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Axial tilt

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The axial tilt of any celestial body is the inclination of its axis from a line perpendicular to the plane of its orbit around its primary. It is also the inclination of its equator from that orbital plane.

Planets with highly tilted axes are problematic for the usual model for the creation of the solar system (the nebular hypothesis). A planet that formed by accretion of matter from a larger spinning disk ought to rotate in the same plane as the disk; its axis should not tilt. Furthermore, the axis of the Sun ought not tilt from the orbital planes of most of its planets—but in fact the Sun's axis is tilted from the ecliptic, and by more than seven degrees.

Planetary Axial Tilts

The NASA Planetary Fact Sheet gives the following information about each planet:[1] The first value is the Axial Tilt in Degrees, then the moment of inertia represented by the letter I, then J2 which is the gravitational moment (if available).

The moment of rotation of a planet can be affected by climate and mass movements, though the effect is generally small [2] The letter I is used to represent the moment of inertia. A solid, uniform sphere would have a moment of inertia of .4, so the lower values for planets show that they generally become more dense close to the center.

The gravitational moment is called J2 (one of a whole series of J values that come from Fourier transforms, but the second J is the most important, even, value for most planets) which equals C-A/(M*R2)[3] where C is the moment of inertia about the spin axis, A is the moment of inertia about an axis through the equator (the two will be different if the planet has an equatorial bulge), M is the mass of the planet, and R is the polar radius. [4] C is sometimes referred to as the Polar Moment of Inertia, since it is calculated presuming an axis of rotation that passes through the poles of the planet. The gravitational moment gives some information about the increase of density of a planet with depth. Scientists warn that slowly rotating bodies (like Mercury, Venus, and the Moon) have a small J2 but that can be more easily related to internal mantle convection than density distribution.[5]

Mercury: 0.01o, I = .33, J2 = 60 x 10-6

Venus: 177.4o. "Venus rotates in a retrograde direction, opposite the other planets, so the tilt is almost 180 degrees, it is considered to be spinning with its "top", or north pole pointing "downward" (southward). [6], I = .33, J2 = 4.458 x 10-6

Earth: 23.5o, I = .3308, J2 = 1,082.63 x 10-6

(Moon) 6.7o, I = .394, J2 = 202.7 x 10-6

Mars: 25.2o, I = .366, J2 = 1,960.45 x 10-6

Jupiter: 3.1o, I = .254, J2 = 14,736 x 10-6

Saturn: 26.7o, I = .210, J2 = 16,298 x 10-6

Uranus: 97.8o "Uranus rotates almost on its side relative to the orbit."[7], I = .225, J2 = 3343.43 x 10-6

Neptune: 28.3o, J2 = 3411 x 10-6

Pluto: 122.5o "Pluto is pointing slightly "down".[8]"


  1. Planetary Fact Sheet. Accessed 21 June 2010.
  2. Changing a Planet's Rotation from Within by Steven Dutch. Accessed 21 June 2010.
  3. Notes on the Facts Sheets. Accessed 21 June 2010.
  4. Planetary science: the Science of Planets around Stars, By G. H. A. Cole, M. M. Woolfson. P. 340
  5. Planetary Sciences, By Imke De Pater, and Jack Lissauer, P. 220
  6. Planetary Fact Sheet Notes. Accessed 21 June 2010.
  7. Planetary Fact Sheet Notes. Accessed 21 June 2010.
  8. Planetary Fact Sheet Notes. Accessed 21 June 2010.