# Why are Sidereal Days and Solar Days different times and what is the purpose of the Sidereal day?

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I’ve researched it but it’s exact purpose eludes me.

In: 5

A Sidereal day is a full rotation of the earth measured by the stars and solar is compared to the Sun. The earth is going around the sun so that orbital rotation means the Earth has to rotate a little bit more for the sun to be at the same point in the sky.

The usage of the Sidereal day it’s tracking where stars will be each night.

A Sidereal day is a full rotation of the earth measured by the stars and solar is compared to the Sun. The earth is going around the sun so that orbital rotation means the Earth has to rotate a little bit more for the sun to be at the same point in the sky.

The usage of the Sidereal day it’s tracking where stars will be each night.

A sidereal day is the time it takes for the Earth to rotate once, about 23 hours, 56 minutes. A solar day is the time it takes for the Sun to return to the same position (well, same east-west position) in the sky, about 24 hours.

To see the difference, let’s imagine the Earth orbited the Sun faster than it does. Let’s make the year four (sidereal) days long.

Let’s picture the Earth orbiting and rotating as though both were in the same flat plane. Imagine a drawing with the Earth above the Sun, and you living on the side of the Earth facing the sun (the “bottom” of the Earth in this drawing). Both the orbit of the Earth, and the rotation of the Earth, are counterclockwise in this drawing, so at this moment, you are being carried to the right by the Earth’s rotation and the Earth is moving to the left in its orbit. It is currently noon in your local solar time, because the Sun is directly overhead.

Now, progress the Earth around by 1/8 of its orbit. It’s now up and to the left of the Sun. 1/8 of an orbit is 1/2 of a (sidereal) day, so the Earth’s rotation has carried you halfway around; you are now on the “top” of the Earth. But you are *not* on the opposite side of the Earth from the Sun – that is, it is *not* [solar] midnight for you right now! If the Earth had stayed put, it would be, but the movement of the Earth in its orbit has moved the position of the Sun relative to the Earth.

Progress the Earth around by another 1/8 of its orbit. It’s now to the left of the Sun. 2/8 = 1/4 of an orbit is 1 full sidereal day, so you’re once again on the “bottom” of the Earth, where you started. But now that the Sun is to the *right* of Earth, it’s not noon for you: it’s dawn! The sun is just rising over your horizon. 1 full sidereal day has passed, but you haven’t yet seen 1 full *solar* day, because the Earth’s orbit is effectively delaying the speed at which the Sun’s apparent position in your sky changes.

It turns out that the total effect of this is that you have 1 less solar day than sidereal day every year*. In our example, there are 4 sidereal days per year, so there are 3 solar days per year. On the real Earth, there are 36**6**.24 sidereal days per year, and thus 365.24 solar days per year (which is what you think of as “the number of days in a year”, because if we say “day”, we usually mean “solar day”). The difference in length (365.24/366.24 = 0.9973) corresponds to the four-minute difference in the time each type of day takes (0.9973 * 24 hours = 23.93 hours = ~23 hours, 56 minutes).

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* provided both rotations are in the same direction, which is true for Earth. If the rotations are in opposite directions, you get one *more* solar day than sidereal day.

A sidereal day is the time it takes for the Earth to rotate once, about 23 hours, 56 minutes. A solar day is the time it takes for the Sun to return to the same position (well, same east-west position) in the sky, about 24 hours.

To see the difference, let’s imagine the Earth orbited the Sun faster than it does. Let’s make the year four (sidereal) days long.

Let’s picture the Earth orbiting and rotating as though both were in the same flat plane. Imagine a drawing with the Earth above the Sun, and you living on the side of the Earth facing the sun (the “bottom” of the Earth in this drawing). Both the orbit of the Earth, and the rotation of the Earth, are counterclockwise in this drawing, so at this moment, you are being carried to the right by the Earth’s rotation and the Earth is moving to the left in its orbit. It is currently noon in your local solar time, because the Sun is directly overhead.

Now, progress the Earth around by 1/8 of its orbit. It’s now up and to the left of the Sun. 1/8 of an orbit is 1/2 of a (sidereal) day, so the Earth’s rotation has carried you halfway around; you are now on the “top” of the Earth. But you are *not* on the opposite side of the Earth from the Sun – that is, it is *not* [solar] midnight for you right now! If the Earth had stayed put, it would be, but the movement of the Earth in its orbit has moved the position of the Sun relative to the Earth.

Progress the Earth around by another 1/8 of its orbit. It’s now to the left of the Sun. 2/8 = 1/4 of an orbit is 1 full sidereal day, so you’re once again on the “bottom” of the Earth, where you started. But now that the Sun is to the *right* of Earth, it’s not noon for you: it’s dawn! The sun is just rising over your horizon. 1 full sidereal day has passed, but you haven’t yet seen 1 full *solar* day, because the Earth’s orbit is effectively delaying the speed at which the Sun’s apparent position in your sky changes.

It turns out that the total effect of this is that you have 1 less solar day than sidereal day every year*. In our example, there are 4 sidereal days per year, so there are 3 solar days per year. On the real Earth, there are 36**6**.24 sidereal days per year, and thus 365.24 solar days per year (which is what you think of as “the number of days in a year”, because if we say “day”, we usually mean “solar day”). The difference in length (365.24/366.24 = 0.9973) corresponds to the four-minute difference in the time each type of day takes (0.9973 * 24 hours = 23.93 hours = ~23 hours, 56 minutes).

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* provided both rotations are in the same direction, which is true for Earth. If the rotations are in opposite directions, you get one *more* solar day than sidereal day.

They’re confusing for Earth because they’re so close, but they’re more useful for other planets

How long is a day on Venus?

Well solar noon to solar noon is 116.75 Earth day, so does Venus rotate on its axis 116x slower than Earth? But its year is 224 earth days so you start getting weird combinations.

Venus actually rotates on its axis in 243 Earth days (1 sidereal Venusian day) but because it rotates backwards relative to most other planets it causes its Solar day to be significantly shorter than its sidereal day while Earths rotation/orbit combine to make the solar day slightly longer than the sidereal day

Sidereal day lets you figure out equatorial velocity and the equatorial bulge that comes from that, its a fixed value that isn’t dependent on other things. Solar days are dependent upon the sidereal day and the orbital period to determine when the sun returns to the same spot over a specific point on the planet but if something like Mercury that’s really booking it around the sun it might take over a solar year for 1 solar day to complete and that gets odd

They’re confusing for Earth because they’re so close, but they’re more useful for other planets

How long is a day on Venus?

Well solar noon to solar noon is 116.75 Earth day, so does Venus rotate on its axis 116x slower than Earth? But its year is 224 earth days so you start getting weird combinations.

Venus actually rotates on its axis in 243 Earth days (1 sidereal Venusian day) but because it rotates backwards relative to most other planets it causes its Solar day to be significantly shorter than its sidereal day while Earths rotation/orbit combine to make the solar day slightly longer than the sidereal day

Sidereal day lets you figure out equatorial velocity and the equatorial bulge that comes from that, its a fixed value that isn’t dependent on other things. Solar days are dependent upon the sidereal day and the orbital period to determine when the sun returns to the same spot over a specific point on the planet but if something like Mercury that’s really booking it around the sun it might take over a solar year for 1 solar day to complete and that gets odd

basically, they are different because of the earths orbit around the sun.

a day is a complete rotation of the planet, yes? well, there are two reference points you can use to mark your progress against. there is the sun, and the stars of the galaxy.

Because we are moving relative to the sun as we orbit it, their is a slight difference the two because we are in not quite the same position, hence the slightly different day lengths. basically, when we have completed a full day relative to the background stars (a sidereal day), the changes in our position relative to the sun mean were are not quite pointing at it, yet, but will do in a few more minutes (a solar day)

basically, they are different because of the earths orbit around the sun.

a day is a complete rotation of the planet, yes? well, there are two reference points you can use to mark your progress against. there is the sun, and the stars of the galaxy.

Because we are moving relative to the sun as we orbit it, their is a slight difference the two because we are in not quite the same position, hence the slightly different day lengths. basically, when we have completed a full day relative to the background stars (a sidereal day), the changes in our position relative to the sun mean were are not quite pointing at it, yet, but will do in a few more minutes (a solar day)

Solar days count between the Sun at identical positions in the sky. Sidereal days count between the *surrounding universe* at identical positions in the sky. It differs by ~1 part in 365.