Fig. 2. Orbits of Earth and Mars
Orbits of Earth and Mars (Click to return to Mars opposition page)
Note: Distances shown are in Astronomical Units (AU) where the Earth's mean distance from the Sun is 1.0 AU (about 93,000,000 mi or 150,000,000 km).

This diagram shows why Mars varies in its distance from the Earth by a factor of about 5 to 1:
  1. The mean distance of Earth from Sun is 1.0 AU.
  2. The mean distance of Mars from Sun is about 1.5 AU.
  3. When Mars is closest to Earth, Mars is on the opposite side of the Earth from Sun (called opposition).
  4. The distance of Mars from Earth is then 1.5 AU - 1.0 AU = 0.5 AU
  5. When Mars is farthest from Earth, Mars is on other side of the Sun from Earth (called conjunction).
  6. The distance of Mars from Earth is then 1.5 AU + 1.0 AU = 2.5 AU
  7. Therefore, Mars changes its apparent size by a factor or ratio of about 2.5 AU/0.5 AU or 5:1!
    8.

Because the orbits of Earth and especially Mars are really elliptical (noncircular), the variation in distance including apparent size, can be even greater.