120px|Photograph of the planet Neptune (large, background) and its moon Triton (small, foreground), taken by [[Voyager 2 as it entered the outer Solar System]]

The definition of planet has comprised many different things. Over the millennia, use of the term was never strict and its meaning has blurred to include or exclude a variety of objects. By the end of the 19th century, the word planet had settled into a comfortable working term. It applied only to objects in the Solar System; a number small enough that any differences could be dealt with on an individual basis. After 1992 however, astronomers began to discover many additional objects beyond the orbit of Neptune, as well as hundreds of objects orbiting other stars. These discoveries not only increased the number of potential planets, but expanded their variety and peculiarity. The issue of a clear definition for planet came to a head in 2005 with the discovery of the trans-Neptunian object Eris, a body larger than the smallest then-accepted planet, Pluto. In its 2006 response, the International Astronomical Union released its decision on the matter. This definition, which applies only to the Solar System, states that a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects around its orbit. Under this new definition, Pluto, along with the other trans-Neptunian objects, does not qualify as a planet. (more...)

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_______________________________ Today's selected anniversaries:

1201:

John Komnenos the Fat briefly seized the throne of the Byzantine Empire from Alexios III Angelos, but he was soon caught and executed. http://en.wikipedia.org/wiki/John_Komnenos_the_Fat

1917:

World War I: The Battle of Passchendaele began near Ypres in West Flanders, Belgium, with the Allied Powers aiming to force German troops to withdraw from the Channel Ports. http://en.wikipedia.org/wiki/Battle_of_Passchendaele

1941:

The Holocaust: Under instructions from Adolf Hitler, Hermann Göring ordered SS General Reinhard Heydrich to settle "the final solution of the Jewish question". http://en.wikipedia.org/wiki/Final_Solution

1991:

The Soviet Union and United States signed the bilteral START I treaty, the largest and most complex arms control treaty in history, which eventually removed 80% of all strategic nuclear weapons then in existence. http://en.wikipedia.org/wiki/START_I

2006:

Following intestinal surgery, Fidel Castro provisionally transferred the duties of the Cuban presidency to his brother Raúl . http://en.wikipedia.org/wiki/2006%E2%80%932008_Cuban_transfer_of_presidential_duties

_____________________________ Wiktionary's word of the day:

orography (n): The scientific study of mountains, or the mountainous features of a given region http://en.wiktionary.org/wiki/orography

___________________________ Wikiquote quote of the day:

Only a crisis — actual or perceived — produces real change. When that crisis occurs, the actions that are taken depend on the ideas that are lying around. That, I believe, is our basic function: to develop alternatives to existing policies, to keep them alive and available until the politically impossible becomes politically inevitable. --Milton Friedman http://en.wikiquote.org/wiki/Milton_Friedman