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Our home planet, third from the sun. Earth is the only planet in our solar system not named for an ancient god. In the image on the right, this is the view Apollo 8 astronauts had of Earth while orbiting the moon.
More on Earth >>

Earth as seen from the moon.

The eccentricity of an ellipse (planetary orbit) is the ratio of the distance between the foci and the major axis.In other words, the more flattened the circle (ellipse), the more 'eccentric' the orbit. Pluto has a highly eccentric orbit around the sun.

Diagram of eccentric orbits

The apparent path of the Sun around the sky.

effusive eruption

a relative quiet volcanic eruption which puts out basaltic lava that moves at about the speed one walks; the lava is fluid in nature; the eruptions at the Kilauea volcano on the island of Hawaii are effusive.


The material thrown from an impact crater during its formation.


A flattened circle or oval. That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Kepler based on the careful observations by Tycho Brahe. An ellipse is essentially a circle that has been stretched out of shape. When describing ellipses, the eccentricity defines how "stretched out" it is. In the case of the moon (diagram), when it is closest it is at perigee and when it is farthest from Earth is is at apogee.

Diagram of an elliptical orbit

The process of physically removing weathered materials; the wearing away of soil and rock by weathering, mass wasting, and the action of streams, glaciers, waves, wind and groundwater.


A change in the gene pool of a population over time.

explosive eruption

A dramatic volcanic eruption which throws debris high into the air for hundreds of miles; lava is low in silicate; can be very dangerous for people near by; an example is Mount St. Helens in 1980.

exponential notation

"1.23e4" means "1.23 times 10 to the fourth power" or 12,300; "5.67e-8" means "5.67 divided by 10 to the eighth power" or 0.0000000567.


The failure of a taxonomic group to produce direct descendants, causing its worldwide disappearance from the record at a given point in time.

extrasolar planet

A planet in orbit around a star other than the Sun.

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Last Updated: 25 Oct 2013