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Answer to Question #59492 in Astronomy | Astrophysics for hamid

Question #59492
As the earth rotates and spins..why don't we observe its motion?
Expert's answer
There are two types of Earth rotation: around its axis and around the Sun. The first period is the amount of time it take for the Earth to turn once on its axis so that it returns to the same orientation compared to the rest of the Universe. The second one is how long it takes for the Earth to turn so that the Sun returns to the same spot in the sky.

For an observer at a fixed position on Earth, the rotation of the earth around its axis makes it appear as if the sky is revolving around the earth. In other words, if you are standing for long enough in a field at night, it looks like the sky is moving, not you. This motion is called "apparent diurnal motion." "Diurnal" means having to do with a day, in the sense of a 24-hour period.

The Earth is orbiting around the Sun, completing one orbit in just over 365 days. If you divide 24 hours by 365 days, you’ll see that you’re left with about 4 minutes per day. In other words, the Earth rotates on its axis, but it’s also orbiting around the Sun, so the Sun’s position in the sky catches up by 4 minutes each day. Such a rotation plays a special role in the seasons changing. We also see differences in distant stars position at winter and at summer, for example.

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