A full disk view of the Earth, courtesy of Meteosat-I 1. Credit: ESA/Meteosat
On Earth, a single solar day lasts 24 hours. That is the time it takes for the Sun to return to the same place in the sky as the day before. The Moon, Earth’s only natural satellite, takes about 27 days to complete a single circuit around our planet and orbits at an average distance of 384,399 km (~238,854.5 mi). Since time immemorial, humans have kept track of the Sun, the Moon, and their sidereal and synodic periods. To the best of our knowledge, the orbital mechanics governing the Earth-Moon system have been the same, and we’ve come to take them for granted.
But there was a time when the Moon orbited significantly closer to Earth, and the average day was much shorter than today. According to a recent study by a pair o researchers from China and Germany, an average day lasted about 19 hours for one billion years during the Proterozoic Epoch – a geological period during the Precambrian that lasted from 2.5 billion years to 541 million years ago. This demonstrates that rather than gradually increasing over time (as previously thought), the length of a day on Earth remained constant for an extended period.
A radar image of the surface of Venus showing one of the surface features used to measure a revised value for the length of the Venusian day. Image Credit: Campbell et al., 2019
There’s a problem with Venus. We don’t know how fast it rotates. For a space-faring civilization like ours, that’s a problem.
Measuring the length of day, or rotation rate, of most bodies is pretty straightforward. Mark a prominent surface feature and time how long it takes to rotate 360 degrees. But Venus is blanketed in thick clouds. Those clouds give it its reflectivity, and make it bright and noticeable in the sky, but they make it hard to measure Venus’ day length.
Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry
Mercury is one of the most unusual planets in our Solar System, at least by the standards of us privileged Earthlings. Despite being the closest planet to our Sun, it is not the hottest (that honor goes to Venus). And because of its virtually non-existence atmosphere and slow rotation, temperatures on its surface range from being extremely hot to extremely cold.
Equally unusual is the diurnal cycle on Mercury – i.e. the cycle of day and night. A single year lasts only 88 days on Mercury, but thanks again to its slow rotation, a day lasts twice as long! That means that if you could stand on the surface of Mercury, it would take a staggering 176 Earth days for the Sun to rise, set and rise again to the same place in the sky just once!
Distance and Orbital Period:
Mercury is the closest planet to our Sun, but it also has the most eccentric orbit (0.2056) of any of the Solar Planets. This means that while its average distance (semi-major axis) from the Sun is 57,909,050 km (35,983,015 mi) or 0.387 AUs, this ranges considerably – from 46,001,200 km (2,8583,820 mi) at perihelion (closet) to 69,816,900 km (43,382,210 mi) at aphelion (farthest).
A timelapse of Mercury transiting across the face of the Sun. Credit: NASA
Because of this proximity, Mercury has a rapid orbital period, which varies depending on where it is in its orbit. Naturally, it moves fastest when it is at its closest to the Sun, and slowest when it is farthest. On average, its orbital velocity is 47.362 km/s (29.43 mi/s), which means it takes only 88 days to complete a single orbit of the Sun.
Astronomers used to suspect that Mercury was tidally locked to the Sun, meaning that it always showed the same face to the Sun – similar to how the Moon is tidally locked to the Earth. But radar-Doppler measurements obtained in 1965 demonstrated that Mercury is actually rotating very slowly compared to the Sun.
Sidereal vs. Solar Day:
Based on data obtained by these radar measurements, Mercury is now known to be in 3:2 orbital resonance with the Sun. This means that the planet completes three rotations on its axis for every two orbits it makes around the Sun. At it’s current rotational velocity – 3.026 m/s, or 10.892 km/h (6.77 mph) – it takes Mercury 58.646 days to complete a single rotation on its axis.
While this might lead some to conclude that a single day on Mercury is about 58 Earth days – thus making the length of a day and year correspond to the same 3:2 ratio – this would be inaccurate. Due to its rapid orbital velocity and slow sidereal rotation, a Solar Day on Mercury (the time it takes for the Sun to return to the same place in the sky) is actually 176 days.
In that respect, the ratio of days to years on Mercury is actually 1:2. The only places that are exempt to this day and night cycle are the polar regions. The cratered northern polar region, for example, exists in a state of perpetual shadow. Temperatures in these craters are also cool enough that significant concentrations of water ice can exist in stable form.
For over 20 years, scientists believed that radar-bright images from Mercury’s northern polar regions might indicate the presence of water ice there. In November of 2012, NASA’s MESSENGER probe examined the northern polar region using its neutron spectrometer and laser altimeter and confirmed the presence of both water ice and organic molecules.
View of Mercury’s north pole. based on MESSENGER probe data, showing polar deposits of water ice. Credit: NASA/JHUAPL/Carnegie Institute of Science/NAIC/Arecibo Observatory
Yes, as if Mercury weren’t strange enough, it turns out that a single day on Mercury lasts as long as two years! Just another oddity for a planet that likes to keep things really hot, really cold, and is really eccentric.
A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL
Venus is often referred to as “Earth’s Sister” planet, because of the various things they have in common. For example, both planets reside within our Sun’s habitable zone (aka. “Goldilocks Zone“). In addition, Earth and Venus are also terrestrial planets, meaning they are primarily composed of metals and silicate rock that are differentiated between a metallic core and a silicate mantle and crust.
Beyond that, Earth and Venus could not be more different. And two ways in which they are in stark contrast is the time it takes for the Sun to rise, set, and return to the same place in the sky (i.e. one day). In Earth’s case, this process takes a full 24 hours. But in Venus’ case, its slow rotation and orbit mean that a single day lasts as long as 116.75 Earth days.
Sidereal Vs. Solar:
Naturally, some clarification is necessary when addressing the question of how long a day lasts. For starters, one must distinguish between a sidereal day and a solar day. A sidereal day is the time it takes for a planet to complete a single rotation on its axis. On the other hand, a solar day is the time it takes for the Sun to return to the same place in the sky.
On Earth, a sidereal days last 23 hours 56 minutes and 4.1 seconds, whereas a solar day lasts exactly 24 hours. In Venus’ case, it takes a whopping 243.025 days for the planet to rotate once on its axis – which is the longest rotational period of any planet in the Solar System. In addition, it rotates in the opposite the direction in which it orbits around the Sun (which it takes about 224.7 Earth days to complete).
In other words, Venus has a retrograde rotation, which means that if you could view the planet from above its northern polar region, it would be seen to rotate in a clockwise direction on its axis, and in a counter-clockwise direction around the Sun. It also means that if you could stand on the surface of Venus, the Sun would rise in the west and set in the east.
From all this, one might assume that a single day lasts longer than a year on Venus. But again, the distinction between a sidereal and solar days means that this is not true. Combined with its orbital period, the time it takes for the Sun to return to the same point in the sky works out to 116.75 Earth days, which is little more than a half a Venusian (or Cytherian) year.
At a closest average distance of 41 million km (25,476,219 mi), Venus is the closest planet to Earth. Credit: NASA/JPL/Magellan
Axial Tilt and Temperatures:
Unlike Earth or Mars, Venus has a very low axial tilt – just 2.64° relative to the ecliptic. In fact, it’s axial tilt is the one of the lowest in the Solar System, second only to Mercury (which has an extremely low tilt of 0.03°). Combined with its slow rotational period and dense atmosphere, this results in the planet being effectively isothermal, with virtually no variation in its surface temperature.
In other words, the planet experiences a mean temperature of 735 K (462 °C; 863.6 °F) – the hottest in the Solar System – with very little change between day and night, or between the equator and the poles. In addition, the planet experiences minimal seasonal temperature variation, with the only appreciable variations occurring with altitude.
Weather Patterns:
It is a well-known fact that Venus’ atmosphere is incredibly dense. In fact, the mass of Venus atmosphere is 93 times that of Earth’s, and the air pressure at the surface is estimated to be as high as 92 bar – i.e. 92 times that of Earth’s at sea level. If it were possible for a human being to stand on the surface of Venus, they would be crushed by the atmosphere.
The composition of the atmosphere is extremely toxic, consisting primarily of carbon dioxide (96.5%) with small amounts of nitrogen (3.5%) and traces of other gases – most notably sulfur dioxide. Combined with its density, the composition generates the strongest greenhouse effect of any planet in the Solar System.
According to multiple Earth-based surveys and space missions to Venus, scientists have learned that its weather is rather extreme. The entire atmosphere of the planet circulates around quickly, with winds reaching speeds of up to 85 m/s (300 km/h; 186.4 mph) at the cloud tops, which circle the planet every four to five Earth days.
At this speed, these winds move up to 60 times the speed of the planet’s rotation, whereas Earth’s fastest winds are only 10-20% of the planet’s rotational speed. Spacecraft equipped with ultraviolet imaging instruments are able to observe the cloud motion around Venus, and see how it moves at different layers of the atmosphere. The winds blow in a retrograde direction, and are the fastest near the poles.
Closer to the equator, the wind speeds die down to almost nothing. Because of the thick atmosphere, the winds move much slower as you get close to the surface of Venus, reaching speeds of about 5 km/h. Because it’s so thick, though, the atmosphere is more like water currents than blowing wind at the surface, so it is still capable of blowing dust around and moving small rocks across the surface of Venus.
Venus flybys have also indicated that its dense clouds are capable of producing lightning, much like the clouds on Earth. Their intermittent appearance indicates a pattern associated with weather activity, and the lightning rate is at least half of that on Earth.
Artist concept of the surface of Venus, showing its dense clouds and lightning storms. Credit: NASA
Yes, Venus is a planet of extremes. Extreme heat, extreme weather, and extremely long days! In short, there’s a reason why nobody lives there. But who knows? Given the right kind of technology, and perhaps even some dedicated terraforming efforts, people could one day being watching the Sun rising in the west and setting in the east.
Artist's conception of Mars, with asteroids nearby. Credit: NASA
Mars represents something of a conundrum for scientists. In many respects, it is significantly different from Earth. It’s cold, it’s dry, there is little atmosphere or precipitation to speak of, and nothing grows there. By our standards, that makes it an incredibly inhospitable place. And yet, in many other respects, it is quite similar to our world.
For instance, Mars’ internal structure is differentiated between a metallic core and a silicate mantle and crust. It also has plenty of water, though the majority of it is concentrated in the polar regions as water ice (and as a permanent layer of permafrost under much of the surface). But perhaps most striking of all, a day on Mars is almost the same as a day here on Earth.
In fact, a day on Mars is roughly 40 minutes longer than a day is here on Earth. Compared to other bodies in our Solar System where a day is either incredibly short (Jupiter’s rotates once on its axis every 9 hours, 55 minutes and 29.69 seconds) or incredibly long (a day on Venus lasts for 116 days and 18 hours), this similarity is quite astounding.
However, there are some things that need to be addressed before we go about declaring just how long a day is on another planet. In fact, there are two ways to determine the length of a day on a celestial body, the sidereal day and the solar day; both of which are used by astronomers for determining the passage on time.
Sidereal vs. Solar:
By definition, a sidereal day on Mars is the length of time that it takes the planet to rotate once on its axis so that stars appear in the same place in the night sky. On Earth, this takes exactly 23 hours, 56 minutes and 4.1 seconds. In comparison, on Mars, a sidereal day lasts 24 hours, 37 minutes, and 22 seconds.
The solar day, by contrast, is the time it takes for the Earth to rotate on its axis so that the Sun appears in the same position in the sky. This position changes slightly each day, but on Earth, a mean solar day works out to being 24 hours long. On Mars, a solar day lasts 24 hours, 39 minutes, and 35 seconds. Rounding that out, we say that a day here on Earth works out to an even 24 hours while on Mars, a day lasts 24 hours and 40 minutes.
Want to know about some other interesting similarities Mars has with Earth? Read on!
Seasonal Changes:
Mars also has a seasonal cycle that is similar to that of Earth’s. This is due in part to the fact that Mars also has a tilted axis, which is inclined 25.19° to its orbital plane (compared to Earth’s axial tilt of approx. 23.44°). It’s also due to Mars orbital eccentricity, which mean that it ranges in distance from 206.7 million to249.2 million kilometers from the Sun.
This change in distance causes significant variations in temperature. While the planet’s average temperature is -46 °C (51 °F), this ranges from a low of -143 °C (-225.4 °F) during the winter at the poles to a high of 35 °C (95 °F) during summer and midday at the equator. This high in temperatures is what allows for liquid water to still flow, albeit intermittently, on the surface of Mars.
It also snows on Mars. In 2008, NASA’s Phoenix Landerfound water ice in the polar regions of the planet. This was an expected finding, but scientists were not prepared to observe snow falling from clouds. The snow, combined with soil chemistry experiments, led scientists to believe that the landing site had a wetter and warmer climate in the past.
The eccentricity in Mars’ orbit is what causes some significant variations in temperature. Credit: NASA
And then in 2012, data obtained by the Mars Reconnaissance Orbiter revealed that carbon-dioxide snowfalls occur in the southern polar region of Mars. For decades, scientists have known that carbon-dioxide ice is a permanent part of Mars’ seasonal cycle and exists in the southern polar caps. But this was the first time that such a phenomena was detected, and it remains the only known example of carbon-dioxide snow falling anywhere in our solar system.
For starters, soil samples and orbital observation have demonstrated conclusively that roughly 3.7 billion years ago, the planet had more water on its surface than is currently in the Atlantic Ocean. Similarly, atmospheric studies conducted on the surface and from space have proven that Mars also had a viable atmosphere at that time, one which was slowly stripped away by solar wind.
Weather Patterns:
Like Earth, Mars can have some pretty extreme weather. In the Red Planet’s case, this takes the form of dust storms that can dominated the surface from time to time. These storms have been known to grow to be thousands of kilometers across, occasionally encircling the entire planet and covering everything in a thick haze of dust. When these storms become that large, they prevent direct observation of the Martian surface.
Case in point: when the Mariner 9 orbiter became the first spacecraft to orbit Mars in 1971, it sent pictures back to Earth of a world consumed in haze. The entire planet was covered by a dust storm so massive that only Olympus Mons, the giant Martian volcano that measures 24 km high, could be seen above the clouds. This storm lasted for a full month, and delayed Mariner 9‘s attempts to photograph the planet in detail.
And then on June 9th, 2001, the Hubble Space Telescope spotted a dust storm in the Hellas Basin on Mars. By July, the storm had died down, but then grew again to become the largest storm in 25 years. So big was the storm that amateur astronomers using small telescopes were able to see it from Earth. And the cloud raised the temperature of the frigid Martian atmosphere by a stunning 30° Celsius.
Therein lies another thing Mars has in common with Earth – global warming! Much like warming trends here on Earth, warming on Mars is caused by the presence of particulate matter in the air that absorbs energy from the Sun and radiates it outward into the atmosphere. causing average temperatures to rise.
These storms tend to occur when Mars is closest to the Sun, and are the result of temperatures rising and triggering changes in the air and soil. As the soil dries, it becomes more easily picked up by air currents, which are caused by pressure changes due to increased heat. The dust storms cause temperatures to rise even further, so you could say Mars has a “greenhouse effect” of its own!
As you have probably concluded from all the facts listed above, Mars can be a harsh and volatile planet. Just knowing the answer to ”how long is a day on Mars?” only provides a small glimpse of what is going on there. At the end of the day (no pun intended!) there is plenty happening on Mars that makes it similar enough to Earth that many people are actually contemplating living there someday. And knowing exactly what sets Mars apart, and what we can work with, will be intrinsic to making that happen!
But the story is a little stranger than that. Venus is actually rotating backwards compared to the rest of the planets in the Solar System. Seen from above the north pole, Venus is slowly rotating in a clockwise direction. Compare this to Earth and the rest of the planets, which rotate in a counter-clockwise direction.
And it gets even stranger, when you consider that a year on Venus only lasts 224.7 days. In other words, a day on Venus is actually longer than a year on Venus. If you could actually stand on the surface and see the Sun, you would see the Sun rise in the West, and pass through the sky over the course of 116.75 days and then set in the East. So a solar day on Venus is 116.75 days.
Astronomers aren’t sure why the length of day on Venus takes so long, and why Venus is rotating backwards. It’s possible that Venus was struck by a large object early on in its history, which flipped it over and caused its strange rotation.
We’ve written many articles about the day length of the planets, here’s an article about a day on Mars, and here’s an article about a day on Saturn.
The length of day on Venus is 243 Earth days. Read that again, it’s not a year, but the length of a single day. In fact, a year on Venus is only 224.7 days, so a day on Venus is longer than its year. And things get even stranger. Venus rotates backwards. All of the planets in the Solar System rotate counter-clockwise when you look at them from above. But Venus turns clockwise.
Of course it’s impossible to stand on the surface of Venus and survive. And even if you could, you wouldn’t be able to see the Sun through the dense clouds. But if you could stand on Venus and see the Sun, you’d see the Sun rise in the West, pass through the sky for 116.75 days and then set in the East. That’s the opposite of what we see here on Earth.
Why does Venus rotate backwards? Astronomers aren’t sure, but it’s possible that Venus suffered a massive impact from a large planetoid billions of years ago. This could have given the planet a kick that set it slowly tumbling, eventually flipping completely over so that it’s now upside-down.
