Planets and Dwarf Planets

As defined by the International Astronomical Union in 2006:

Planet (noun): a celestial body that orbits the Sun, has sufficient mass to assume hydrostatic equilibrium and to clear its orbital path of debris, and is not a moon.

Dwarf planet (noun): a celestial body that orbits the Sun, is not a moon, has sufficient mass to assume hydrostatic equilibrium but not to clear its orbital path of debris.

A dwarf planet, thus, has almost exactly the same definition as a planet, except that it does not have enough gravity to clear its orbit. Because gravity is dependent on mass, the essential difference between a planet and a dwarf planet is mass.

"Planet" comes from the Greek planetoi, "wanderers." This is because the stargazing Greeks saw the planets and noted that they did not follow the same path in the sky as the stars; they wandered through the skies. Thus the name meaning "wanderer."

Bibliography

1. Moche, Dinah L. Astronomy: A Self-Teaching Guide. Hoboken, NJ: John Wiley & Sons, Inc., 2009.

2. Wikipedia. "IAU definition of planet." Wikipedia. 16 May 2013. Wikipedia. May 21, 2013. <http://en.wikipedia.org/wiki/IAU_definition_of_planet>

Syzygy

Syzygy (noun): the nearly straight-line configuration of three celestial bodies (as the Sun, Moon and Earth during a solar or lunar eclipse) in a gravitational system.

Syzygy is when three of the bodies in the solar system line up in a row. This term is most often used to refer to a Sun-Moon-Earth lineup. During one of these, the gravitational stress on the moon can cause a Moonquake, the Moon's equivalent of an earthquake. However, it is highly unlikely that a Sun-Moon-Earth syzygy can cause an earthquake, since the Earth is 82 times more massive than the Moon.

This word comes from the Latin syzygia, "conjunction," and the Greek syzygos, "yolked together."

Bibliography

1. Moche, Dinah L. Astronomy: A Self-Teaching Guide. Hoboken, NJ: John Wiley & Sons, Inc., 2009.

2. Wikipedia. "Syzygy (astronomy)." Wikipedia. 31 March 2013. Wikipedia. 21 May 2013. <http://en.wikipedia.org/wiki/Syzygy_(astronomy)>