Archive for May 28th, 2009

Longitude, Part I

May 28, 2009

So yesterday we talked about how to find your latitude if you’re in the Northern Hemisphere (and I’m pretty sure there are ways to find it in the southern hemisphere with an astrolabe, but I don’t think they’re as straightforward).  Longitude, however, turns out to be tough no matter where you are.

For one thing, although the Earth spins so there are two natural poles, and therefore you can measure Latitude in relation to those poles, there isn’t any natural Longitude:  it’s all in relation to an arbitrary point.  According to this official site at Greenwich, Hipparchos was the first guy to use longitude formally, about 150 BCE,  and he used Rhodes in southern Greece as the key spot.  About 250 or 300 years later Ptolemy joined the fun, but he used the Canary Islands in Spain as the starting point.    Mecca was used as a Prime Meridian for many mathematicians.  France had one, as did the US and Canada.  Altogether there were lots and lots of 0°, which could get a little confusing, and so about 130 years ago a bunch of countries got together and decided that Greenwich could be #1 Number 0, and everyone was happy.  Except France.  France didn’t want to give up its own Prime Meridian through Paris, so they refused to vote for Greenwich and kept using their own meridian until…until… actually, I don’t know if they ever formally adopted the Greenwich one.  Here’s a picture of the Paris one:

Paris-meridienneFredA took this picture (published under GNU-FDL).

So the first problem is deciding on a reference point, and clearly that was a big one.  The next problem is figuring out how far away from that reference point you are, and that’s hard too because the earth keeps spinning.  The simplest way is actually to use time: the Earth makes one complete rotation in 24 hours 23 hours and 56 minutes, so the Earth turns 15° in just under an hour.  Which means that if your buddy calls you and says, “Dude, did you see that amazing eclipse that happened at 1:23?” and you say, “Yes I did, but here it happened at 2:15!” then it means that you are 52 minutes ahead, so that’s (52 minutes)*(1 hour/60 minutes)*(15°/1 hour)= 13° longitude further east than your friend (assuming the AM/PM match up).    You could also set a clock to noon, and then if your clock keeps working  you could compare it to local noon (when the sun is at the highest point in the sky) as you travel away from your starting point.

You don’t HAVE to use time to determine longitude, and in fact some of the great minds did look into ways to use the stars to find a way, but it did turn out that clocks were the key (as we’ll see in part II).