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We have discussed the rising and setting of the stars and rotation of the Earth without going into detail about when these events take place. First of all, the Earth rotates around its axis once every 23 hours and 56 minutes. If you really want to be exact, it takes 23 hours, 56 minutes and 4.098904 seconds. This may sound a bit strange at first. Nobody, when pressured to work faster, shouts back, “For goodness sake, there are only 23 hours, 56 minutes and 4.098904 seconds in a day.” So let’s take a deep breath and a closer look at what type of day we are talking about. It takes 23 hours and 56 minutes for the Earth to rotate in respect to the far distant stars. This time period is called a sidereal day. This differs from the 24-hour solar day which measures the rotation of the Earth in respect to the nearest star, our Sun.
One way to visualize the difference between a solar and a sidereal day is to imagine looking at the Sun and the Earth from high above the North Pole. Furthermore, imagine that center of the Sun, a point on the Earth’s equator such as Quito, Ecuador, and the North Pole all lay along a straight line. Since all these are on a straight line, it is high noon in Quito, which is directly facing the Sun. Twenty-three hours and 56 minutes later, the Earth will have rotated around its axis exactly one time and once again will be facing the same distant stars. But during this time, the Earth has revolved slightly around the Sun and Quito no longer forms a straight line with the Sun’s center and the North Pole. It will take another 4 minutes of the Earth’s rotation around its axis for Quito once again to directly face the Sun and for local time to be high noon.
Sidereal time is like solar time but with a clock that runs 4 minutes fast every day. If you look at the sky at this moment, wait 23 hours and 56 minutes and look again, the stars will be in exactly the same position. The sidereal time was exactly the same both times you looked at the sky.
If sidereal time is so convenient for looking at the stars, why do we base our lives on solar time? The main difficulty would be that the Sun would shine at a different time each day. During certain times of the year, sidereal midnight falls in the middle of the night, or at the break of dawn or even in the middle of the day. As long as we wish to base our lives on the rising and setting of the Sun and not the stars, we have to abandon sidereal and accept solar time.
The sky would be much less interesting if there was not the 4 minute gap between the solar and sidereal day. Imagine that you do your sky watching every evening at nine o’clock. Every night you look up at the sky after a hiatus of 24 solar hours or about 24 hours and 4 minutes of sidereal time. Each night, the stars have not only revolved around to their previous locations, but have advanced another 4 minutes worth. The advances itself about 4 minutes daily, 2 hours monthly and a complete turn every year. Another way of looking at all this is to remember that the celestial sphere circles around 366 ¼ times each year, 365 ¼ times because of the Earth’s rotation around its axis and one additional time because of the Earth’s revolution around the Sun. By the way, if you divide the length of the solar year by the number 366 ¼ you get the exact length of a sidereal day.
The daily 4-minute creep of the celestial sphere provides us with a different sky to look at during each season. The constellation of Scorpio is best seen during the summer, Leo during the spring and Orion during the winter. “Best” in this case does not only mean high in the sky, but also at about nine o’clock in the evening. Of course, there is no rule restricting us to looking at the sky at only nine in the evening. One night I suddenly wake up at about four in the morning. Instead of turning over and going back to sleep, I slip out of bed, go into my kitchen and look out the window facing east. I am now looking at stars, which I won’t be able to during the early evening for another three months -- a sneak preview of the coming sky. There are many signs that give us a sense of season. We occasionally forget the day of the week, but the change of season is too fundamental to ignore. Most people develop their sense of season by the weather or by what sport is on television. To a skygazer there is a different feeling to the winter sky with Orion above, from the sky of summer with Scorpio peeking over the southern horizon. The sky can become so much a part of you that by staying up late one night you can get the feeling of seeing ahead into spring while still in the middle of frigid winter.