why does Mercury spin so slowly?

Question

1 day in Mercury = 176 Earth days.

Answer 1

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It actually spins roughly in 58 days and 15 hours, not 176 days. 176 days is the solar day, which due to its slow rotation, eccentric orbit, and closeness to the Sun ends up being longer than the sidereal day.

The reason it is so slow is because of the tidal effects of the Sun's gravity acting on it. It is the same reason that we always see the same side of the Moon from the Earth. The Earth's gravity has slowed down the Moon's rotation to the point that it takes the same time to spin on its axis as it does to revolve around the Earth. The Moon's gravity is doing the same thing to the Earth, but because the Earth's gravity is so much stronger the process occurred to the Moon much faster. But there is a difference with Mercury. The problem with Mercury is that its orbit is so eccentric. As a result the speed it travels around the Sun is greater at perihelion (its closest point) than aphelion (its farthest point). Mercury's rotation is locked to the faster rate so as a result it is in a 3:2 spin-orbit resonance as opposed to 1:1 like the Moon.

Answer 2

Its day is locked to its year (1 day = 2 years).

As a rule 'day' length (period of its spin) has no rule for planets, while year length follows Kepler's Third Law.

Answer 3

It s tidally locked to the sun, in the same way that the moon is tidally locked to the earth.

The same face of Mercury always faces the sun. which means it spins the same speed as it orbits.

There is no night and day cycle on Mercury, as the sun never rises and sets. The is a light side where the sun always shines, and a dark side where it never shines.

Answer 4

Physics