by Nitpicker » Thu Oct 31, 2013 1:10 am
geckzilla wrote:BMAONE23 wrote:CharliePatriot wrote:Need some help here. Exactly what distance is denoted by "Light Year".? Have heard several and don't now which is true.
It is the distance that light travels in one year
Through a vacuum. An important but often neglected variable.
Well, if we're getting
technical, it is the distance that light travels in a vacuum in one
Julian year (365.25 days [86,400 SI seconds per day]).
Chris Peterson wrote:Or in civilized units that the rest of the world understands, one light year is
9 454 254 955 500 kilometers
Not that we're that civilised, nor civilized, but the rest of us know them as
kilometres. (Picky, picky.)
For a unit that is truly civilised, militarised, internationalised and also rather natural to planet Earth, there is the
nautical mile:
http://en.wikipedia.org/wiki/Nautical_mile
"
The nautical mile (symbol M, NM or nmi) is a unit of length that is about one minute of arc of latitude measured along any meridian, or about one minute of arc of longitude at the equator (both at sea level). By international agreement it has been set at 1,852 metres exactly (about 6,076 feet)."
But I digress. Like
kilometres,
nautical miles are not much chop for astronomy (and
statute miles can take a hike too -- they're not bad for hiking). The more convenient
light years and
astronomical units, both have
exact definitions in units of the SI
metre (which is itself defined relative to the speed of light in a vacuum). And the (pretty nifty)
parsec is defined exactly as 648000/π
astronomical units.
[quote="geckzilla"][quote="BMAONE23"][quote="CharliePatriot"]Need some help here. Exactly what distance is denoted by "Light Year".? Have heard several and don't now which is true.[/quote]
It is the distance that light travels in one year[/quote]
Through a vacuum. An important but often neglected variable.[/quote]
Well, if we're getting [i]technical[/i], it is the distance that light travels in a vacuum in one [i]Julian [/i]year (365.25 days [86,400 SI seconds per day]).
[quote="Chris Peterson"]Or in civilized units that the rest of the world understands, one light year is
9 454 254 955 500 kilometers[/quote]
Not that we're that civilised, nor civilized, but the rest of us know them as [i]kilometres[/i]. (Picky, picky.)
For a unit that is truly civilised, militarised, internationalised and also rather natural to planet Earth, there is the [i]nautical mile[/i]:
http://en.wikipedia.org/wiki/Nautical_mile
"[i]The nautical mile (symbol M, NM or nmi) is a unit of length that is about one minute of arc of latitude measured along any meridian, or about one minute of arc of longitude at the equator (both at sea level). By international agreement it has been set at 1,852 metres exactly (about 6,076 feet).[/i]"
But I digress. Like [i]kilometres[/i], [i]nautical miles [/i]are not much chop for astronomy (and [i]statute miles[/i] can take a hike too -- they're not bad for hiking). The more convenient [i]light years[/i] and[i] astronomical units[/i], both have [u]exact[/u] definitions in units of the SI [i]metre[/i] (which is itself defined relative to the speed of light in a vacuum). And the (pretty nifty) [i]parsec [/i]is defined exactly as 648000/π [i]astronomical units[/i].