An orbit is the path of any object moving under the influence of gravity, including spacecraft, planets, stars, or galaxies. Once an orbit is determined, future positions can be predicted.
Special points in an orbit (in any solar system):
Perihelion: the point where a planet is closest to the Sun and moves fastest along its orbit. The name comes from Greek helios for the Sun.
Aphelion: the point where a planet is farthest from the Sun and moves slowest along its orbit.
For Moon or any satellite orbiting a planet (geek term): perigee (closest to Earth) and apogee (farthest from Earth).
Distinguishing terms:
Moon: a natural object that orbits a planet.
Satellite: an object (often human-made) that orbits a planet.
Orbits of the Planets
With Newtonian physics, planetary orbits can be calculated and predicted with high precision.
There are eight planets, ordered from the Sun as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Ceres is the largest asteroid and is now classified as a dwarf planet.
Orbital data summarized (Table 3.2 data referenced in the transcript):
Mercury has the shortest orbital period and the highest orbital speed among the planets.
Neptune has the longest orbital period and the slowest average orbital speed.
Orbital characteristics:
All planets have orbits with relatively low eccentricity (e).
Mercury has the most eccentric planetary orbit among the eight planets, with Iightly cited value around e ≈ 0.21; the other planets have eccentricities smaller than 0.1 in the typical summary.
Mars has a comparatively larger eccentricity relative to many other planets.
The planetary orbits lie close to a common plane, near the ecliptic (the plane of Earth’s orbit).
Pluto’s orbit is inclined by about 17° to the ecliptic; Eris’ orbit is inclined by about 44°.
All major planets lie within about 10° of the common plane.
Key speeds and periods (examples):
Mercury: very short period, very high speed; approximate values: $$T \