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Moons in the Solar System

The planets of our solar System are orbited by moons, except the innermost planets Mercury and Venus:

Planet Earth Mars Jupiter Saturn Uranus Neptun Pluto
Moons 1 2 64 62 27 13 4

Solar System Planet Moon

The force of gravitation between the planet (mass mP) and its moon (mass mM) is given by:

gravitation force planet moon
and the force of gravitation between the Sun (mass mS) and the moon (mass mM) by:

gravitational force sun moon
The mean value of the distance of the moon from the Sun (rSM) is approximatly equal to the mean distance of the planet from the Sun (semi-major axis rSP).
The ratio of the forces is
ratio of gravitation forces

The values of the ratio (table below) are showing some interesting results:

the extraordinary ratio of 0.45 for the Moon of our Earth means that the attraction by the Sun is about 2.2 greater the the attraction by the Earth!

earth sun moon
The ratio of the gravitation forces F(Earth_Moon)/F(Sun_Moon),
fmax 0.53, min. 0.40, mean 0.46,
computed by my Moon Distance Applet

table moons

curvature radius of lunar orbit
The geometric curvature radius of the lunar orbit at time t is computed from 3 positions at t-12h, t, t+12h.
It is always positive,
passing a maximum (about 2.6
·108 km) at New Moon (2011: Jan 4, Feb 3, etc),
and a minimum
(about 1.1·108 km) at Full Moon (2011: Jan 19, Feb 18, etc),
the mean value in 2011 is
1.65·108 km,
the radius of the Earth's orbit is r=1.50·108 km.
Computed by my Planet Applet

The European Space Agency has referred to the Earth–Moon system as a type of double planet.
The moon does not actually revolve around the Earth; it revolves around the Sun in concert with the Earth (like a double planet system). It is the only moon in the solar system that is always falling toward the Sun (orbit always convex).

sun planet moon

The orbit of a moon orbiting a planet of mass m orbiting the Sun (mass M) at a distance R is always convex if and only if  the radius r of the moon's orbit exceeds a certain value (MathPages):
convex orbit
The Earth's Moon (r =384.400 km) meets the condition
r > 259.000 km

curvature radius earth orbit
Computed by my Planet Applet

Lunar orbit moon

Heliocentric Orbit of the Moon, June 2010 (x-y-coordinates, AU), time interval 12 hours.

sun distance
Heliocentric distance of the Moon, Jun 2010 (New Moon June 12, Full Moon June 26)


New Moon June 12, Full Moon June 26

Please visit my Moon Applet Collection

Visit my page: The Moon: orbit and phases

Web Links

Moons of Mars (Wikipedia)

Moons of Jupiter (Wikipedia)

Moons of Saturn (Wikipedia)

Moons of Uranus (Wikipedia)

Moons of Neptune (Wikipedia)

Moons of Pluto (Wikipedia)

Double Planet (Wikipedia)

Origin of the Moon (University of Arizona)

The Moon Always Falls Toward the Sun (MathPages)

The Formation of the Moon (Windows to the Universe)

Theories of Formation for the Moon (University of Tennessee)

On The Formation of Earth's Moon (ScienceWeek)

Planetary Satellite Physical Parameter (NASA)

Lunar Constants and Models Document (JPL)

E. Hantzsche: Doppelplanet Erde-Mond, Teubner, Leipzig 1973.

Isaac Asimov: Von Raum und Zeit, Ullstein, Frankfurt/M 1978.

© 2011 J. Giesen

Last modified: 2011, Sep 26