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6.5 Mercury's Orbit

Mercury takes 88 Earth days to go around the Sun. In 59 Earth days, it makes one complete turn on its axis, with respect to very distant stars (in other words, it takes 59 days for any one star to return to the same place in the Mercury night sky). Do you notice a relationship between those two numbers? The 59-day rotation period (a Mercury "day") is exactly two thirds of Mercury's 88-day "year." Is it only chance that the ratio is exactly 2/3? No. Due to tidal forces and the high eccentricity of Mercury’s orbit, the planet’s rotation period has stabilized at two thirds of the 88-day “year.” This phenomenon, called tidal locking, is also observed in the orbits of planets and satellites elsewhere in the solar system.

Orbits of Earth, Venus, and Mercury as well as an illustration of the Vulcanoid Zone. Click here for original source URL

The result of this orbital resonance is a very odd-looking "day” on Mercury. The combination of the 59-day spin and the 88-day "year" means that the time from one noontime to the next at any spot on Mercury's surface averages 176 Earth days! The number 176 is not random; it’s the lowest number that’s an even multiple of both the planet’s spin period and its orbital period around the Sun. Mercury experiences about 88 Earth days of burning daylight, followed by 88 days of frigid night. Even stranger, the Sun's movement across Mercury's sky is not regular and steady, like its motion across Earth's sky. This is because Mercury's orbit is more eccentric than the Earth's. This creates a "wobble" in the Sun’s slow movement across the sky, relative to the horizon.

Mercury and Earth size comparison. Click here for original source URL.

Mercury's orbit is highly eccentric. This means that at the closest point to the Sun, called perihelion (from the Greek roots peri = around, and helios = the Sun), Mercury is only 46 million kilometers away from the Sun. Compare this to aphelion (the farthest point from the Sun), when the distance is 70 million kilometers! Mercury’s orbit has the highest eccentricity of any planet except Pluto. The planets all accreted from the same rotating disk of material, and that process made most of their orbits nearly circular. But Mercury’s is not. What caused this anomaly? Scientists think a large object hit Mercury early in solar system history. This impact may have affected the planet’s orbit, as well as its composition.

tylized and exaggerated picture of the precession of the orbit of Mercury around the sun. Click here for original source URL.

In the 1800s, scientists were surprised to find that Mercury’s perihelion shifts in position slowly around the Sun, by a tiny angle from year to year. This shift, called precession, could not be explained by Newton's laws of gravity. In the late 1800s, scientists thought the precession must come from the gravitational force of an unknown planet between Mercury and the Sun. This hypothetical planet was called Vulcan. (This is the origin of the name for Mr. Spock's home planet in the classic science fiction show Star Trek.) However, later observations showed that no such planet exists.

The solution to the mystery of Mercury’s orbit came in 1915, when Albert Einstein modified and improved Newton's laws with his new theory of relativity. Einstein's theory describes the relationships between gravity, space, and time differently than Newton’s laws did. Einstein's equations exactly predicted the rate of precession that was observed in Mercury's orbit. Solving the mystery of Mercury's motion led to worldwide acceptance of Einstein's theory of relativity. This is a dramatic example of how science works. A single observation rarely leads to a new theory. But in this case, a small shift in the orbit of a single planet led to a new conception of gravity.