# The Earth

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 Mass 5.98 × 1024 kg Equatorial Diameter 12,756 km Polar Diameter 12,714 km Mean Density 5.52 × 103 kg m-3 Rotational Period (Siderial) 23 hours 56 minutes 04 seconds Rotational Period (Solar) 24 hours Mean Distance from Sun 149.6 ×106 km Orbital Period 365.25 days Orbital Eccentricity 0.0167 Mean Orbital Velocity 29.78 km s-1 Axial Tilt 23.45° Surface Gravity 9.80665 m s-2 Escape Velocity 11.2 km s-1 Albedo 37% Solar Irradiance 1367.6 W m-2

### Mass

Mass is the amount of matter that an object contains. On the Earth mass can be measured by weight.

### Equatorial and Polar Diameters

The equatorial diameter is measured through the Earth's equator. The polar diameter is measured through its poles. The Earth is flattened at its poles. Its shape is described as an oblate spheroid.

### Density

Density tells how concentrated the matter in a planet is. The Earth is over five times as dense as water.

### Rotational Periods

Relative to the stars, the Earth completes a rotation on its axis in 23 hours 56 minutes and 4 seconds. Relative to the Sun, it requires 24 hours by definition. The difference is caused by the Earth moving a little around the Sun during a day.

### Mean Distance From Sun

The Earth travels around the Sun in an elliptical orbit. The figure given is the average distance. The Earth is at its closest to the Sun in early January and farthest in early July.

### Orbital Period

This is how long the Earth takes to revolve around the Sun. It is the period defined as a year.

### Orbital Eccentricity

The orbits of most objects in the Solar System are ellipses. This curve resembles a flattened circle. The eccentricity describes how much the ellipse differs from a circle. An orbit with an eccentricity of 0 is a circle. An orbit with an eccentricity of 1 would be an open curve called a parabola. The Earth's orbit has a low eccentricity, close to zero.

### Mean Orbital Velocity

This is the Earth's average velocity in orbit around the Sun. The Earth changes its velocity as it travels in its elliptical orbit. It moves faster when it is closer to the Sun in accordance with Kepler's Second Law.

### Axial Tilt

The Earth rotates on its axis. This is the angle that the axis is tilted relative to the plane of the Earth's orbit. This tilt causes the seasons. One hemisphere is tilted towards the Sun for half the year and away from the Sun for the other half year.

### Surface Gravity

On the Earth, the acceleration due to gravity is 9.806 65 meters per second per second. Surface Gravity depends on a planet's mass and radius.

### Escape Velocity

This is the speed that an object must attain in order to escape from the Earth's gravitational field. Escape Velocity also depends on a planet's mass and radius.

### Albedo

This is the percentage of sunlight that is reflected by the Earth. It is 37% because of the mixture of land, sea and clouds. Cloud covered Venus has a larger albedo, cloudless Mars has a lower value.