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The Elusive Planet

Around the Sun

Each planet orbits the Sun on an elliptical path, so that its distance from the Sun varies regularly from maximum to minimum and back again with each round trip. Astronomers refer to the point of the orbit where the distance is a maximum as the aphelion, and the point where the distance is a minimum as the perihelion.

Diagram of Mercury’s orbit around the sun

The orbits of some planets, such as Earth’s, are nearly circular; the Earth-Sun distance at aphelion is only 3.5% larger than at perihelion. Mercury’s orbit, on the other hand, is much more elongated; the Sun-Mercury distance increases by over 50% (46 million to 70 million km) as it moves from perihelion to aphelion. (Notice that in the diagram the diameters of Mercury and the Sun are not drawn to scale).

The closer a planet is to the Sun, the faster it travels along its orbit. So at perihelion a planet is moving faster than at aphelion. At perihelion Mercury’s orbital speed is 56.6 km/s. At aphelion, half a Mercury year later, it has slowed to 38.7 km/s.

A Mercury year is 88 Earth days long, but a Mercury day (the time from noon to noon) takes twice that time. To an observer on Mercury, the Sun’s apparent motion across the sky is due to both the planet’s rotation and its motion along the orbit. These two have the opposite effect – the spin by itself would make the Sun appear to move from east to west, and the orbital motion would make it appear to move from west to east. On Mercury, the competition between these two means that a full day takes two full years. Generally the Sun appears to move from east to west (the spin generally wins), but near perihelion, where the planet is moving fastest in its orbit, the Sun briefly reverses its apparent motion in the sky.

Check out the animation to see this happen!

A full rotation of the planet Mercury takes exactly two-thirds of a full orbit (a Mercury year). Why? Because of the difference in distance, the Sun’s gravity is stronger on the closest side of Mercury (the side facing the Sun) and weaker on the opposite side. The result is that the planet tends to elongate or "bulge" along a line toward the Sun. This strenght of this effect varies regularly as the planet travels around its orbit. It is strongest at perihelion and weakest at aphelion, and has “locked” the planet’s spin to its orbital period. Since the ratio of orbital period to the spin period is 3:2, planetary scientists call it a 3:2 spin-orbit coupling or 3:2 resonance.

Look carefully at the animation to see the 3:2 spin-orbit resonance.

 
Odd but True... heading
Pluto has the most elongated orbit, two-thirds further from the Sun at aphelion than at perihelion. Pluto’s orbit is so elongated that for 20 of its 248-year orbital period, it is closer to the Sun than is Neptune. From 1979 until 1999, Neptune was the farthest planet from the Sun, but this won’t happen again for over 200 years!

 
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The same kind of resonance or coupling occurs between the Earth and Moon (see left for Mercury and Sun resonance description). In this case, though, the Moon’s spin period is exactly the same as its orbital period about the Earth, so from Earth we always see the same side of the Moon.