Astronomy

# What causes the solar day of a planet to be different from its sidereal day?

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###### 2013-02-22 04:56:16

The sidereal day of a planet is the time it takes to rotate once on it's axis. The solar day is the time from sunrise to sunset.

To see why they are different, let's image a planet that rotates very slowly. Every time it goes around its star once, it also rotates once on its axis. Since it rotates once on its axis per year, there is one sidereal day per year. Now, in order for this to work, one side of the planet must be facing the star at all times. This means that there is no sunrise or sunset, so on this planet, there are zero solar days in a year.

Now let's image a planet that rotates twice a year (has two sidereal days a year). At the beginning of the year, a side of that planet is facing the star. Halfway through that year, the planet has rotates once, but is on the other side of the star, so that side of the planet is now facing away from the star. At the end of the year, the planet is back where it started. There has been one sunrise and one sunset, so only one solar day.

From this we can see that a planet (as long as it has at least one sidereal day per year) has one more sidereal day per year than solar day per year. There are 365.242 solar days in an Earth year, but there are 366.242 sidereal days in an Earth year.

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## Related Questions

A solar day is usually longer. However a planet like Venus has a longer sidereal day than solar day. That's because of its "retrograde" rotation.

A solar day is 24 hours whereas a sidereal day is 23 hours and 56 minutes

A day on Jupiter lasts about 10 hours. This is known as its sidereal rotation period. Jupiter is the largest planet in the Solar system.

Venus has the longest "sidereal day". One sidereal day on Venus is 243 Earth days. A sidereal day is the time for a planet to spin once. There's another important definition of "day" and that's the "solar day". The solar day is based on the position of the Sun in the sky and depends on a planet's orbit as well as its spin. Using the solar day definition, Mercury has the longest day. (On Earth, the solar day is exactly 24 hours and the sidereal day is about four minutes shorter.)

The Earth rotates on its axis in one day. Strictly speaking that's the "sidereal day" not the "solar day". Also, by definition, each planet rotates once in a period that's the "sidereal day" for that particular planet.

Venus, has the longest rotation period. That's called its "sidereal day". However, Mercury has the longest "solar day". A solar day on Earth is exactly 24 hours and Earth's sidereal day is about 23 hours and 56 minutes.

It is about 117 Earth days (for the "solar day") or 243 Earth days (for the "sidereal day").

The Sun crosses the meridian (the imaginaryline in the sky from due north to due south and passing directly overhead) at solar noon. The period between two successive solar noons is a "solar day". When you reckon the period of time between two successive meridian transits of a distant star, or the vernal equinox, that is a "sidereal day". In simple terms the sidereal day is the rotation period of a planet.

There are two main definitions of "day" for a planet:1) Sidereal day. That's the time taken to rotate once.2) Solar day. That's the time taken for the Sun to complete one apparent journeyaround the sky. Usually it's given as the "mean solar day", which is the average value.The solar day depends on the planet's orbital motion, as well as its rotation.Here are the details, with slight approximations:Mercury : sidereal day, 58.65 Earth dayssolar day, 175.94 Earth daysVenus: sidereal day, 243.02 Earth dayssolar day, 116.75 Earth daysEarth: sidereal day, 23.934 hourssolar day, 24.000 hoursMars: sidereal day, 24.623 hourssolar day, 24.660 hoursFor these planets both days are nearly the same length:Jupiter: 9.92 hoursSaturn: 10.66 hoursUranus: 17.24 hoursNeptune: 16.11 hours

The sidereal day is about 243 Earth days. That is the time for the planet to rotate once . The solar day is 117 Earth days,roughly. That is a big difference. On Earth the difference between these two days is only about 4 minutes. The reason for the big difference on Venus is that Venus rotates very slowly. The solar day depends on the rotation of a planet AND its orbital motion. The slow rotation makes the effect of the orbital motion much greater on the length of the solar day. Incidentally Venus rotates in the opposite direction to Earth. That's why the solar day is shorter than the sidereal day. On Earth it's the other way round.

Mercury has the 2nd longest "sidereal day" with a sidereal rotation period of 58.646 Earth days. The longest "sidereal day" day is Venus, with a sidereal rotation period of 243.018 Earth daysIf you use the "solar day" as your definition of "day", the order is reversed. Mercury then has the longest day and Venus has the second longest day.

There is no such planet in our solar system. The nearest case would be Venus. Venus has a "sidereal" day of about 243 Earth days and a year of about 225 Earth days.

For the planet Earth, the difference is about 4 minutes and a few seconds; the sidereal day is shorter by that amount. The reason is that one sidereal day is the time that it takes the Earth to rotate 360 degrees on its axis. But in that time, the Earth has continued to move along in its orbit around the Sun. So in order to get back to the point where the Sun is in the same position in the sky, the Earth has to spin for about another four minutes. That's a solar (or Sun referenced) day. Interestingly, on the planet Venus it is the solar day that's shorter. That's because of the "retrograde rotation" of Venus.

Solar energy causes temperature differences on our planet that results in different air pressures and hence causes wind from high pressure areas to low pressure ones.

Venus "sidereal day" is longer than its year. Mercury's "solar day" is longer than its year. However, there is no planet in our solar system with a day longer than our year on Earth.

Venus. That's the "sidereal day" (rotation period). However, Venus has a "solar day" that's shorter than its year. Mercury is the planet with a "solar day" that's longer than its year.

That's Mercury: "sidereal day" is more than half its year. Or, it could be Venus: "solar day" is just over half its year.

There are a few different things that affect the amount of solar energy that the planet receives. The atmosphere is one of these factors.

Mercury's Solar Day is about 176 Earth days. That's the time taken for the Sun to complete one apparent journey around the sky. Mercury's Sidereal day (rotation period) about 58.6 Earth days. Earth's solar day is 24 hours. Earth's sidereal day is about 23 hours and 56 minutes.

Jupiter is the largest planet in the solar system

Because Venus's spin is retrograde, that is to say it spins "backwards". (Also it travels around the sun faster than it spins on its axis, meaning the Venusian "day" is longer than a Venusian "year".) That is the Venusian SIDEREAL day which is its rotation period.The SOLAR day is how long the Sun takes to apparently travel once round the sky.That's from one Venusian "noon" to the next.The combination of retrograde spin and very slow spin causes the Solar dayto be much less than the time taken to spin once (the sidereal day).On Earth the solar day is more than the sidereal day, but not by much.

On Earth, a sidereal day is shorter than a solar day. The sidereal day is 23 hours 56 minutes and some odd seconds. This is the time it takes for the Earth to rotate once on its axis relative to the far distant stars. In this period of time, the Earth also revolves around the Sun a little bit, enough to require nearly four additional minutes for the Earth to "catch up" with the Sun and arrive at the same point relative to the Sun, completing its Solar day. For most planets this is typical. However for a planet like Venus, which has "retrograde" rotation, the sidereal day is the longer day.

The definition of a planet has a wide range of different types of bodies. A planet in this solar system will refer to a body that orbits the sun.

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