What is sidereal time?

What is sidereal time?

By Christopher Crockett in ASTRONOMY ESSENTIALS 

June 10, 2012

Explanation of sidereal time, or star time. New Year’s celebrations on Mercury. How Venus spins backward and is the slowest rotating planet in the solar system.

Sidereal versus solar day

When Earth is at position 1, it is noon at the red dot. One sidereal day later, Earth has rotated once and moved along its orbit (position 2). For it to be noon again, Earth has to continue rotating for another four minutes (position 3). Credit: Wikipedia

sidereal day measures the rotation of Earth relative to the stars rather than the sun.  It helps astronomers keep time and know where to point their telescopes without worrying about where Earth is in its orbit.

Every 24 hours, the Earth spins once around its axis and the sun loops around the sky.  From noon to noon – or the time it takes the sun to return to its highest point in the sky – is how we define the days of the week.  Astronomers call this a solar day.

But the time it takes for the sun to make one circuit around the sky and the time it takes our planet to complete one rotation is not the same thing.  If you’ve spent your life thinking that 24 hours is how long it takes Earth to rotate, you might be in for a surprise.Sunrise at Srah Srang, Cambodia

Earth turns to face the sun in Cambodia. Credit: chauromano via Fotopedia

In the time it takes the Earth to spin once about its axis, it also moves along its orbit by over 2.5 million kilometers.  Because Earth has moved, the sun will not appear in the same part of the sky at the end of that rotation.  To end up facing the sun again, the Earth has to rotate for another four minutes.

In other words, a solar day is how long it takes Earth to rotate once – and then some.  A sidereal day – 23 hours 56 minutes and 4.1 seconds – is the amount of time needed to complete one rotation.

In this system, the stars always appear at the same place in the sky at the same time each sidereal day.  Sidereal noon is when the vernal equinox – where the sun sits in the sky at the first moment of northern hemisphere spring – passes directly overhead.  The four-minute difference between sidereal and solar days can be seen by watching the stars rise four minutes earlier every night.  If Vega is rising at 9 P.M. tonight, then it will rise at 8:56 P.M. tomorrow, and 8:52 P.M. the following night, and so on.  As Earth travels about the sun, we see each star earlier and earlier.Mercury orbiting the sun

Mercury has to orbit the sun twice from one Mercurian noon to the next. Credit: Wikipedia

Sidereal days are also how astronomers define the rotation periods of other planets.  It helps isolate how quickly the planet is actually spinning from how fast it’s traveling about the sun.  In most cases, like Earth, the difference between a solar day and a sidereal one is pretty small.  But our solar system does have some notable exceptions.

Mercury’s rotation rate is two-thirds of its orbital period: a 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, an inhabitant of Mercury has to wait for about 170 Earth-days from one noon to the next.

But this means that a solar day on Mercury is longer than its year!

One Mercury year is about one-half of a Mercury solar day.  Imagine ringing in the year 2012 at midnight, and then gearing up for the next New Year’s celebration at noon!Venus over the Pacific Ocean

Venus over the Pacific Ocean. Our closest neighbor in space and about the same size and density as Earth. But the sun only rises twice in a Venusian year. Plus Venus spins backward, relative to other worlds in our solar system. Credit: Brocken Inaglory via Wikipedia

Venus is a particularly odd case.  She goes around the sun faster than she spins on her axis: a 225 Earth-day orbit versus 243 to complete one rotation.  This is why Venus is the slowest spinning planet in the solar system.  At Venus’ equator, the planet is spinning at about 6 km/hr while Earth’s equator is hurtling along at nearly 1700 km/hr.

What’s more, Venus does this while spinning backward.  If there were ever to be a break in Venus’ stifling cloud layer, the native Venusians would watch the sunrise in the west and set in the east.

The backward rotation makes Venus the only planet in the solar system where the sidereal day is actually longer than the solar one.   The sun returns to its highest point in the sky before the planet has completed one rotation.

Combining all this together leaves Venus with a solar day that takes 117 Earth-days.  Put another way, the sun only rises twice in a Venusian year.

Sidereal time measures the rotation of our planet relative to the stars.  It allows astronomers to keep time without worrying about the motion of Earth around the sun.  And it reveals some of the quirky motions of our planetary brothers and sisters.  Next time your clock strikes noon, try and imagine what life might be like in a world where the sun moves backward or doesn’t get a chance to set before the year is over.  Turns out, such alien environments are right next door!


Christopher has a Ph.D. in astronomy from the University of California, Los Angeles. After eight years of searching for exoplanets, probing distant galaxies and exploring comets, Chris realized he enjoyed talking about astronomy a lot more than actually doing it. After being awarded a 2013 AAAS Mass Media Fellowship to write for Scientific American, he left a research career at the U.S. Naval Observatory to pursue a new life writing about anything and everything within the local cosmological horizon. Since 2014, he’s been working with Science News.

How Long Is a Tropical Year / Solar Year?

By Vigdis Hocken

The length of a tropical year is the time it takes the Earth to complete a full orbit around the Sun, but it varies from year to year.

Illustration image
A tropical, common, and leap year.
A Year Is Never 365 Days Long

A tropical year, also known as a solar year, an astronomical year, or an equinoctial year, is, on average, approximately 365 days, 5 hours, 48 minutes and 45 seconds long (365.24219 days).

How Long Is a Day on Earth?

A tropical year can be measured from either the vernal or autumnal equinox to the next one, or from the Summer or the winter solstice to the next one.

On timeanddate.com, we calculate a tropical year from the March equinox to the next March equinox (see table below).

Leap Day Synchronizes Calendar

Although a common year has 365 days in today’s Gregorian calendar, we add a leap day nearly every four years to stay in sync with the tropical year.

Is There a Perfect Calendar?

Without the correct amount of leap years, our calendar would quickly become out of sync. This happened to the Julian calendar, which had too many leap years. Eventually, it was replaced with the Gregorian calendar.

Can Vary by 30 Minutes

The exact length of a tropical year can vary by up to around half an hour. For instance, the tropical year 2032 will last longer than 365 days and 6 hours. 2027, however, will only last 365 days, 5 hours, and 39 minutes.

List of all tropical years 1900–2100

Length of Tropical Year 2010–2030

Tropical year measured from/to the March equinox. Accuracy believed to be within a few seconds. Leap seconds are accounted for.

March 2010 – March 201136554823
March 2011 – March 201236555356
March 2012 – March 201336554722
March 2013 – March 201436555514
March 2014 – March 20153655482
March 2015 – March 201636554456
March 2016 – March 201736555836
March 2017 – March 201836554641
March 2018 – March 201936554312
March 2019 – March 20203655514
March 2020 – March 202136554755
March 2021 – March 202236555554
March 2022 – March 202336555055
March 2023 – March 20243655428
March 2024 – March 202536555453
March 2025 – March 202636554439
March 2026 – March 202736553839
March 2027 – March 202836555227
March 2028 – March 202936554457
March 2029 – March 203036554956

Tropical years from 1900 to 2100

Topics: TimekeepingCalendarLeap YearAstronomySeasons


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Writer and Journalist living in Canada since 1987. Tamil activist.

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