There was a discussion today in Twitter if a Venusian year is longer than a Venusian day. The original tweet said it was. But as in many things in Astronomy, the correct answer is Yes and No. With some help from my son, I tried to explain that in a series of tweets. This is a slightly expanded version of those tweets.
What is a day?
By default we would say it is the time taken for earth to rotate upon its axis. Some would be a little bit more specific and say it is the time taken between successive noons, noon being the time when sun is at its highest point in the sky.
That is a correct definition but it refers to what is called as the solar day because the definition is in relation to the Sun. But there is also another type of day called Sidereal day. Sidereal means in relation to different stars. If you take the time between two times when a distant star is at its highest point in the sky that is called the sidereal day. The sidereal day is the time it takes for the Earth to complete one rotation about its axis with respect to the 'fixed' stars.
Simply put a sidereal day is the time it takes for a planet to complete a single rotation on its axis. A solar day is the time it takes for the Sun to return to the same place in the sky.
There is a difference in the length of a solar day and a sidereal day. For our Earth, the solar day is calculated to be 24 hours but the sidereal day is about four minutes shorter. Not much of a difference, but this is more pronounced for some planets than the others. The difference between the sidereal and solar day is due to the orbital motion of the planet.
What grade is your planet?
Based on the direction of rotation a planet is called either a prograde planet or a retrograde planet. If the rotation of the planet is in the same direction of its orbital motion it is called a prograde planet. If the rotation of the planet is in the opposite direction to its orbital direction, it is a retrograde planet.
In the solar system, if you see from the top of the Sun's North Pole, the planets would revolve around it in a counterclockwise direction. So if a planet rotates in a counterclockwise direction, it is a prograde planet. In our solar system, only Venus and Uranus are retrograde and all other six are prograde planets.
For prograde planets in our solar system, the sidereal day is The difference between the sidereal and solar day is due to the orbital motion of the planet. than the solar day. For the retrograde planets, the solar day is shorter than the sidereal day.
Picture this yourself
To explain this I took the help of my son to create an image.
Image Courtesy @AstroAceVikare
The top panels show the starting position of the planet with the arrow pointing at the Sun and at a star which is very far from the planet. Then the planet starts to move in its orbit and also rotate around itself in a counterclockwise direction. The stars are so far away that Earth's movement along its orbit makes nearly no difference to their apparent direction. In the lower left panel, the angle to the further star is already reached after one full rotation on the axis, while it takes a bit more rotation for the spot on the planet to face the Sun. This difference in time is the difference between the sidereal and solar days.
Image Courtesy @AstroAceVikare
This image is for the retrograde planets. Note the solar and sidereal sides are swapped. Here, because of the clockwise rotation, the point of reference faces the sun first before it rotates further to the distant star. This is the reason, for retrograde planets, the solar day is shorter than the sidereal day.
Mercury, Venus and much more
Venus goes around the sun faster than it spins on its axis. Because of this speed, the Venusian Solar Day is about 117 earth days but the Venusian Sidereal Day is about 243 earth days. The time taken for one trip around the sun, however, is about 225 earth days. Therefore, a Venusian year is shorter than its sidereal day but is almost twice its solar day.
Mercury is another odd ball. Mercurian sidereal day is 58 earth days while its year is 88. Because the sidereal day is a considerable fraction of the planet’s orbital period, a solar day is about 170 earth days. In Mercury, a solar day is longer than a year.
Since we refer to the solar day when we talk about a day in Earth, we need to understand this difference.
Who needs Sidereal Times anyways?
Astronomers!
Because sidereal time is calculated relative to distant stars, it helps astronomers keep time to locate celestial objects without worrying about where the planet is in its orbit. It is the clock of the astronomers.
Watch this video by Dr. James O'Donoghue (@physicsj ) that shows selected objects in our solar system to scale in size and rotation speed.
