Length of a Da2

# Length of a Da2 - minutes from the previous day If you want...

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Length of a Day - Solar versus Sidereal Why is there a difference between a Solar day and a Sidereal day? The cause is the motion of the Earth, in this case our orbital motion around the Sun. To illustrate what's going on, follow the stick in Figure 6. It starts out one day pointing directly at the Sun (at noon) and at a very distant star (a star way off to the right). Figure 6. The time it takes for a position on the Earth to line up with a distant star (way off to the right) is 23 hours and 56 minutes. However, the Sun will not be lined up with the position on the Earth, and an additional four minutes are needed. After 23 hours and 56 minutes, the Earth will have not only made one complete rotation, but will have also moved in its orbit. The stick will no longer be pointing toward the Sun, but it will again be pointing toward the star - this tells us that the Earth has made exactly (no more, no less) one rotation. The time on our watches is 11:56 AM - NOT NOON! - since it is 23 hours and 56
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Unformatted text preview: minutes from the previous day. If you want to see the stick pointing again at the Sun, you must wait four more minutes for the Earth to spin a little bit further around. When that happens, the time will again be noon. You might be a bit amazed at how I was able to easily draw up the motions of the Earth and such so quickly, but how did I know which way it was going? There is a rule about how things in the solar system move and you can use it to draw similar diagrams. All major motions in the solar system are in a COUNTER-CLOCK WISE direction when observed from above the North Pole. This includes the motions associated with the Earth, the Moon, the orbital motions of the planets, and most of their rotation motions as well. There are of course some exceptions, but they aren't that common. If you have to quickly draw any solar system motions, you'll know which way the stuff is moving - again, there are a few exceptions and I'll tell you what they are if necessary....
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