Longtitude?

[ste]

Hi guys.

Now then, I was watching a Ray Mears program the other night and it was on sea survival. There was a "feature" about a guy who survived in a life raft for some ridiculous amount of time.
He was attempting to work out his position at differnet times. He could work out latitude (using a make shift sextant - I can go into more detail if people wish), but not longtitude, so couldn't get an exact position.
This got me thinking as to how longtitude is calculated. I'm guessing that you need to know what time zone you're in and what the time zone is. But what if you don't know the time? (If you need the time) How can you work it out? I considered the shadow stick method, but if it was used on a raft it wouldn't work.
Any ideas?

 

[Stryver]

Latitude would have fixed references regardless of where Greenwich happened to be, Longitude requires an invented reference point.

The easiest method would be to know a time and location reference. This could be GMT, or a local time that you knew the longitude for. Then correct for daylight savings. Apply the difference between your known time/longitude and local Noon, and you get your longitude. This does require having an accurate, set, timekeeping device. For all practical purposes, it means know when Noon is at some fixed reference point.

The second method merely requires a mostly accurate timekeeping device, but it doesn't have to be set to any particular time, you'll set it to local Noon. It does, however, require a fairly in-depth celestial knowledge. You would have to have charts for one or more stars, and compare the azimuth time for a given star to your local noon, and the time of year. You would be comparing the star azimuth : noon of your locality, and the same, predicted, time for a known reference point.


When ships first hit the oceans, star charts were more accurate. Today, I think the average watch is accurate to within a few minutes each year. A minute worth of time innaccuracy would put you 15 minutes off your location (1/4 degree longitude).


Hope that answers your question.


Stryver

 

[Walt Welch]

There was a great program on TV recently about this very subject. There was a prize offered by Queen Anne of a few thousand pounds (a lot of money back then) for the first person to invent a practical time piece which would allow ships to determine their longitude.

A rather unknown and poor clockmaker made the first (and then two more) timepieces, about the size of an old alarm clock, which were accurate enough to serve the purpose.

Unfortunately, the Longitude Committee wasn't about to give this rather common person the prize; it obviously needed to go to a prominent English scientist. Finally Parliament had to award the money to him by direct act.

Timepieces accurate enough to be used for determining longitude are to this day called chronometers.

Walt