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What does the term limb of the earth (see this question, for example) or atmospheric limb mean? The phrase strikes me as very odd, since earth is nearly spherical. Do other planets with atmospheres also have a limb?

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1  
see earthobservatory.nasa.gov/IOTD/view.php?id=3338 . It talks about the limb of the moon. – anna v Oct 8 '12 at 15:51
up vote 8 down vote accepted

The etymology of limb for an astral object comes from the original Latin root. Limbus means border in Latin, a language in which all good students of last centuries were proficient, including astronomers; when describing heavenly bodies viewed through the telescopes they shortened it to limb.

  1. (astronomy) The apparent visual edge of a celestial body.
  2. (on a measuring instrument) The graduated edge of a circle or arc

Now limb as an appendage of a body or tree has a different etymology in English. It comes from Middle English lim and acquired the "b" later.

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A slightly pedantic point. Usually when someone cites etymology of an English word, unless you want to show the whole path (as in the link), one doesn't usually cite "Middle English" as an abbreviated history of the word, because Middle English drew heavily on both English and French - so "ME" alone doesn't really tell you where the word came from. A good pocket dictionary, where space is costly, would simply say "from Old English / Anglo Saxon / OE / AS" or "French / Fr" or "Latin /Lt" as appropriate. An exception is made if the word genuinely seems to have begun its life in the ME era. – WetSavannaAnimal aka Rod Vance Jun 26 at 0:43
    
But hey, +1, I've read that word a quite few times and been stricken by its oddness, but have always been in too much of a hurry to look it up. Now I know what it means! – WetSavannaAnimal aka Rod Vance Jun 26 at 0:44

The limb of the Earth is simply the horizon, or the edge of the Earth at the horizon, especially if you see it from a spacecraft. When you also explicitly see the thin layer near the horizon corresponding to the atmosphere, it's the atmospheric limb.

There's no problem with the Earth's being spherical because the spherical symmetry is respected by both phrases containing "limb". On the other hand, the "limbs" are limb-like because in the one-dimensional model of the Earth where the altitude is the only coordinate, the limb (in both senses) is at the end point, much like hands and feet are "end points of a body".

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protected by Qmechanic Jun 25 at 22:36

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