# Planetary Fact Sheet Notes

Mass or Weight (1024kg or 1021tons) - This is the mass in septillion (1 followed by 24 zeros) kilograms or weight in sextillion (1 followed by 21 zeros) tons. Tons are measures of weight, not mass, but are used here to represent the mass of one ton of material under Earth gravity.

Diameter (km or miles) - The mean diameter of the body, the distance through the center of the planet from one point to the point on the opposite side, in kilometers or miles.

Volume (km3 or miles3) - The volume of the body in kilometers cubed or miles cubed.

Density (kg/m3 or lbs/ft3) - The average density (mass divided by volume) of the whole planet (not including the atmosphere for the Earth and Venus) in kilograms per cubic meter or pounds per cubic foot.

Gravity (m/s2 or ft/s2) - The gravitational acceleration on the surface at the equator in meters per second squared or feet per second squared, including the effects of rotation. For the gas giant planets the gravity is given at the 1 bar pressure level in the atmosphere. The gravity on Earth is designated as 1 "G", so the ratio gives the gravity in G's.

Escape Velocity (km/s) - Initial velocity, in kilometers per second or miles per second, needed at the surface or top of photosphere to escape the body's gravitational pull, ignoring atmospheric drag.

Rotation Period (hours) - This is the time it takes for the body to complete one rotation relative to the fixed background stars (not relative to the Sun) in hours. Negative numbers indicate retrograde (backwards relative to the Earth) rotation.

Length of Day (hours) - The average time in hours for the Sun to move from the noon position in the sky at a point on the equator back to the same position.

Distance from Sun (106 km or 106 miles) - This is the average distance from the planet to the Sun in millions of kilometers or millions of miles, also known as the semi-major axis. All planets have orbits which are elliptical, not perfectly circular, so there is a point in the orbit at which the planet is closest to the Sun, the perihelion, and a point furthest from the Sun, the aphelion. The average distance from the Sun is midway between these two values. The average distance from the Earth to the Sun is defined as 1 Astronomical Unit (AU), so the ratio gives this distance in AU.

Orbital Period (days) - This is the time in Earth days for a planet to orbit the Sun from one vernal equinox to the next. Also known as the tropical orbit period, this is equal to a year on Earth.

Orbital Velocity (km/s or miles/s) - The average velocity or speed of the planet as it orbits the Sun, in kilometers per second or miles per second.

Orbital Inclination (degrees) - The angle in degrees at which a planets orbit around the Sun is tilted relative to the ecliptic plane. The ecliptic plane is defined as the plane containing the Earth's orbit, so the Earth's inclination is 0.

Orbital Eccentricity - This is a measure of how far a planet's orbit about the Sun (or the Moon's orbit about the Earth) is from being circular. The larger the eccentricity, the more elongated is the orbit, an eccentricity of 0 means the orbit is a perfect circle. There are no units for eccentricity.

Axial Tilt (degrees) - The angle in degrees the axis of a planet (the imaginary line running through the center of the planet from the north to south poles) is tilted relative to a line perpendicular to the planet's orbit around the Sun.
*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).

Surface Pressure (bars or atmospheres) - This is the atmospheric pressure (the weight of the atmosphere per unit area) at the surface of the planet in bars or atmospheres.
*The surfaces of Jupiter, Saturn, Uranus, and Neptune are deep in the atmosphere and the location and pressures are not known.

Visual Magnitude - The visual brightness of the body if it were one AU from the Earth at a phase angle of zero, dimensionless. The smaller (more negative) the number, the brighter the object.

Planetary Fact Sheet - Metric Units
Planetary Fact Sheet - U.S. Units
Planetary Fact Sheet - Values compared to Earth
Index of Planetary Fact Sheets - More detailed fact sheets for each planet

Author/Curator:
Dr. David R. Williams, dave.williams@nasa.gov
NSSDC, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771
+1-301-286-1258