A Day on Mercury Lasts Longer Than a Year

If somebody asked you "how long is a day?" you'd probably say "24 hours." That's how long it takes Earth to spin once. But on Mercury, the answer is much weirder — and it depends on what you mean by "day."

Mercury spins on its axis very slowly. It also races around the Sun very quickly. Those two motions don't line up the way they do on Earth, and the result is a planet where one day lasts longer than one year.

How Long Is a Day on Mercury?

There are actually two answers, depending on which kind of "day" you mean.

If you measure how long Mercury takes to spin once on its axis — the same way you'd say Earth spins once every 24 hours — then a day on Mercury is 59 Earth days. That's how long it takes Mercury to make one full rotation. Astronomers call this a sidereal day.

If you measure how long it takes the Sun to go from one sunrise to the next sunrise — which is what most of us mean by "a day" — then a day on Mercury is 176 Earth days. That's almost six Earth months. Astronomers call this a solar day.

Why the difference? Because Mercury is also moving around the Sun while it spins. By the time Mercury has finished one full rotation (59 Earth days), it has also traveled a long way along its orbit. So even though it has spun once, the Sun is no longer overhead at the same place. Mercury has to keep spinning for another 117 Earth days before the Sun catches up and rises again.

How Long Is a Year on Mercury?

A year is the time a planet takes to orbit the Sun once. Earth's year is 365.25 days. Mercury's year is just 88 Earth days.

That's the shortest year of any planet. Mercury has the shortest year because it's closest to the Sun, so it doesn't have far to travel, and because the Sun's gravity is strongest there, so it pulls Mercury along faster.

If you lived on Mercury, you'd celebrate a new birthday every three Earth months. A 10-year-old on Earth would already be over 41 in Mercury years. By the time you graduated high school on Earth at age 18, you'd be 75 in Mercury years.

So a Day Is Twice as Long as a Year?

Yes. This is the part most people find hardest to believe.

One Mercury solar day = 176 Earth days. One Mercury year = 88 Earth days. The day is exactly two years long.

Imagine being born on Mercury at sunrise. By the time the Sun finally set on your first day of life, you'd already be one Mercury year old. You'd live through your second birthday before sunrise on day two.

Mercury Is the Fastest Planet

Mercury orbits the Sun faster than any other planet in the solar system. It moves at about 29 miles per second (47 km/s). That's roughly 105,000 miles per hour — thousands of times faster than a jet plane.

Speed-wise, here's how the planets stack up:

• Mercury: 29 mi/s (fastest)
• Venus: 22 mi/s
• Earth: 19 mi/s
• Mars: 15 mi/s
• Jupiter: 8 mi/s
• Neptune: 3.4 mi/s (slowest)

Mercury's speed is one of the reasons it got its name. Ancient astronomers watched it move quickly across the sky and named it after Mercury, the Roman messenger god famous for being the fastest of all the gods.

The Strange Mercury Sunrise

Now here's where it gets really weird.

Mercury's orbit isn't a perfect circle. It's an oval, called an ellipse. So sometimes Mercury is closer to the Sun (about 29 million miles away) and sometimes farther (about 43 million miles away). Astronomers have names for these two points: the closest point is called perihelion, and the farthest point is called aphelion. When Mercury is at perihelion (closest to the Sun), it speeds up. When it's at aphelion (farther away), it slows down.

At perihelion, Mercury is moving so fast around the Sun that its orbital speed actually beats its rotation speed. From certain spots on the surface, this means the Sun appears to rise, slow down, stop, go backwards, set, and then rise again. It's the only planet in the solar system where the Sun does this strange dance.

If you stood at one of those spots on Mercury and watched the Sun rise, it would briefly come up over the horizon, pause, dip back down for a while, and then rise properly. The same thing happens in reverse at sunset.

The 3:2 Spin-Orbit Resonance

Mercury's spin and orbit are locked together in a really specific pattern. Mercury spins exactly three times for every two trips around the Sun. Astronomers call this a 3:2 spin-orbit resonance, and Mercury is the only planet that does it.

For a long time, scientists thought Mercury was tidally locked to the Sun, meaning one side always faced toward it (the way one side of our Moon always faces Earth). But in 1965, astronomers used radar from Earth to measure how Mercury was actually spinning, and they got a surprise: it wasn't tidally locked at all. It was in this strange 3:2 pattern instead.

The resonance is probably caused by Mercury's lopsided orbit and the Sun's strong gravity working together. The mathematician who figured it out, an Italian named Giuseppe "Bepi" Colombo, is the same person the BepiColombo mission is named after. Read more about BepiColombo.

Why Mercury Has No Real Seasons

Earth has seasons because it tilts on its axis. We tilt 23.5 degrees, so different parts of the planet point toward the Sun at different times of year. That's why we get summer and winter.

Mercury barely tilts at all. Its axis is tilted only 2 degrees — almost perfectly upright. So Mercury doesn't have summer, autumn, winter, or spring the way Earth does. The temperature on a given spot of Mercury depends almost entirely on whether the Sun is up or down, not on the time of year.

That tiny axial tilt is also part of why Mercury's poles can hold water ice. With almost no tilt, the bottom of polar craters never sees sunlight at any time. More on Mercury's surprising polar ice.

Frequently Asked Questions