Orbit of Mercury
Since Pluto isn't a planet, Mercury now takes the record for the planet with the most eccentric orbit. In fact, its orbit ranges quite dramatically, ranging between 46 million and 70 million kilometers from the Sun.
Mercury's orbital period is 88 days.
In addition to this highly eccentric orbit, Mercury also has a fairly steep inclination in its orbit. If you set the plane of the Earth's orbit at 0°, Mercury's orbit is inclined at 7°.
Transits of Mercury happen when the orbits of Mercury and Earth line up so that Mercury is seen to pass in front of the Sun (a solar eclipse is an example of a transit). The impact of this axial tilt means that Mercury only makes transits across the surface of the Sun every 7 years or so.
While Mercury's orbit and inclination are extreme for the Solar System, its axial tilt is not. While we have a 23.4° axial tilt here on Earth, which accounts for all the seasons, Mercury's tilt is only 0.01°. If you could stand on equator of Mercury, and watch the Sun rise and set, it would always follow a line directly above the crater. And if you stood on the poles, the Sun would never get higher than 0.01° above the horizon.
Astronomers used to think that the rotation of Mercury was the same as its orbital period. It would take 88 days to turn once and go around the Sun. They thought it was tidally locked to the Sun; always facing one side. This turned out to be wrong.
But Mercury orbit and length of day do have a resonance. For every 2 orbits around the Sun, Mercury completes 3 rotations on its axis. This is known as a 3:2 spin-orbit resonance.
Filed under: Astronomy
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