What Is the Length of Day on Mercury?

What Is the Length of Day on Mercury?
••• Photo by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

In the solar system, a year is determined by how long it takes for a planet to revolve around the sun, and a day is determined by how long it takes for a planet to completely rotate on its axis. Mercury has an unusual length of day when compared to most of the other planets in the solar system.

Time Frame

Mercury rotates on its axis very slowly, while it orbits the sun quickly. In fact, one day is actually as long as two years on Mercury. It takes Mercury about 88 Earth days to orbit the sun, while Earth takes 365 days. It takes about 176 Earth days for Mercury to rotate on its axis (from sunrise to sunrise), while Earth takes only 24 hours. On Mercury, it is daytime for one year, and night for one year.

Considerations

Venus, the planet between Earth and Mercury, also has a day that is longer than its year. Venus has the longest day of any planet in our solar system. One day on Venus lasts for 243 Earth days, while a year lasts about 225 Earth days.

Features

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Because Mercury has an elliptical orbit around the sun--compared with Earth's circular orbit--and because it has such a slow rotation, this makes the sun appear to move in a way that people from Earth would find highly unusual. Sometimes the sun comes to a complete stop, then seems to move backward for a while, then moves forward again in a loop back to the position where it had stopped at previously, before moving forward. The sun also sometimes looks larger and sometimes smaller, and decreases in size to the point where background stars can be seen. Sometimes the background stars appear to be moving three times faster than the sun. Mercury has a very thin atmosphere and looks a lot like Earth's moon.

Geography

Mercury is the closest planet to the sun, about 58 million kilometers away. Earth is about 150 million kilometers from the sun. Because of its proximity to the sun, here on Earth we can only see Mercury with the naked eye or with binoculars when it appears as a bright star close to the horizon just after sunset or just before sunrise.

Identification

Mercury is either the smallest planet, or the second smallest if Pluto is counted as a planet. Since Pluto was re-categorized as a dwarf planet in 2006, debate continues about whether Pluto still actually should be considered a planet or rather an asteroid body. Mercury's diameter is 4,879 kilometers, which is not much larger than Earth's moon at 3,475 kilometers. Both are larger than Pluto, with a diameter of 2,390 kilometers.

Related Articles

Similarities of the Sun & Moon
What Is Mercury's Rotation Period?
What Is Saturn's Orbit in Earth Days?
What Is Venus's Revolution Period in Earth Days?
How to View Venus in the Night Sky
Similarities of the Sun & Moon
Why Are Days Longer and Shorter?
What Are Some Unique Characteristics of Saturn?
How Many Earth Days Equals One Year on Venus?
Saturn Facts for Children
Difference Between Rotate & Revolve
What is the Distance From Saturn to the Sun?
What Is the Revolution Time of the Earth?
Which Planets Are Known to Have Satellites?
Eccentricity of Planet Mars' Orbit
Which Planet Moves the Slowest Along Its Orbital Path?
What Is the Distance From the Sun to Mercury?
What Types of Measurements Are Used for Measuring in...
What Causes the Day/Night Cycle on Earth?