Planetary Time: How long is a day on each planet and Why?
Last Updated: November 9, 2023
What is a day? While this may seem like an obvious question with an obvious answer, it gets a little complicated when we begin to consider anywhere other than Earth.
Let’s discuss the planetary mechanics that make what we see as a day, explore what a day is like on each of the other planets of our solar system, and discuss some of the reasons behind the current day lengths and the effects of longer or shorter days on planets.
Definitions of “Day” and Planetary Mechanics
We learn the concept of day and night pretty early in life. A new day starts when the sun rises and when it sets, the night takes over until the sun rises again the next morning bringing a new day. We use these demarcations to schedule activities like meals and work.
We even learn quickly that a day is 24 hours. This doesn’t mean that the sun rises or sets at the 24-hour or 0:00 mark. Instead, the rising and setting of the sun occur within the 24-hour period with moments of day, dusk, and night. Every location on Earth receives an average of about 12 hours of sunlight every day, but the amount varies based on location and time of year.
We see the sun rising in the Eastern horizon because the Earth is turning on its axis. Think of a globe with a stick going through its center so that the stick comes out of each of the poles, marking the axis. If you twirl the stick between your fingers, the globe spins around.
As your particular point on the Earth spins, it travels from facing the Sun to facing away from it, meaning it does a full rotation (sunrise to sunrise) in about 23.9 hours, rounded to 24. The tilt of our axis is also very important at about 23.5 degrees because it means that some of the planet will be facing the Sun more directly than others at certain times of the year, creating seasons.
Locations at the equator always receive 12 hours of daylight but the locations above the Arctic and Antarctic circles (66.5 degrees latitude) actually experience 24 hours of daylight during their respective summer and 24 hours of night during their respective winter for a couple of months.
It is important to remember a few points when comparing Earth’s days to the days on other planets. The distance around Earth’s equator (equatorial circumference) is 24,873.6 miles (40,030.2 km). Its rotational speed is determined by how long it takes for a point to travel the full circumference back to its starting point.
Therefore, both its speed and its size determine its rotational period or its day. In addition, Earth is also circling the Sun about 93 million miles away or 1 astronomical unit (AU), taking about 365.25 days which is known as a year.
Days on Other Planets
So, now that we understand why our day is 24 hours long, is that rotational speed consistent with other planets in our solar system? Let’s find out!
Mercurian days
Mercury is small with an equatorial circumference of 9,525.1 miles (15,329.1 km), a little more than 1/3 the width of Earth. It is the closest planet to our Sun at an average distance of 36 million miles (58 million kilometers or 0.4 AU). Mercury’s axis is tilted just 2 degrees, meaning little to no seasonal variations.
Compared to Earth’s fairly circular orbit, Mercury’s orbit is highly eccentric and egg-shaped, meaning although its average orbit is 36 million miles from the Sun, it is 29 million miles (47 million kilometers) away at its closest point in the orbit and 43 million miles (70 million kilometers) at its furthest. Zipping around the Sun every 88 days, the planet spins slowly on its axis, completing a rotation every 59 Earth days.
However, when Mercury is moving the fastest in its orbit and is closest to the Sun, the Sun’s movements get a little wonky and each rotation is not accompanied by sunrise and sunset. The morning Sun appears to rise briefly, set, and rise again from parts of the planet’s surface, and the same happens in reverse at sunset for other parts of the surface.
Due to these weird mechanics, one full-day night cycle or solar day actually equals 176 Earth days, which at 88 days per orbit around the Sun means it is technically over 2 years on Mercury.
Summary:
- 1 Rotation of Mercury’s axis: about 59 Earth days
- 1 Solar Day (full day-night cycle): about 176 Earth days
- 1 Year: 88 days
Venusian days
Venus is approximately as big around as Earth at 23,627.4 miles (38,024.6 kilometers) in equatorial circumference, versus 24,873.6 miles (40,030.2 kilometers) for Earth, and closer to the sun at an average of 67,238,251 miles (108,209,475 km), but rotates slowly on its axis. Venus takes 243 Earth days to rotate on its axis, but only 225 Earth days to orbit around the Sun, meaning its day is longer than its year. In fact, sunrise to sunset would take 117 Earth days.
In addition to its slow rotation, Venus spins backward compared to Earth meaning the Sun rises in the West and sets in the East. This is why you may see the rotational period or day designated with a negative value. Venus’s axial tilt is very small at only about 3 degrees, meaning it does experience seasons, but they are much milder, less drastic changes than for Earth, which are then lessened even more by its runaway greenhouse effect from the dense, acidic atmosphere making it the hottest planet in our solar system.
Summary:
- 1 Day: 243 Earth days
- 1 Year: 225 Earth days
Martian days (sols)
Mars is about half the size of Earth, but completes a full rotation in 24.6 hours. With an average distance from the Sun of 142 million miles (228 million kilometers), Mars is 1.5 AU from the Sun, and takes 687 Earth days or 669.6 Mars days called sols to complete its orbit.
However, it is worth noting that Mars’s orbit is one of the most eccentric in the solar system with its distance from the Sun varying between 1.38 and 1.67 AU throughout its orbit around the Sun. Its axial tilt is slightly greater than Earth, creating more extreme seasonal changes than we see here on Earth.
Summary:
- 1 Day: 24.6 hours
- 1 Year: 687 Earth days (669.6 sols/ Mars days)
Jovian days
Does Jupiter’s massiveness also create longer days? Does its 5.1 AU distance from the Sun create longer years?
Actually, Jupiter rotates very quickly with a day of only 9.93 hours, and takes 11.86 Earth years to complete one orbit around the sun (1 Jupiter year). Jupiter’s axis is tilted at only 3 degrees (like Venus) so its change in day and night length as well as the season as it moves around the year are much less extreme than Earth’s.
Summary:
- 1 Day: 9.93 hours
- 1 Year: 11.86 Earth years
Saturnian days
Saturn is about 9 times wider than Earth with an equatorial circumference of 227,348.8 miles (365,882.4 kilometers) and orbits at an average distance of 886 million miles (1.4 billion kilometers) or over 9 AU.
However, similar to Jupiter, Saturn rotates extremely quickly, taking only 10.7 hours, and has an understandably longer year for its larger orbit at 29 Earth years. Saturn’s axial tilt is almost 27 degrees, so slightly larger than Mars’s.
Summary:
- Day: 10.7 hours
- Year: 29 Earth years
Uranian days
Uranus rotates at nearly 97 degrees from its plane of orbit meaning it appears to spin sideways, orbiting the Sun like a rolling ball as opposed to the other planets which act more like spinning tops with various degrees of tilt.
This extreme tilt means that when the north pole is pointed at the Sun during the north polar summer, the south pole is in complete darkness, and vice versa for the north polar winter. When its axis is perpendicular to the light from the sun (a.k.a. spring and fall), it experiences full day-night cycles.
With an equatorial circumference of 99,018.1 miles (159,354.1 kilometers), Uranus is 4 times wider than Earth and an average of 1.8 billion miles (2.9 billion kilometers) away from the Sun (about 19.8 AU). One day on Uranus or the time it takes for a full rotation is a little over 17 hours and it takes about 84 Earth years to orbit the Sun once.
Similar to Venus, Uranus also rotates “backwards” with the Sun rising in the West and setting in the East, and the rotational period or day may be designated as a negative value.
Summary:
- Day: 17 hours, 14 minutes
- Year: 84 Earth years
Neptunian days
The furthest planet from the Sun is also about four times wider than Earth, similar to its twin Uranus, but a little smaller and slower. It has an equatorial circumference of 96,129 miles (154,704.6 kilometers), and an average distance of 2.8 billion miles (4.5 billion kilometers) or 30 AU from the Sun. In comparison, sunlight takes 4 hours to arrive at Neptune versus 8 minutes to arrive on Earth. Check out our previous article to learn more about the speed of light.
Neptune takes about 16 hours to complete a full rotation around its axis and about 60,193 Earth days or 165 Earth years to complete an orbit around the Sun (1 year on Neptune). Neptune’s axis is tilted at 28 degrees, but its extreme distance from the Sun means seasonal variation is relatively low.
Summary:
- Day: 16 hours
- Year: 165 Earth years
Length of days rankings
How do they rank up? In order from the shortest day to the longest, we have:
- Jupiter at 9.93 hours or 9 hours and 55.8 minutes
- Saturn at 10.7 hours or 10 hours and 42 minutes
- Neptune at 16 hours
- Uranus at 17.23 hours or 17 hours, 14 minutes
- Earth at about 24 hours
- Mars at 24.6 hours
- Mercury at about 59 Earth days to turn once on its axis and about 176 Earth days for 1 full day-night cycle
- Venus at 243 Earth days
Reasons Behind and Effects of Different Planetary Mechanics
The size of the planet as well as the distance to and around the Sun both affect the rotational and orbital period, but why do some planets spin faster than others? What effects does this speed have on the planet?
Explanations for spin speed
While the reasons behind why some planets rotate faster still puzzle planetary scientists, there are many factors that are known to affect this.
The speed at which a planet spins around its axis is primarily determined by two factors during the early solar system when the planets were forming. The more mass a developing planet had, the faster it would spin, which is why the outer planets spin faster than the inner planets. Second, the material that was forming into various planets in the early solar system was moving at different speeds and in different ways than others. The planet’s evolution from its early formation to today could also have some effect on the rotational spin. For instance, we know that Uranus’s axis is at a severe tilt, likely due to a collision that could have also impacted its spin as well.
The reality is that we are still learning about how early star and planetary systems form. While our understanding of this process and our animations have improved over the decades, we still don’t understand everything. In addition, it can be hard to reconstruct exactly what happened when much of the planets’ histories happened long before humans developed and we don’t have enough evidence to figure out everything since we are still collecting evidence about each planet and different processes have erased much of that evidence over the billions of years.
It is also important to note that a planet’s spin is not necessarily constant over its life. For instance, Earth’s spin has been slowing down at a rate of approximately 2 seconds every 100,000 years based on fossil coral records that show a decrease in the number of daily cycles in a year between the Devonian Period (380 million years ago) and the Pennsylvanian Period (290 million years ago) implying that a day was about 21.8 hours long in the former and about 22.4 hours in the latter. While we are not certain, current research points to a “drag” on the Earth from tidal cycles caused by the gravitational pull of the moon.
Effects of the rotational spin on the planet
There are a number of effects on a planet from the speed of its rotation including weather, shape, and surface due to a number of different factors. The number of hours in the sunlight versus out of it in combination with the distance from the Sun, the eccentricity of the orbit, and the season due to axial tilt and time of year determines how hot the planet’s surface (or upper layers for gas giants) can become and how drastic the change in temperature from day to night can be.
In addition, on planets with a high speed of rotation and an atmosphere, these gases can be whipped around the planet, up to monumental speeds in contrast to planets with lower rotational speeds. Here on Earth, the highest gusts ever recorded were about 300 mph (483 km/h) in a tornado. Venus’s slower spin in addition to the thicker atmosphere means the highest recorded wind speeds are about 250 mph (402 km/h). In contrast, Mars’s winds rarely exceed about 60 mph (97 km/h) due in part to its very thin atmosphere.
As the fastest spinning planet in our solar system, Jupiter’s poles are slightly flattened, and its equator bulges. Combined with its dense, turbulent atmosphere, Jupiter’s fast spin creates dramatic storms, and not just the famous Great Red Spot which clocks in with windspeeds over 400 mph (644 km/h) and has likely been raging for over 300 years.
The fastest winds are near the poles, approaching about 900 mph (1,448 km/h). However, the planet’s rotational spin is not the only factor for wind speed, meaning that although Jupiter is the fastest spinning planet, it does not break the record for the fastest winds, which is Neptune at more than 1,200 miles per hour (2,000 km/h) in its upper atmosphere.
The speed of a planet’s spin helps to shape the surface. Faster spins could mean flatter surfaces due to increased wind erosion as well as a different distribution of any liquid. For instance, if Earth’s rotational speed doubled, creating a 12-hour day instead of 24, the water would be pulled away from the poles to the equator by its centrifugal force and water levels would raise 100 meters at the equator.
This would drastically alter the landscapes of our continents, submerging Indonesia and much of northern South America as well as splitting Africa into two islands (with Mount Kilimanjaro poking out from the ocean).
Conclusion
While the length of our day may seem shorter or longer depending on a variety of factors, it is a constant in our lives. Our 24-hour day with on average 12 hours of daylight is something so consistent that it often falls into the background of normal.
And yet, this is only true on our planet. The sun rises in the morning because our planet turns toward it and sets because it turns away from it. While the speed for our planet equals one full rotation every 24 hours, that is not the same on any other planet, although Mars gets pretty close. It’s a stark reminder that our understanding of normal is only applicable here on Earth.
In addition, what we see as an easy demarcation of time also impacts other aspects of our planet including our weather, temperature, shape, and surface. Our world would be very different with a drastically different day such as the other planets’ days in our solar system. While we know that this speed likely won’t stay the exact same forever, it does impact the world we see today.
All in all, a day is so much more than a mere 24 hours.
Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at https://medium.com/@sarah-marie and authors self-help and children’s books.
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