Asteroids: Also known as minor planets, Asteroids are small rocky bodies that usually orbit the Sun between the orbits of Mars and Jupiter.

Average distance from the Sun: An AU is an Astronomical Unit, which is defined as the average distance from the Earth to the Sun, or 149.6 million kilometers (93 million miles).

Average orbital speed around the Sun: This is a measure of how fast a planet moves through space, in kilometers per hour.

Axial inclination: Axial inclination is the angle at which a planet's axis of rotation is tilted, with respect to that planet's own orbit. On Earth, as well as other planets, this tilt is responsible for the seasons.

Average temperature: Given in degrees centigrade.

Black Hole: The remains of the death and collapse of an extremely massive star. The gravitational pull of a black hole is so strong that light itself cannot escape.

Earth: Third planet from the Sun

Jupiter: Fifth planet from the Sun

Mass: The mass is a measure of how much 'stuff' something contains. This shouldn't be confused with weight, which is a measure of the force of gravity on an object. An apple weighs more on Jupiter than it does on Earth because Jupiter's gravity is stronger. However, the apple always has the same mass, no matter where it is. We can't measure the weight of a planet simply because in space, the concept of weight is meaningless! Weight is measured in terms of the attraction of one body to another. In other words, in order to "weigh" Jupiter, you'd have to have it sitting on a scale on the Earth!

Mars: Fourth planet from the Sun.

Mercury: Closest planet to the Sun.

Neptune: Eighth planet from the Sun (usually)

Orbital inclination: The orbits of the planets are tilted with respect to each other. Orbital inclination is a measure of the orbit's tilt with respect to the Earth's orbit.

Orbital period: The length of time for a planet to circle the Sun is known as its orbital period, or its year. Planets far away from the Sun have longer years than those close to the Sun.

Pluto: Ninth planet from the Sun (usually)

Relative surface gravity: This is the measure of the strength of a planet's gravity as felt from its surface, compared to the Earth.

Rotational period: The time it takes for a planet to rotate once on its axis, with respect to the stars (not the Sun.) Except for Mercury and Venus, this is comparable with what we would call its day. Venus, Pluto, and Uranus spin retrograde, in the opposite direction from the other planets.

Saturn: Sixth planet from the Sun

The Sun: The yellow star at the center of our Solar System.

Uranus: Seventh planet from the Sun

Venus: Second planet from the Sun

Last Updated 3/12/96
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