By Sabine Stanley, Ph.D., John Hopkins University
How is Mercury’s year only 1½ Mercury days long while the solar days on Mercury last for two whole years? How does the Sun follow an unusual motion in the Mercury sky, and how one can stay in the Sun’s terminal line? Let’s uncover the strange patterns of Mercury’s sky.
Mercury’s year is only 1½ Mercury days long
The Sun’s motion across the Mercury sky is very strange. First, Mercury’s rotation is really slow. Or in another way, Mercury’s day is really long. That’s because when a planet is so close, the Sun’s gravity exerts strong tidal forces on the near and far sides of the planet that have slowed Mercury’s spin way down. It takes about 59 Earth days for Mercury to complete just one rotation on its axis. The time for one rotation is only barely faster than Mercury’s year, which is 88 Earth days. So that means Mercury’s year is only 1 1⁄2 Mercury days long.
Learn more about orbiting Earth: up through the atmosphere.
One Solar day on Mercury is Two Whole Mercury Years Long
Second, the Sun doesn’t rise every day. It doesn’t even rise every year. On Earth, we are used to our time of day is well marked by the position of the Sun in our sky, but this doesn’t happen on Mercury. Sundials would not work well on Mercury. Ultimately, this is because the length of the day is so close to the length of a year.
Let’s unpack this a bit. Start by considering the situation we take for granted on Earth. If I stand in one spot and time how long it takes between sunrises, I get 24 hours. We call that a solar day. And if I stand in the same spot and pick a faraway star, then I can time how long it is before I see that star in the same spot in the sky again. And I get almost the same result: That time would be 23 hours and 56 minutes. Astronomers call this ‘day with the stars’—a sidereal day.
The stars act as a good fixed reference frame since they are so far away. This is a good way to ignore the Sun and see how long it took the Earth to complete a whole 360-degree revolution. This difference of 4 minutes between the sidereal and solar days happens because as the Earth rotates, it is also moving in the same direction along its orbit. So even though it will have completed 360° of revolution, it still has to rotate a little bit more to be facing the Sun once again.
Now, on Earth, the rotation period for a day is much shorter than the orbit period for a year, so the sidereal and solar days are quite similar—only a 4-minute difference. But on Mercury, the difference between the solar day and the’stars day’ is much bigger.
Let’s look at what happens on Mercury, you stand in one spot on Mercury and pick a faraway star and wait until that star reappears in the same spot. That takes 59 Earth days. That’s the length of Mercury’s sidereal day. But after one Mercury sidereal day, Mercury has moved about of the way around its orbit. That means it’s going to take a lot of extra rotation to get to the point where the Sun appears in the same spot in the sky again. Moreover, Mercury rotates very slowly, so it’s going to take a long time for this catch-up rotation to finish.
In fact, one solar day on Mercury is 2 whole Mercury years long! Mercury’s solar day lasts 176 Earth days, while Mercury’s year is 88 Earth days long. The idea of a solar day is longer than a year seems quite strange to us, but our planet has very different orbital and rotation periods.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.
The Sun’s Strange Retrograde Motion in Mercury Sky
There is a third strange feature of the Sun’s motion across Mercury’s sky: The Sun’s motion in the sky can change direction. If you were standing on Mercury watching the Sun, it would rise, move westward across the horizon, but at certain intervals along Mercury’s orbit, it would stop moving, reverse direction, and move eastward for a while. It would then stop and reverse direction again and continue its normal westward motion across the sky.
Why does this happen? This is because of how Mercury’s very elliptical orbit combines with the fact that Mercury rotates very slowly. Planets on elliptical orbits change their orbital speed when they change their distance from the Sun. When Mercury is further from the Sun, it is moving more slowly in its orbit, and when Mercury is closer to the Sun, it is moving faster. Moreover, for Mercury, although its rotational speed is usually greater than its orbital speed, that’s not always true.
Sometimes Mercury is orbiting faster than it’s rotating. This change in which type of motion is faster—orbiting or rotating—is what causes the Sun to change direction in Mercury’s sky.
Let’s look at a changeover point: roughly 4 Earth days before Mercury reaches its closest point to the Sun, which is called perihelion. Here, Mercury’s orbital motion has sped up to the point that it matches its rotational speed. Here is where things get spooky: At this point, the Sun stops moving in the sky. Then, as Mercury continues orbiting even closer to the Sun, its orbital speed becomes faster than its rotational speed. Now, because the slow rotation of Mercury cannot keep up with the faster orbital motion, the Sun appears to move backward in the sky.
We call this retrograde motion, and it continues until another 4 Earth days after Mercury reached perihelion. By this point, the orbital motion has slowed down again, enough, so that the rotational motion is once again faster than the orbital motion. So now the Sun resumes moving in the normal westward direction across the sky.
Learn more about Mars from space and the ground.
Staying Inside the Sun’s Terminator Line or the Twilight Zone
Science fiction writers have enjoyed exploring what living on Mercury would be like. For example, Kim Stanley Robinson’s novel 2312 imagines a city built on rails that move around the planet, staying near the terminator, the boundary between day and night.
How fast would this rolling city have to move to continue the same pattern of Mercury sky? Mercury’s circumference is about 15,000 kilometers. In order for the city to stay at the terminator, it must cover that distance in one Mercury solar day, which, as we’ve seen, is 176 Earth days long, which is 4224 hours. That means that on average, a city at Mercury’s equator would need to travel about 31⁄2 kilometers per hour. That’s walking speed—just a little over 2 miles per hour.
At Earth’s equator, by contrast, the Sun’s terminator line moves at roughly 1000 miles per hour.
Learn more about exploring the Earth-Moon system.
Common Questions about Strange Patterns of Mercury Sky
Mercury completes one rotation every 59 Earth days. Mercury slowly spins on its axis, and its every rotation is not accompanied by sunrise and sunset like on most other planets.
Mercury is tidally locked with the Sun in a 3:2 spin-orbit resonance, which means that relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.
Mercury’s day lasts 59 days because its rotation around its axis lasts 59 days.
Mercury takes 59 Earth days to turn once around its axis, with respect to the background stars. This period is also known as the sidereal day.
