finding out what season the Earth is in is by tracking the motion of the stars on the night sky, more precisely the position of our planet on the orbit around the Sun.
Ancient civilizations might have not known that stars are not actually moving on the night sky in relation to Earth, but still knew when a particular season was approaching.
For example, during the winter season, the north hemisphere can easily observe the constellation of Orion on the night sky, while during the summer season Orion is not available for view, but rather the stars in the constellation of Scorpius, which are located in the southern part of the sky. The same can be said about spring, when the constellation Leo dominates the northern skies, while during the fall we are able to view the constellation of Pegasus.
The phenomenon is caused by Earth's rotation around its axis and around the Sun. The change is subtle, but given a considerable amount of time, something strange begins to happen. Within the period of only a few weeks, a whole portion of the night sky disappears under the horizon and is replaced by another one, which was previously high up in the sky. To understand the whole process, one must first realize that the 24 hours day, defined as a complete rotation of our planet around its axis, does in fact require 23 hours 56 minutes and 4 seconds to complete. This is called a sidereal day.
At the same time however, the Earth travels about 960 kilometers in its elliptical orbit around the Sun, meaning that the Sun chances its position in relation to the background stars by one degree each day. To make up for the 24 hours in a day, however, the Earth has to rotate a little more around its axis to compensate, 4 minutes and 56 seconds more precisely, which corresponds to the Earth rotating by one degree. This motion is provided by the Earth's rotation around the Sun, thus completing a full circle in a 24 hour day, which is also why the stars on the night sky appear to shift positions in relation to the Earth and its seasons.
Which brings us to the question: why do clocks have 12 hours dialed instead of 24? We have already established that the day has 24 hours, haven't we? As it turns out, the 24 hour dial is only available for sidereal clocks, which, opposed to regular clocks, make a single complete rotation in one day and has no notion of ante meridian and post meridian. These clocks are adjusted in relation to the motion of the stars on the night sky, starts with the hour 00 and is numbered all the way to 23. Thus except of running 4 minutes and 56 seconds faster that regular clocks, sidereal clocks are not so different from clocks we use on a daily basis.
If we were to use sidereal clocks in every day life, something even stranger would star to happen with the passing of time. The rising of the Sun in the middle of the night might appear as something out of a science fiction movie, but would actually be only grave error in time measurement.
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