Sidereal vs Solar Day: How Earth's Rotation Defines Our Time
The rotation of the Earth in relation to the stars, rather than the sun, is determined by a sidereal day. It allows astronomers to keep track of time and determine where to aim their telescopes without having to worry about the Earth's orbit.
The Planet rotates once every 24 hours on its axis, and the sun loops through the sky.
The Earth rotates once entirely on its axis in 23 hours, 56 minutes, and 4 seconds, which astronomers refer to as a Sidereal Day.
Sidereal Period of Earth
The Earth rotates once every 24 hours on its axis, and the sun loops through the sky. The days of the week are described by the time it takes the sun to return to its highest point in the sky, or from dawn to noon. This is referred to as a solar day by astronomers.
The time it takes the sun to complete one circuit around the sky and the time it takes our world to complete one rotation, on the other hand, are not the same. If you've spent your whole life believing that the Earth rotates in 24 hours, you're in for a big shock.
The Planet travels about 2.5 million kilometers along its orbit in the time it takes to rotate once around its axis. The sun will not appear in the same part of the sky at the end of the rotation because the Earth has shifted. The Earth must rotate for another four minutes to return to facing the sun.
A solar day, in other words, is the time it takes the Earth to spin once – and then some. A sidereal day is the time it takes to complete one rotation, which is 23 hours 56 minutes and 4.1 seconds.
Every sidereal day, the stars appear in the same position in the sky at the same time in this scheme. The vernal equinox – when the sun sits in the atmosphere at the first moment of spring in the northern hemisphere – passes directly overhead at sidereal noon. Every night, watch the stars rise four minutes earlier to see the four-minute gap between sidereal and solar days. If Vega rises at 9 p.m. tonight, it will rise at 8:56 p.m. the next night, and 8:52 p.m. the night after that, and so on. We see each star earlier and earlier as the Earth orbits the sun.
The disparity between a solar day and a sidereal day is generally very slight in most situations, such as on Earth. However, there are a few significant exceptions in our solar system.
Mercury rotates at two-thirds the speed of light, with a sidereal day of 58 Earth days and an orbital period of 88 Earth days. Since the sidereal day is such a small part of Mercury's orbital cycle, an inhabitant must wait approximately 170 Earth days from one noon to the next.
However, this means that on Mercury, a solar day is longer than a year!
A Mercury year is about half the length of a Mercury solar day. Imagine ringing in the new year at midnight, then getting ready for the next New Year's Eve party at noon!
The planet Venus is an outlier. She orbits the sun faster than she rotates on her axis, with a 225 Earth-day orbit compared to 243 Earth-days to complete one rotation. This is why Venus is the solar system's slowest-spinning planet. The planet Venus is spinning at around 6 km/hr at its equator, while Earth's equator is hurling along at about 1700 km/hr.
Venus, on the other hand, does so by rotating backwards. The native Venusians would watch the sunrise in the west and set in the east if the suffocating cloud layer on Venus ever broke.
Venus is the only planet in the solar system where the sidereal day is longer than the solar day due to its backward rotation. Before the earth has completed one rotation, the sun hits its highest point in the sky.
When all of this is summed up, Venus has a solar day that lasts 117 Earth days. In other words, in a Venusian year, the sun only rises twice.
Sidereal Day Definition in Short
Note - The time it takes for the Planet to rotate around its axis enough for distant stars to appear in the same position in the sky is known as a sidereal day. The time it takes for the Earth to rotate around its axis so that the Sun remains in the same spot in the sky is known as a solar day. The solar day is four minutes less than the sidereal day.
Synodic Period of the Moon
Because of the Earth's rotation around the Sun, the Moon takes 29.5 days to return to the same position on the celestial sphere as referenced to the Sun; this is known as a synodic month (lunar phases as observed from the Earth are correlated with the synodic month).
Synodic Period of Mercury
Mercury has an 88-day sidereal period, but a 116-day synodic period; Venus has a 225-day sidereal period, but a 584-day synodic period.
FAQs on Sidereal Day Explained: Definition, Importance & Key Differences
1. What is a sidereal day in simple terms?
A sidereal day is the time it takes for the Earth to complete one full 360-degree rotation on its axis relative to distant, 'fixed' stars. It is considered the true rotational period of the Earth and has a duration of approximately 23 hours, 56 minutes, and 4.091 seconds.
2. What is the main difference between a sidereal day and a solar day?
The main difference lies in the reference point used for measurement. A sidereal day is measured against distant stars, while a solar day is measured against the Sun. This leads to a difference in duration.
Sidereal Day: Measures a 360° rotation. Lasts 23 hours and 56 minutes.
Solar Day: Measures the time for the Sun to return to the same position in the sky. Lasts 24 hours.
3. Why is a sidereal day approximately 4 minutes shorter than a solar day?
A sidereal day is shorter because it only accounts for Earth's rotation. While the Earth completes one 360° spin (a sidereal day), it also moves about 1 degree along its orbit around the Sun. To get the Sun back to the same position in the sky (to complete a solar day), the Earth must rotate for an additional 4 minutes to cover this extra orbital distance.
4. What is the importance of using sidereal time in astronomy?
Sidereal time is crucial for astronomers because it provides a consistent framework for locating celestial objects. An object's coordinate in the sky (its Right Ascension) directly corresponds to the Local Sidereal Time when it crosses the meridian (the highest point in the sky). This allows astronomers to precisely point telescopes and predict when a star or galaxy will be best positioned for observation, regardless of the time of year.
5. How does Earth's orbit around the Sun affect the length of a solar day but not a sidereal day?
The sidereal day is unaffected because its reference points—distant stars—are so far away that Earth's movement in its orbit is negligible from that perspective. The solar day, however, uses the much closer Sun as its reference. Therefore, Earth's daily orbital motion must be accounted for, requiring the planet to rotate slightly more than 360° each day to 'catch up' with the Sun's apparent position.
6. Is a sidereal day the true period of Earth's rotation?
Yes, the sidereal day is considered the true rotational period of the Earth. It measures the precise time taken to complete one full 360° spin on its axis relative to a fixed frame of reference (the distant stars), isolated from the effect of Earth's revolution around the Sun.
7. If a specific star is directly overhead at 9 PM tonight, when can I expect to see it overhead tomorrow?
You can expect to see the star directly overhead about four minutes earlier tomorrow night, at approximately 8:56 PM. This is because stars follow sidereal time, which is four minutes shorter than the 24-hour solar day our clocks use. This daily shift is why we see different constellations throughout the year.
