Sidereal Day and Solar Day: Why Earth Turns in 23 Hours 56 Minutes but Your Day Lasts 24
A sidereal day is one turn of Earth measured against the stars, about 23 hours 56 minutes 4 seconds. A solar day lasts 24 hours because Earth must turn a little extra to bring the Sun back to the same place.
Also called
sidereal rotation period, sidereal period, mean sidereal day, mean solar day, length of day
A simplified model
The drawings are computed from the numbers in the sources. They simplify the real sky. For exact times and dates, use official tables.
Ask anyone how long a day is and the answer comes fast: 24 hours. That is the right answer for the clock on your wall. It is not the time Earth needs to spin once. Measured against the distant stars, one full turn takes a few minutes less, and the gap is small enough to hide in plain sight.
This guide explains the two kinds of day, lists the official numbers and shows why Earth must turn a little extra each day to bring the Sun back to the same place. It ends with a short task in the Clock Dial.
Two ways to count a day
A solar day follows the Sun. NASA describes the length of day as the average time for the Sun to move from the noon position in the sky back to the same position. The mean solar day is the version we use for clocks. The U.S. Naval Observatory defines a day as 86,400 seconds, and NIST explains that the mean solar day is useful for timekeeping because it averages the length of the day over a year.
A sidereal day follows the stars. NASA defines Earth's rotation period as the time to complete one turn relative to the fixed background stars, and it adds that this is not measured relative to the Sun. The Naval Observatory gives the mean sidereal day as approximately 23 hours, 56 minutes, 4 seconds.
| Kind of day | What it follows | Length |
|---|---|---|
| Mean solar day | The Sun | 24 hours, or 86,400 seconds |
| Sidereal day | The distant stars | About 23 hours 56 minutes 4 seconds, or 23.9345 hours |
Even the Sun-based day is an average. The Naval Observatory explains that the Sun crosses the sky at slightly different rates through the year, so a true solar day can differ from 24 hours by as much as about half a minute. In late December each solar day runs almost half a minute longer than 24 hours. The clock day of exactly 24 hours is the yearly average. The guide on the equation of time shows how these small daily differences pile up into many minutes over several weeks.
Why the Sun needs a few extra minutes
Earth does not only spin. It also travels around the Sun. NASA's Ask an Astrophysicist team describes the result. By the time Earth has turned once, it has also moved a little along its orbit, so it must keep turning for about another 4 minutes before the Sun seems to be back in the same place it held a day earlier.
The NASA Glenn Research Center turns that idea into numbers. Compared with the Sun, the background of stars goes around once per tropical year, which is 365.2422 days. That is 0.9856 degree per day. Glenn adds it to Earth's 360 degrees per day and gets 360.9856 degrees per day. In other words, one trip from noon to noon needs a bit more than a full turn.
Here is a quick check of the size of the gap. It is a division built from two sourced numbers, not a figure printed by NASA. The Naval Observatory says Earth's rotation carries the Sun across the sky at 15.04 degrees per hour. The extra 0.9856 degree therefore takes 0.9856 divided by 15.04, which is about 0.0655 hour, or roughly 3.9 minutes. That matches NASA Glenn, which says the sidereal day is about 3 minutes 56 seconds shorter than the solar day, and it matches NOAA, which says 3 minutes 55.91 seconds.
The numbers, side by side
Several official pages give the sidereal day. They agree on 23 hours 56 minutes and round the seconds in different ways.
| Source | How it states the sidereal day |
|---|---|
| U.S. Naval Observatory glossary | Approximately 23 hours, 56 minutes, 4 seconds |
| NASA Earth Fact Sheet | 23.9345 hours (sidereal rotation period) |
| NASA Jet Propulsion Laboratory | 86,164.09054 seconds, written as 23:56:04.09054 |
| NASA Glenn Research Center | 23.9345 hours, written as 23 hr 56 min 4.2 sec |
| NOAA solar glossary | 3 minutes 55.91 seconds shorter than 24 hours |
Notice the seconds: 4, 4.09 and 4.2. The last one is simply a rounded conversion of 23.9345 hours. If you subtract NOAA's 3 minutes 55.91 seconds from 24 hours, you get 23 hours 56 minutes 4.09 seconds, which is the JPL value. So the sources do not truly disagree. They just stop at different decimal places.
NOAA's tide glossary adds a fine point. It measures the sidereal day against the vernal equinox, a reference point on the sky, and says it equals about 0.99727 of a mean solar day. Because of the precession of the equinoxes, that is slightly less than a turn measured against the fixed stars, but the difference is less than a hundredth of a second.
What the gap does to the night sky
The few minutes matter after dark. NIST notes that a mean solar day is about four minutes longer than a sidereal day. As a result, a given star appears to rise about four minutes earlier each night, relative to solar time. If a star clears the trees at 9:00 PM tonight, expect it about 8:56 PM tomorrow.
Those minutes pile up. Four minutes a night, repeated for about 365 nights, comes to 1,460 minutes, a little more than 24 hours. NIST adds that different stars are visible at different times of the year. NASA Glenn states the same thing in a count of days: a tropical year of 365.2422 solar days equals 366.2284 sidereal days. Earth turns one more time against the stars than the Sun-based calendar counts. The extra turn is the sum of all those small daily gaps.
For a closer look at the Sun's own daily timing, read the guide on solar noon. For the same two-clock idea applied to the Moon, see the two lunar months.
Common mistakes
- Calling 24 hours Earth's spin time. Earth completes one turn against the stars in about 23 hours 56 minutes. The 24 hours is the time from noon to the next noon, which needs a little extra turning.
- Treating "4 minutes" as exact. NIST says "about four minutes." NOAA gives 3 minutes 55.91 seconds. Use the longer figure when the dial asks for precision.
- Believing every solar day is exactly 24 hours. The 24 hours is an average. The Naval Observatory says real solar days can be up to about half a minute off.
- Mixing up the sidereal day with the lunar day. NOAA's tide glossary gives the mean lunar day as approximately 24.84 solar hours. That is Earth's turn measured against the Moon, a third kind of day.
- Mixing up the sidereal day with the sidereal year. The sidereal year is Earth's trip around the Sun, about 365.256 days in NASA's Earth Fact Sheet. The sidereal day is one spin, about 23.93 hours.
Try it in the instrument
Open the Clock Dial and set it to the length of one sidereal day. The target is about 23 hours 56 minutes 4 seconds. In decimal hours that is 23.9345, the value in NASA's Earth Fact Sheet. The dial is a simplified model with a window of accepted answers, so you do not need the last second, but 24.0 hours is outside the idea.
A good way to land on the answer is to start from the solar day and take away the gap. Begin at 24 hours. Subtract about 3 minutes 56 seconds, the extra turning Earth needs each day because it keeps moving around the Sun. You arrive at 23 hours 56 minutes 4 seconds. If you like decimals, a gap of about 0.0655 hour leaves 23.9345.
Questions people ask
How long is a sidereal day?
About 23 hours 56 minutes 4 seconds, according to the U.S. Naval Observatory. NASA's Earth Fact Sheet lists 23.9345 hours, and NASA's Jet Propulsion Laboratory lists 86,164.09054 seconds.
Why is a sidereal day shorter than a solar day?
Earth moves along its orbit while it spins. To bring the Sun back to the same spot in the sky, Earth must turn a little more than one full turn against the stars. NASA puts the extra turning at about 4 minutes.
Do the stars rise at the same time every night?
No. They rise about four minutes earlier each night by the clock, according to NIST. Over weeks the shift adds up, and NIST notes that different stars are visible at different times of the year.
How many sidereal days are in a year?
NASA Glenn counts 366.2284 sidereal days in a tropical year of 365.2422 solar days. That is one more turn against the stars than the number of Sun-based days.
Is a solar day always 24 hours?
On average, yes. In practice a true solar day can run up to about half a minute longer or shorter, says the Naval Observatory, and the longest ones come near late December. The 24-hour day is the yearly average.
Sources
- Glossary, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/asa_glossary. Opened November 10, 2026.
- The Equation of Time, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/eqtime. Opened November 9, 2026.
- Sunrise and Sunset Times Near the Solstices, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/rs_solstices. Opened November 9, 2026.
- Earth Fact Sheet, NASA Space Science Data Coordinated Archive. https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html. Opened November 9, 2026.
- Planetary Fact Sheet Notes, NASA Space Science Data Coordinated Archive. https://nssdc.gsfc.nasa.gov/planetary/factsheet/planetfact_notes.html. Opened November 9, 2026.
- Telling Time by the Stars - Sidereal Time, NASA Glenn Research Center. https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/telling_time_by_the_stars.htm. Opened November 9, 2026.
- Imagine the Universe!, NASA Goddard Space Flight Center (Ask an Astrophysicist, Earth questions). https://imagine.gsfc.nasa.gov/ask_astro/earth.html. Opened November 9, 2026.
- Astrodynamic Parameters, NASA Jet Propulsion Laboratory, Solar System Dynamics. https://ssd.jpl.nasa.gov/astro_par.html. Opened November 9, 2026.
- Solar Calculator Glossary, NOAA Global Monitoring Laboratory. https://gml.noaa.gov/grad/solcalc/glossary.html. Opened November 10, 2026.
- Time and Frequency from A to Z, S to So, NIST Time and Frequency Division. https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-s-so. Opened November 10, 2026.
- Tides and Currents Glossary, NOAA Tides & Currents. https://tidesandcurrents.noaa.gov/glossary.html. Opened November 10, 2026.
We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.
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