Что такое length of the day

How Long Is One Day on Other Planets?

Mercury1,408 hoursVenus5,832 hoursEarth24 hoursMars25 hoursJupiter10 hoursSaturn11 hoursUranus17 hoursNeptune16 hours

When you think of a day, you normally think of one cycle of daytime to nighttime. That is called a solar day. On Earth, a solar day is around 24 hours. However, Earth’s orbit is elliptical, meaning it’s not a perfect circle. That means some solar days on Earth are a few minutes longer than 24 hours and some are a few minutes shorter.

Another way to measure a day is to count the amount of time it takes for a planet to completely spin around and make one full rotation. This is called a sidereal day. On Earth, a sidereal day is almost exactly 23 hours and 56 minutes.

We know how long an Earth day is, but how about the other planets in our solar system? How long does it take for those planets to spin one full rotation? And what is the best way to show the answer to this question?

Let’s look at a few options.

Option 1: A Paragraph

We can write a paragraph about how long days last on other planets.

On Mercury a day lasts 1,408 hours, and on Venus it lasts 5,832 hours. On Earth and Mars it’s very similar. Earth takes 24 hours to complete one spin, and Mars takes 25 hours. The gas giants rotate really fast. Jupiter takes just 10 hours to complete one rotation. Saturn takes 11 hours, Uranus takes 17 hours, and Neptune takes 16 hours.

Reading that paragraph took a while, and it’s hard to find all the numbers. Let’s see how it looks if we put it in a table.

Option 2: A Table

Mercury1,408 hoursVenus5,832 hoursEarth24 hoursMars25 hoursJupiter10 hoursSaturn11 hoursUranus17 hoursNeptune16 hours

That’s a little bit better. We can look up and down at the numbers and can compare them more easily. But wouldn’t it be nice if we could see how big those differences are?

Let’s make a graph with these numbers!

Option 3: A Graph

To start, make a number line that starts at 0 and goes to the highest number you need to include. This first graph will only have Earth, Mars, Jupiter, Saturn, Uranus, and Neptune on it. We’ll save Mercury and Venus for later. You’ll see why in a minute.

The longest day among those planets is 25 hours. That means our number line will go up to 25.

Label the number line so you remember it represents hours. And write what information the graph will have at the top.

Along the bottom, write the names of the planets.

Make a dot above the name of the planet next to the number of hours its day takes.

Our graph is coming together!

What do we do now? Color in the area below the dot to make a bar graph.

Now we can easily see which planet has the longest day, the shortest day, and everything in between. This is much easier than reading a list of numbers, don’t you think?

But what do we do about Mercury and Venus? Their days are thousands of hours long. How do we make a graph for those?

We make a number line, but instead of numbers 1, 2, 3… we will use 1,000, 2,000, 3,000…

Make a number line that goes all the way up to 6,000. Along the bottom, write Mercury and Venus. Above Mercury, mark a dot a little less than halfway between 1,000 and 2,000. It’s not going to be perfect, but that’s okay. Now make a dot a little under 6,000 for Venus.

Do you think we can include Earth, Mars, Jupiter, Saturn, Uranus, and Neptune on here? Their dots would be so close to 0 it would be hard to tell the difference between them. Graphs work best when the numbers are similar in size, usually with the same number of digits.

Now that you know how to make a graph, you can show all kinds of information this way. You can graph the time it takes to get to school each day, the number of pieces of pizza your friends can eat, and how many people like the color blue or green. Go on and get graphing!

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A Day Is Not Exactly 24 Hours

Exact Day Length* — вск 26 дек 2021 г.

Today’s prediction: 24 hours, 0 minutes, 0,0004985 seconds (0,4985 milliseconds)

Yesterday’s prediction: 24 hours, 0 minutes, 0,0003040 seconds (0,3040 milliseconds)

At the start of today, UT1 was 0.1076900 seconds behind UTC.

* Based on mean solar day. Numbers provided by the International Earth Rotation and Reference System Service (IERS).

The Earth’s rotation slows down over time.

Earth’s Rotation Defines Length of Day

Modern timekeeping defines a day as the sum of 24 hours—but that is not entirely correct. The Earth’s rotation is not constant, so in terms of solar time, most days are a little longer or shorter than that.

The Moon is—very gradually—slowing the Earth’s rotation because of friction produced by tides. Over the course of a century, the length of a day increases by a couple of milliseconds (where 1 millisecond equals 0.001 seconds).

Within this general trend, however, there is fluctuation: sometimes the Earth spins a bit faster, sometimes a bit slower. Recently, our planet has been speeding up a little, making for slightly shorter days.

How Long Is Today?

Today is predicted to be 0,4985 ms (milliseconds) or 0,0004985 seconds longer than 24 hours. This is the time it takes Earth to rotate 23,19 cm (9,13 in), as measured at the equator.

This means that today lasts:

If every day were as long as today, a leap second would have to be added every 2006,02 days.

Average Day Lengths & Leap Seconds

Overall, the Earth is a good timekeeper: the length of a day is consistently within a few milliseconds of 86,400 seconds, which is equivalent to 24 hours. However, over the course of months and years, these small differences can add up and put our clocks out of sync with the Earth’s spin. When this happens, a leap second is used to bring them back into alignment.

Leap seconds can be positive or negative. A positive leap second adds a second to our clocks, while a negative leap second subtracts a second.

The system of leap seconds was introduced in 1972. So far, there have been 27 leap seconds, and they have all been positive. The table below shows the yearly average day lengths since 1973.

How Is True Day Length Measured?

Astronomers and timekeepers express mean solar time as Universal Time (UT1), a time standard based on the average speed of the Earth’s rotation. UT1 is then compared to International Atomic Time (TAI), a super-precise time scale calculated by a network of atomic clocks.

The actual length of a day is expressed as the deviation of UT1 from TAI over 24 hours.

Why Isn’t Earth’s Rotation Constant?

The speed of the Earth’s rotation varies from day to day. One of the main factors are the celestial bodies surrounding us.

For example, the Moon’s gravitational pull causes tides and changes the Earth’s shape, ultimately resulting in a lower rotational speed. The distance between Earth and Moon changes constantly, which makes for daily variations in the speed our planet rotates around its axis.

Find Day Length for Any Date

How Far Back Does the Data Go?

Super-accurate atomic clocks were first developed in the 1950s and 1960s. So measurements of the Earth’s rotation using atomic clocks only go back as far as then.

However, telescopic timings of stellar occultations by the Moon provide information about the Earth’s rotation going back to the 17th century. An occultation is when the Moon, as seen from the Earth, passes in front of a star.

Ancient Records Give Away Earth’s Speed

Going back even further, records of solar and lunar eclipses provide information from the 8th century BCE onwards.

For example, a Babylonian clay tablet tells us that a total solar eclipse was observable in the ancient city of Babylon on April 15, 136 BCE.

Modern computer models can calculate the path of totality for this eclipse with a high degree of accuracy. From this, we can work out the Earth’s spin. For instance, if the Earth had been spinning a bit faster at that time, the path of totality would have passed to the west of Babylon—not directly over the city.

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Day length

Day length, or length of day, or length of daytime, refers to the time each day from the moment the upper limb of the sun’s disk appears above the horizon during sunrise to the moment when the upper limb disappears below the horizon during sunset. Due to the diffusion and refraction of sunlight by the atmosphere, there is actually daylight even when the sun is slightly below the horizon. The period when it is still somewhat light even though the sun is below the horizon is called twilight.

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In general, the length of a day varies throughout the year, and depends upon latitude. This variation is caused by the tilt of the Earth’s axis of rotation with respect to the ecliptic plane of the earth around the sun. At the solstice occurring about June 20–22, the north pole is tilted toward the sun, and therefore the northern hemisphere has days ranging in duration from just over 12 hours in the southern portion of the Tropic of Cancer to 24 hours in the Arctic Circle, while the southern hemisphere has days ranging in duration from just under 12 hours in the northern portion of the Tropic of Capricorn to zero in the Antarctic Circle. At the equinox occurring about September 22–23, the poles are neither tilted toward nor away from the sun, and the duration of a day is generally about 12 hours all over the earth. At the solstice occurring about December 20–22, the south pole is tilted toward the sun, and therefore the southern hemisphere has days ranging in duration from just over 12 hours in the northern portion of the Tropic of Capricorn to 24 hours in the Antarctic Circle, whereas the northern hemisphere has days ranging in duration from just under 12 hours in the southern portion of the Tropic of Cancer to zero in the Arctic Circle. At the equinox occurring about March 19–21, the poles are again aligned so that the duration of a day is generally about 12 hours all over the earth.

In each hemisphere, the higher the latitude, the shorter the day during winter. Between winter and summer solstice, the day’s duration increases, and the rate of increase is larger the higher the latitude. A fast increase of day length is what allows a very short day on winter solstice at 60 degrees latitude (either north or south) to reach about 12 hours by the spring equinox, while a slower increase is required for a much longer day on winter solstice at 20 degrees latitude (again, either north or south) to reach 12 hours by the spring equinox. The rate of change of day duration is generally fastest at the equinoxes, although at high latitudes the change is similar for several weeks before and after the equinoxes. The rate of change of day duration at each solstice is zero as the change goes from positive to negative, or vice versa.

Some interesting facts are as follows:

Alternate definition

More conveniently, atmospheric refraction is ignored and the center of the sun is often used in place of the upper limb for computing a day’s duration. When sunrise and sunset do occur, the day duration can be computed as 2ω_o/15°, where ω_o is the sunset hour angle in degrees (°) given by the sunset equation. When sunrise and sunset do not occur during the course of a day, the day duration is either 0 or 24 hours.

To considerable accuracy, all the points at the same latitude on the same calendar date can be considered to have the same day length. The contour plot in the figure is computed using the sunrise equation.

With this definition the following features can be observed:

Historical variation of day length because of tidal acceleration

The day length was less in the past. About 320 million years ago there were 400 days in the year.

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How Long Is a Day on Other Planets?

The Earth is the only planet with an approximately 24-hour day

The definition of a day is the amount of time it takes an astronomical object to complete one full spin on its axis. On Earth, a day is 23 hours and 56 minutes, but other planets and bodies rotate at different rates. The Moon, for example, spins on its axis once every 29.5 days. That means future lunar inhabitants will have to get used to a sunlight «day» that lasts for about 14 Earth days and a «night» that lasts about the same time.

Scientists typically measure days on other planets and astronomical objects in reference to Earth’s day. This standard is applied across the solar system to avoid confusion when discussing events that occur on those worlds. However, each celestial body’s day is a different length, whether it’s a planet, moon, or asteroid. If it turns on its axis, it has a «day and night» cycle.

The following table depicts the day lengths of the planets in the solar system.

Planet	Length of Day
Mercury	58.6 Earth days
Venus	243 Earth days
Earth	23 hours, 56 minutes
Mars	24 hours, 37 minutes
Jupiter	9 hours, 55 minutes
Saturn	10 hours, 33 minutes
Uranus	17 hours, 14 minutes
Neptune	15 hours, 57 minutes
Pluto	6.4 Earth days

Mercury

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NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Wikimedia Commons/Public Domain

The planet Mercury takes 58.6 Earth days to spin once on its axis. That may seem long, but think about this: its year is only 88 Earth days long! That’s because it orbits very close to the Sun.

There’s a twist, however. Mercury is gravitationally locked with the Sun in such a way that it rotates three times on its axis for every two times it goes around the Sun. If people could live on Mercury, they’d experience one full day (sunrise to sunrise) every two Mercurian years.

Venus

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Kevin Gill/Flickr/CC BY 2.0

Planet Venus spins so slowly on its axis that one day on the planet lasts nearly 243 Earth days. Because it’s closer to the Sun than Earth is, the planet has a 225-day year. So, the day is actually longer than a year, which means that Venus residents would only get to see two sunrises per year. One more fact to remember: Venus spins «backward» on its axis compared to Earth, which means those two yearly sunrises take place in the west and sunsets occur in the east.

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At 24 hours and 37 minutes, the Mars day length is very similar to Earth’s, which is one of the reasons that Mars is often thought of as something of a twin to Earth. Because Mars is farther than Earth from the Sun, however, its year is longer than Earth’s at 687 Earth days.

Jupiter

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When it comes to gas giant worlds, «day length» is a more difficult thing to determine. The outer worlds don’t have solid surfaces, although they do have solid cores covered with huge layers of clouds and layers of liquid metallic hydrogen and helium beneath the clouds. On the gas giant planet Jupiter, the equatorial region of the cloud belts rotates at a rate of nine hours and 56 minutes, while the poles rotate quite a bit faster, at nine hours and 50 minutes. The «canonical» (that is, commonly accepted) day length on Jupiter is determined by the rotation rate of its magnetic field, which is nine hours, 55 minutes long.

Saturn

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NASA / JPL / Space Science Institute/Wikimedia Commons/Public Domain

Based on measurements of various parts of gas giant Saturn (including its cloud layers and magnetic field) by the Cassini spacecraft, planetary scientists determined that the official length of Saturn’s day is ten hours and 33 minutes.

Uranus

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Orange-kun (old version user: Brian0918)/Wikimedia Commons/Public Domain

Uranus is a weird world in many ways. The most unusual thing about Uranus is that it’s tipped over on its side, and «rolls» around the Sun on its side. That means one axis or the other is pointed at the Sun during part of its 84-year orbit. The planet does rotate on its axis once every 17 hours and 14 minutes. The length of day and the length of the Uranian year and the weird axial tilt all combine to create a day that’s as long as a season on this planet.

Neptune

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Kevin Gill from Los Angeles, CA, United States/Wikimedia Commons/CC BY 2.0

The gas giant planet Neptune has a day length of approximately 15 hours. It took scientists a number of years to calculate the rotation rate of this gas giant. They accomplished the task by studying images of the planet as features rotated around in its atmosphere. No spacecraft has visited Neptune since Voyager 2 in 1989, so Neptune’s day must be studied from the ground.

Pluto

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NASA/JHUAPL/SwRI/Wikimedia Commons/Public Domain

Dwarf planet Pluto has the longest year of all the known planets (so far), at 248 years. Its day is a lot shorter, but still longer than Earth’s, at six Earth days and 9.5 hours. Pluto is tipped over on its side at an angle of 122 degrees with respect to the Sun. As a result, during part of its year, portions of Pluto’s surface are either in continuous daylight or constant night-time.

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How does the length of the day vary from one day to the next throughout the year?

January 11, 2011

By Nick Lomb

The change from one day to the next in the length of daylight durind 2011. Graph and calculation Nick Lomb

The length of the day is the interval between sunrise and sunset. We have discussed in a previous post how that varies during the year. Briefly, days are longest at the time of the summer solstice in December and the shortest at the winter solstice in June. At the two equinoxes in March and September the length of the day is about 12 hours, a mean value for the year.

Let us now ask what is the change in the day length from day-to-day? You may have noticed that the time of sunset since 2 January has not changed so far and will not change for a few more days until Friday 14 January 2011. The time of sunrise has been becoming later by about a minute each day so that the length of the day has been shortening accordingly.

The diagram above makes things clearer. The change from one day to the next at the time of the summer solstice in late December is around zero. Similarly, there is little change from day-to-day at the time of the winter solstice in June. This is not surprising as solstice means ‘the day the Sun stood still’, so at those two times of the year we expect little change from day-to-day.

There is more change at the time of the equinoxes – autumn and spring – in March and September respectively. At those times the changes day-to-day can be up to three minutes.

The curve above is surprisingly messy and not the smooth curve that would be expected. The probable explanation is that it is due to the rounding to the nearest minute in the calculation of sunrise and set times. If they were calculated to the nearest second then the curves calculated from them would be smoother. However, there is no point in giving sunrise and set times to any higher precision than a minute as atmospheric conditions each day can make the actual times vary slightly from the calculated times.

33 responses to “ How does the length of the day vary from one day to the next throughout the year? ”

You wrote: “Briefly, days are longest at the time of the summer solstice in December and the shortest at the winter solstice in June.” This appears to be a typo – error: You should exchange December and June to correct it.

Sandy, thank you for noticing this. However, in the southern hemisphere, where Sydney Observatory is located, the statement is correct.

Hi Andrew, it looks as though at this time of year sunrise is slightly earlier each day but sunset is later at a greater rate. Is this correct? If so, why is this?

Peter, Yes that is correct. It is due to the “equation of time” effect. This is explained in the links noted in my answer below, dated ‘Jan 8, 2019’.

Where I live our earliest sunrise this year is 5:11. Does this mean every year our earliest sunrise is at 5:11 or does it vary from year to year?

Adam, Yes, it will be the same each year, to within a minute or so. Only over thousands of years will it vary as Earth moves wit the Milankovitch cycles.

How many more minutes daytime in Hereford UK have my wife and I gained today since the solstice on 21st Dec 2019. Approx answer PLEASE

Peter, this is easy to estimate using the website time and date. Enter your location and navigate to the ‘Sunrise & Sunset’ page. There you will find tabulated the ‘daylength’. For your location – and as an example for anyone else to follow – the daylength on Jan 15 2020 was 8:15:12 [h:m:s] and on December 21 2019 daylength was 7:44:01. So you have gained 31min 11sec of daytime. In fact, it is only reasonable to estimate this time to the nearest minute because the actual time of sunrise is dependent on atmospheric refraction, your height above sea level and other effects. Also the the daytime gained is independent of the year, so will be the same next year.

Is the amount of change in daylight even during a yearly cycle? It would seem to me that it would be even as the axis change alters at an even pace but charts make it look like a bell curve.

Cathy, Maybe I have misunderstood your question but the post makes clear that at the solstices the change in day-length is almost zero around the solstices and up to 3-minutes around the equinoxes.

Why The change in day length each day is more on higher lattitudes than at equator or tropics. I did not get satisfactory answer still by anyone

Ajay, I can answer this by considering the extremes of daytime length, i.e. sunrise to sunset. At the poles the daytime length varies from 0 to 24 hours per 24-hour day. In the tropics the daytime length varies by much less, for example at Rockhampton in Queensland, just on the Tropic of Capricorn it varies only from 10h42m up to 13h35m, a 2h53m diference (according to timeanddate.com). Since these changes in daytime length at both locations occur over exactly the same number of days during the year then at the poles the daily change in day length has to be greater there.
This is certainly the case around the equinoxes. However, around the times of the solstices the daily change in daytime length is close to zero for a few days everywhere.

How do sunrise and sunset and twilight change from year to year? i’m interested in times in far north queensland in 1893. Are they basically the same every year? Would it be safe to say that the sun rose about 7 and set about 6 (18.00) then? Would twilight have been about the same? To be precise, the subjects of my article left Croydon in Qld on 16/7/1893 and cycled to Sydney, arriving 15/9/1893. They camped out a lot. They often started cycling about 6 a.m. Would they have been travelling in the dark? Would there be any sunlight?

Rosemary, Yes sunset, sunrise and twilight times remain basically the same, on the same calendar date, from year to year. There is a small cyclical shift of around half a minute that resets every four years, i.e. with the leap years, but that is insignificant in this context. However, over such a great distance (Croydon to Sydney) and time (July to September) the rise, set and twilight times may have been very different, you couldn’t say it rose at 7am every day of their journey. You could use an astronomy app or even time and date to determine if it was light or not each day when they started cycling. One imagines they would only have cycled when it was light enough. You might want to check the phases of the Moon too – maybe they could start earlier (or finish later) on some days if a nearly-full Moon was up.

(If you’re still watching)

I can’t attach my graph (it IS a smooth sine wave) but the NOAA spreadsheet is at:

that sounds great! I think I will try this for in the uk. I was wondering if u could answer this or not but I am finding this website is not giving me my direct answer so may I ask u a question and get a reply/ answer? my question is why do the times when it is light or dark vary in length? it would be much needed if I could have a reply today however if not please do it at another possible time that is near this day of my reply. greatest thanks from me. sorry if I spelt anything wrong but I was in a rush.

user, You can use this TimeAndDate page to see how day and night length vary during the year (try jumping through the year in “+4 weeks” intervals). It is caused by the Earth being tilted, which leads to the seasonal changes in day-length and the change from winter to summer.

Mike, No, the gains (or losses) are not split equally. Once again this is due to the Equation of Time. You can use the spreadsheet in Ken’s comment below to investigate this.

Can you help me understand how it is that around winter solstice, the change in sunrise time is less than the change in sunset time in the northern hemisphere? I fail to see how the changes would not be identical

David, this is caused by the effect of the Equation of Time. This defines the time when the Sun is due north, or equivalently the time of day exactly half way between sunrise and sunset. Read about the Equation of Time and the changes in sunrise/set times.

The amount of daylight change day to day is the product of the earth’s poles’ tilt with respect to the sun— the same thing that gives us seasons. The best visual aid I can think of is a swinging pendulum. When it changes direction at each side analogous to the winter and summer solstices) the speed is lowest. But it picks up speed until it gets to the bottom of the swing (analogous to the vernal and fall equinoxes it is fastest). As the pendulum approaches the furthest point right or left it slows essentially to a stop when it changes direction. These ate cyclical motions.

Winter solistic is in Dec and summer solistic is in june
Spring equinox is in march and autumn equinox is in September.
Some how article above is call ing it other way round??

Nidhi, Sydney Observatory is in the southern hemisphere. Down here things are the right way around 😉

Depends on whether you’re in the northern or southern hemisphere.

I’m just wondering how day length occurs.

Andrew, I’m not quite sure what you are asking. As the post says a day (as opposed to a night) is the time between sunrise and sunset. Actually that’s a little ambiguous because we also define a ‘day’ as the time taken for Earth to rotate on its axis once. But you can find that definition anywhere, so have I misunderstood your question?

It occurs from sunrise to sunset and I also think that the Earth’s movement could be faster or slower.

Thank you for help me to understand it.

I have a basic grasp of the content above.

I don’t understand why the percentage of rate (day light) increases or decreases through out the year. Why isn’t it some sort of constant?
Speed of the planet in an elliptical orbit combined with the tile of the earth?

I live near the 47 parallel and the change is quite apparent over time. In watching the astronomic date on my weather channel I note that the daylight comes and goes at this different percentage at different times.

I’d appreciate some direction as to where to look next. I ask that you keep the vocabulary simple in that I have no degrees in this, simply a curiosity,
thank-you,
terry

> You have to remember that the length of day light varies due to the 23.5 degree tilt that the Earth rotates on. Day length would always be equal if the Earth wasn’t tilted, but because it is, as the Earth travels around the sun, different parts of the planet will be tilted further towards (Summer time, which results in more daylight hours) or further away (Winter time, which results in more night time hours).
Speed of the planet is irrelevant here.
The picture on this may help: http://www.lpi.usra.edu/education/skytellers/day_night/about.shtml
However, it is contextualised towards America, so wherever you see a season, swap it in your mind for the opposite season.

We could look at this mathematically: this post displays the change-in-length-of-day which is the gradient or slope of the length-of-day plot shown in the previous post mentioned. The length-of-day plot is ‘cosinusoidal’ therefore its gradient plot is sinusoidal.

Thanks for helping me understand what was right in front of me all the time.

In being an elliptical orbit around the sun, the velocity is not at a constant rate. Therefore what I’m viewing in the rate of change is an effect driven by this difference.
terry

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