Tug of war (astronomy)

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search

The tug of war in astronomy is the ratio of planetary and solar attractions on a natural satellite. The term was coined by Isaac Asimov in The Magazine of Fantasy and Science Fiction in 1963.[1]

Law of universal gravitation[edit]

According to Isaac Newton's law of universal gravitation

In this equation

F is the force of attraction
G is the gravitational constant
m1 and m2 are the masses of two bodies
d is the distance between the two bodies

The two main attraction forces on a satellite are the attraction of the Sun and the satellite's primary (the planet the satellite orbits). Therefore, the two forces are

where the subscripts p and s represent the primary and the sun respectively, and m is the mass of the satellite.

The ratio of the two is


Callisto is a satellite of Jupiter. The parameters in the equation are [2]

  • Callisto–Jupiter distance (dp) is 1.883 · 106 km.
  • Mass of Jupiter (Mp) is 1.9 · 1027 kg
  • Jupiter–Sun distance (i.e. mean distance of Callisto from the Sun, ds) is 778.3 · 106 km.
  • The solar mass (Ms) is 1.989 · 1030 kg

The table of planets[edit]

Asimov lists tug-of-war ratio for 32 satellites (then known in 1963) of the Solar System. The list below shows one example from each planet.

Primary Satellite Tug-of-war ratio
Neptune Triton 8400
Uranus Titania 1750
Saturn Titan 380
Jupiter Ganymede 490
Mars Phobos 195
Earth Moon 0.46

The special case of the Moon[edit]

Unlike other satellites of the solar system, the solar attraction on the Moon is more than that of its primary. According to Asimov, the Moon is a planet moving around the Sun in careful step with the Earth.[1]


  1. ^ a b Asimov, Isaac (1976). Asimov on Astronomy. Coronet Books. pp. 125–139. ISBN 0-340-20015-4.
  2. ^ Arny, Thomas. Explorations. Mc Graw Hill. pp. 543–545. ISBN 0-07-561112-0.