Sidereal Day vs Solar Day
Geography › Earth's Origin, Interior and Motions · Topic 1.20
A solar day is the time between two successive noons, when the Sun is at its highest, and averages exactly 24 hours. A sidereal day is the time the Earth takes to rotate once relative to the distant stars, which is 23 hours 56 minutes 4 seconds. The solar day is nearly four minutes longer because the Earth also moves along its orbit by about 1 degree each day, so it has to turn a little more for the Sun to come back to the same position in the sky.
5 min read · UPSC Prelims · Geography · Earth's Origin, Interior and Motions
At a Glance
- Solar day 24 hours; noon to noon
- Sidereal day 23 h 56 min 4 s; star to star
- Difference About 3 minutes 56 seconds
- Cause Earth’s motion along its orbit
- Year About 365.24 solar days; about 366.24 sidereal days
- Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions
Where Does It Fit?
This topic sharpens the idea of rotation by showing that there is more than one way to measure a day. It supports the earlier topics on rotation and time and is the last of the group on the motions of the Earth, before the topic on equinoxes and solstices. Previous topic: Seismic Discontinuities (Conrad, Mohorovicic, Repetti, Gutenberg, Lehmann). Next topic: Tropic of Cancer Winter solstice About 22 December, Sun over the…">Equinoxes and Solstices (Summer Solstice, Winter Solstice, Vernal and Autumnal Equinox).
What Are the Solar Day and the Sidereal Day?
In short: the sidereal day measures the Earth’s true spin against the stars, while the solar day measures it against the Sun, which is what daily life uses.
| Feature | Solar day | Sidereal day |
|---|---|---|
| Reference | The Sun | A distant star |
| Length | 24 hours on average (mean solar day) | 23 hours 56 minutes 4 seconds |
| Used for | Civil time, clocks and calendars | Astronomy and telescope pointing |
| Rotation angle | About 361 degrees | 360 degrees |
| Stars rise | About 4 minutes later each day by the clock | At the same time by sidereal clock |
Why Is the Solar Day Longer Than the Sidereal Day?
Imagine that the Sun and a distant star are in line with a place on the Earth at noon. After one full rotation of 360 degrees, the star is again overhead, which completes the sidereal day. But in that time the Earth has moved about 1 degree along its orbit around the Sun, so the Sun appears to have shifted by about 1 degree eastward against the stars. The Earth must turn an extra degree, which takes about 4 minutes, before the Sun is overhead again.
- Extra rotation: 1 degree is 1/360 of a day, or about 4 minutes.
- In a year: these 4 minutes add up to one full day, so there are about 366.24 sidereal days in a year but only 365.24 solar days.
- Result: a star rises about 4 minutes earlier each night, and so different constellations are seen in different seasons, as the Earth revolves, as in Motions of the Earth: Rotation and Revolution and Their Effects.
What Are the Mean Solar Day, the Equation of Time and the Year?
The true solar day is not exactly equal throughout the year, because the Earth’s orbit is elliptical and the axis is tilted, as explained in Inclination of the Earth’s Axis and Its Effects. The actual interval from noon to noon varies by up to about 30 seconds. To get a steady clock, a mean solar day of 24 hours was defined from the yearly average, and the difference between the Sun’s real position and the mean Sun is called the equation of time. It reaches about 16 minutes in early November, when a sundial runs ahead of the clock.
| Type of year | Length (days) | Meaning |
|---|---|---|
| Sidereal year | About 365.256 | Time to return to the same place against the stars |
| Tropical year | About 365.242 | Time between two vernal equinoxes; the basis of the calendar and seasons |
| Calendar year | 365 or 366 | Whole days with the leap day rule, as in Local and Standard Time, International Date Line, Calendar |
The slight gap between the sidereal and tropical years comes from the slow wobble, or precession, of the Earth’s axis.
Does the Length of the Day Change?
The Earth’s spin is slowing very slowly, mainly because of the tides raised by the Moon. The day lengthens by about 1.8 milliseconds a century, and fossil corals suggest that the day was about 22 hours long some 400 million years ago. Because the spin is a little irregular, clocks based on atomic time and the Earth’s rotation can differ by fractions of a second, so a leap second has been added at times since 1972 to keep civil time in step. In 2022, the international body for measures decided to stop adding leap seconds by 2035.
Other planets give striking cases. On Venus, which rotates very slowly in the opposite direction, the sidereal day of about 243 Earth days is longer than its year, but the solar day from one noon to the next is only about 117 Earth days. A Moon-based day, the lunar day from one moonrise to the next, is about 24 hours 50 minutes.
Why Is This Topic Important?
- Concept Distinguishes two ways of measuring rotation
- Facts Exact lengths are asked
- Link Calendar and leap seconds
What Came Next?
The next topic fixes the four dates of the year on which the Sun reaches its extreme positions. Next topic: Equinoxes and Solstices (Summer Solstice, Winter Solstice, Vernal and Autumnal Equinox).
Key People and Terms
- Solar day Noon to noon
- Sidereal day Rotation against the stars
- Mean solar day 24 hours, the average
- Equation of time Difference between real and mean Sun
- Tropical year Equinox to equinox
- Leap second Second added to keep civil time in step
Exam Corner
Points to Remember
- The sidereal day is 23 h 56 min 4 s.
- The mean solar day is 24 h.
- The gap is about 4 minutes, due to the orbit.
- A year has about 366.24 sidereal days.
- Tropical year: about 365.24 days.
Do Not Confuse With
- Rotation period and solar day 360 degrees versus 361 degrees of turning
- Sidereal year and tropical year Against the stars versus against the equinox
- Solar day and lunar day 24 hours versus about 24 hours 50 minutes
Memory Hook
Star-to-star is short, Sun-to-Sun is long; four minutes lost to the orbit’s song.
Mains Angle
Questions are mostly factual at Prelims. These points add depth for a short note.
- Cause Orbital motion adds an extra degree
- Use Civil time versus astronomy
- Trend Slowing rotation
A Question You May Face
An original practice question, not a past paper question.
“Why is a solar day longer than a sidereal day, and how does this difference affect the sky we see through the year?”
How to Answer
- Define Both days.
- Explain Orbital shift of one degree.
- Effect Stars rise four minutes earlier.
- Add Mean solar time.
GS Relevance
Frequently Asked Questions
What is the difference between a sidereal day and a solar day?
A sidereal day is the time the Earth takes to rotate once relative to the distant stars, about 23 hours 56 minutes 4 seconds. A solar day is the interval between two successive noons, averaging 24 hours, and it is longer by nearly four minutes because the Earth also moves along its orbit.
Why is the solar day longer than the sidereal day?
While the Earth completes one rotation, it also moves about one degree along its orbit around the Sun, so the Sun appears to have shifted against the stars. The Earth must rotate an extra degree, which takes about four minutes, to bring the Sun back to the same position in the sky.
How many sidereal days are there in a year?
A year has about 366.24 sidereal days but only about 365.24 solar days. The extra sidereal day arises because the Earth turns once more relative to the stars than it does relative to the Sun during one trip around the orbit.
What is the mean solar day?
The mean solar day is the average length of the solar day over a year, defined as exactly 24 hours. The real interval from noon to noon varies slightly through the year because of the elliptical orbit and the tilt of the axis, so a uniform average is used for clocks.
What is the equation of time?
The equation of time is the difference between apparent solar time, shown by a sundial, and mean solar time, shown by a clock. It varies through the year and reaches about 16 minutes in early November, because of the Earth's elliptical orbit and tilted axis.
PYQ Practice — Statement Analysis
1 The sidereal day is shorter than the solar day.
It is 23 hours 56 minutes 4 seconds.
2 The Earth rotates through exactly 360 degrees between two successive noons.
It rotates through about 361 degrees.
3 A year has more sidereal days than solar days.
About 366.24 against 365.24.
4 The mean solar day is 24 hours.
It is the yearly average.
Past Mains Questions on This Topic
This topic comes under Earth's Origin, Interior and Motions. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
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