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.
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).
| Odd
but true: 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! |
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.
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.
| Odd
but true: The same kind of coupling
occurs between the Earth and Moon. 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. |
|
