A Day on Another Planet

A day refers to the amount of time it takes for a planet to complete a single 360-degree rotation on its axis. On Earth, that’s 24 hours. But the other planets in the solar system rotate at faster or slower speeds, and their day lengths vary accordingly.



On Mercury, a single day amounts to 58 days and 15 hours Earth time. In contrast, the planet’s years are extremely short: only 88 Earth days. This is because Mercury moves more quickly around the sun, abbreviating its years, while its speed of rotation is slow enough to prolong its days.


Of all the planets in the solar system, Venus has the longest day. Its equivalent is 243 Earth days, while its year only consists of 224.7 days, essentially making its days longer than its years. The reason for the discrepancy is its slow rotation speed: while the velocity of the other planets have flattened their poles, Venus has not leveled out anywhere. It also rotates backwards, an oddity that some astronomers attribute to a massive impact with another planet billions of years ago.


A day on Mars, which is referred to as a √ęsol’, consists of 24 hours, 39 minutes, and 35 seconds, making its days similar to Earth time. This is one of many reasons why scientists asserted that plant and animal life could exist on the Red Planet, as Mars is also called. A Martian year lasts 686.98 Earth days, which translates to approximately 1.88 years.


Jupiter is the largest planet in our solar system, but it also happens to have the shortest day (9.9 Earth hours), because it rotates more quickly than any other body in the solar system. Its rapid rotation speed has caused extreme flattening at the poles and a bulging equator line.

Scientists had difficulty determining day length on Jupiter because it lacked surface features that could be used to calculate its rotational speed. At first storm centers were used, but Jupiter’s storms moved so rapidly that results were inconclusive. Rotational period and speed were finally determined by assessing radio emissions from the planet’s magnetic field.


The length of a Saturn day has proven to be extremely difficult to calculate, as the plant is a giant gas entity as opposed to terrestrial like Earth or Mars. The first estimation, which was attempted back in the 1980s, was 10 hours, 39 minutes and 24 seconds in Earth time. A second attempt delivered a result of 10 hours, 45 minutes, and 45 seconds. Using more advanced equipment in 2006, astronomers came up with a measurement of approximately 10 hours and 47 minutes. Some members of the scientific community assert that the length of a day on Saturn will never be known for certain.


A day on Uranus is the equivalent of 17 hours, 14 minutes, and 24 seconds on Earth. Because its axis is tilted to nearly 90 degrees, Uranus actually rotates on its side instead of spinning like a top. As a year progresses, one of its hemispheres faces the sun’s light while the other is in darkness for a complete season. What this means is that the planet’s days and seasons are one and the same. Day is as long as summer, and night is as long as winter.


On Neptune, a day translates to 16 hours, 6 minutes and 36 seconds on Earth. Since the planet is mainly gas, different parts actually rotate at varying speeds, a process known as differential rotation. Neptune’s equator zone takes around 18 hours to rotate, while the polar regions complete a cycle in 12 hours. Scientists attribute the large difference in rotational rate to Neptune’s winds, which can go up to 2,400 km/hour, making them the strongest in the solar system.

The Leap Second: a Jump in Time

Did you know? Leap seconds are adjustments made to Coordinated Universal Time (UTC) so that the UTC time standard, which is measured by atomic clocks and used for international timekeeping, can be synchronized with astronomical time to within 0.9 seconds.

The Earth’s rate of rotation around its axis is irregular, while atomic clocks are engineered to tick at the same speed for eons. The addition of leap seconds ensures astronomical time and UTC (otherwise known as Greenwich Mean Time) remain in accord.

earth rotation

The standard allows leap seconds to be applied at the end of any month, but so far all have been implemented on June 30 or December 31. Since their adoption in 1972, 25 leap seconds have been inserted, the last of which took place on June 30, 2012 at 23:59:60 UTC. The next one is scheduled for June 30, 2015 at the same time.

What’s in a Second?

The average day has 86,400 seconds, but atomic clocks do not define one second as 1/86,400 of the time it takes the Earth to travel around its axis. In atomic terms, one second is 9,192, 631,770 cycles of the standard Cesium-133 transition.

It’s an intricate calculation that’s incredibly precise, whereas the Earth’s rotation is slowing down over time, making the days irregular in length. An Earth day averages 0.002 seconds longer than the time tabulated by the atomic clocks. The result is a discrepancy of about one second every year and a half. Leap seconds ensure this discrepancy does not get too vast over time.

In theory, at least, leap seconds can be positive (with one second added) or negative (one second omitted), depending on the status of astronomical time results. All leap seconds have been positive so far, and the current pace of the Earth’s rotation makes it unlikely that a negative one will ever come into effect.

The Future of Leap Seconds

Some scientists want to abolish leap seconds, which would effectively redefine the way time is measured, but a consensus has yet to be reached on the subject. In 2012, attendees at the World Radiocommunication Assembly in Geneva scheduled a new vote on the matter for 2015.

Arguments against leap seconds include the following:

  • They are an anomaly, making them a cause for concern with safety-oriented real-time systems, such as air-traffic control programs that use satellite navigation.
  • Leap seconds are potential disruptions in computer systems that are closely synchronized with UTC.

2012’s leap second played havoc with LinkedIn, Reddit, Yelp, and other sites and applications. The Qantas Airlines computer system even went down for hours, forcing staff to check in passengers manually.

Coding for these apps and systems are based on UNIX, which appeared in 1970, before leap seconds came into effect. When the International Earth Rotation and Reference Systems Service, which maintains global time, signals to these computers that a certain minute has 61 seconds, Unix-based software systems become unstable.

Google developed a solution after the leap second of 2005 caused system issues. It slowly adds a couple of milliseconds to the clocks on its servers throughout the day of an impending leap second, which bypasses the security settings without triggering disaster.


Google’s fix has not been universally applied, and opponents of the leap second remain insistent that any time calibration benefits are overshadowed by the technological crises they cause. They point out that even if a leap second were applied every year, astronomical time would only be 16 minutes behind atomic schedule by 3015.