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Astro*Dictionary by Michael Erlewine





4 articles for "Axis"

Axis [Astro*Index]

The line about which a body rotates, such as the Earth's spin axis; a line perpendicular to a plane, such as the the axis of the ecliptic; a line used as a reference position, such as the horizontal axis of a graph.

See also:
♦ Ecliptic
Axis [Munkasey M.]

The Polar Axis of the Earth. That line which cuts throughthe Earth and joins the North Pole of the Earth with the South Pole of the Earth. An extension of this line infinitely into space.

See also:
♦ Ecliptic
Axis, Inclination of [Astro*Index]

The inclination of the axis of a rotating body (such as the Earth) refers to the inclination (tilt) of the polar axis to the normal to plane of the orbit. The Earth has its polar axis tilted at about 23.45° (the Obliquity of the Ecliptic) from the normal to the plane of its orbit. If we imagine a planet to be like a child's toy top spinning on a table, and the table top to be the plane of the planet's orbit, then the top is not standing upright on the table. Instead, it is tilted one way or another as it spins across the table's surface. All planets in our solar system have their polar axes tilted this way.


Mercury   072°
Venus   060°
Earth   023.45°
Mars   025°
Jupiter   003°
Saturn   026°
Uranus   102°
Neptune   155°
Pluto   ???°


See also:
♦ Obliquity of the Ecliptic
Axis, Inclination of [DeVore]

The equators of rotating bodies appear never to parallel their orbits. Hence there is an inclination of the axis when considered in reference to the plane of the orbit. Within the solar system these inclinations arc, at this epoch, as follows: Mercury 72°, Venus 60°, the Earth 23°, Mars 25°, Jupiter 3°, Saturn 26°, Uranus 102°, Neptune 155°, Pluto unknown. The inclination of the Sun's axis to the plane of the Earth's orbit is about 7°. Its inclination to the plane of its own orbit is unknown, because the Sun's orbit is itself unknown. It is claimed by some that there is an additional motion of the Earth's axis amounting to 50" a century, making an orbit of about 2½ million years, in the course of which the North Pole and the South Pole successively point to the Sun instead of as at present to the Pole Star. This theory is advanced by way of explanation for successive Ice Ages.

See also:
♦ Obliquity of the Ecliptic


Astro*Index Copyright © 1997 Michael Erlewine


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