Finding longitude

[Jens]

I'm reading the Island of the Day Before, by Umberto Eco, so of course I've been thinking a bit about longitude.

I'm sort of confirming this, but if you wanted to find your longitude (compared to say, Greenwich), and didn't have a good clock, one way to do it would be to find your latitude, and then use a universal measure of time (i.e., an event that would be seen at the same time wherever you are on the earth, as long as you can see it). And the one event that is pretty much seen at the same time (sinceI think there is relatively little parallax) is the transit of Jupiter's moons.

So I guess that you would need three things. (1) a small telescope, (2) a chart to know what time it is at your latitude, say a chart that shows what stars rise at what time at what latitude at what time of year, and (3) a chart showing what time Jupiter's moons transit.

Is this pretty much correct? Maybe there's an easier way to tell local time. And also, I think that there wouldn't be much parallax with Jupiter, because again, I'm not positive.

[Chuck]

Sounds good unless it's cloudy or Jupiter is below the horizon.

[Senor Molinero]

The positions of Jupiter's moons was actually first posited as a method of determining time accurately enough to measure your longitude. Then a discrepancy was found in the predicted and actual positions. The length of light travel time between when we are closest to and furthest from Jupiter. The discrepancy was used to calculate the speed of light once the scientists had worked out what was going on.

[Acolyte]

Um... the time at your latitude is the purpose for the Chronograph isn't it? If you're at latitude 30º North you need the time at your start point to get the difference between where you are & whatever event you're using to determine the longitude. For any given longitude, the time is the same no matter what latitude you're standing at...

[astromark]

Protractors, sextants and a good watch...A stick and some string. A sunny day and a sundial... lots of charts for all the information required.. but wait Jens, I know this is cheating but...
Just dial up Google Earth and zoom in to your location. Bottom left of your screen is the position of the cursor. Or obtain a GPS device from a local sports store...
This is 2008. Use the technology at our finger tips. Mark.

[Acolyte]

...but if you wanted to find your longitude (compared to say, Greenwich), and didn't have a good clock...

About that 'good watch... *grins*

also, sundial will not work. 6AM on the dial will show as 6AM no matter what longitude you are at. Not sure what you plan to do with the stick & string - beat the heck out of some poor native till he tells you the longitude perhaps?

[Hornblower]

A good ephemeris of the Moon, and measurements of its position relative to background stars with a sextant, will enable you to determine Greenwich time within a couple of minutes, and thus find your longitude within about half a degree. This is assuming a precision of one arcminute in reading the sextant. Since true north, your latitude and the Moon's separation from the zenith can be measured accurately, you can correct for the parallax.

If I am not mistaken, this was the preferred method of the British Admiralty when John Harrison was developing his famous chronometer.

Jupiter's moons make a good clock for long term reckoning, and could be used to pin down a previously uncharted position on land. For a quick finding at sea, the lunar position method above would be faster unless you were fortunate in having a Jovian shadow transit or eclipse event at just the right time.

[schlaugh]

You also need to measure your speed and keep track of direction (so many knots on a SSE bearing) so that you can establish duration as one input into locating current position using a relative timepiece such as a half-hour sand clock or stopwatch.

It does no good to know that Ganymede is in a given position if you don't know how fast you were moving - or in which direction - the last time you noted Ganymede's position (or Io, Europa, etc.).

[Nick Theodorakis]

I recommend Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, by Dava Sobel, for those with a historical interest in the problem.

Nick

[Hornblower]

You also need to measure your speed and keep track of direction (so many knots on a SSE bearing) so that you can establish duration as one input into locating current position using a relative timepiece such as a half-hour sand clock or stopwatch.

It does no good to know that Ganymede is in a given position if you don't know how fast you were moving - or in which direction - the last time you noted Ganymede's position (or Io, Europa, etc.).

Seaworthy captains and navigators do that as standard procedure. They could get away with not doing it if they were anchored in shallow water or ashore on an uncharted island.

[grant hutchison]

I'm sort of confirming this, but if you wanted to find your longitude (compared to say, Greenwich), and didn't have a good clock, one way to do it would be to find your latitude, and then use a universal measure of time (i.e., an event that would be seen at the same time wherever you are on the earth, as long as you can see it). And the one event that is pretty much seen at the same time (sinceI think there is relatively little parallax) is the transit of Jupiter's moons.

Yep, that's how Jean Richer resynchronized his clock in Cayenne with the master clock at the Paris observatory, so that he could measure the position of Mars at the same time as a measurement was taken in Paris. That allowed the first triangulation of the distance to Mars, and therefore the first good estimate of the absolute size of an astronomical unit. He used ane ephemeris of the Galilean satellites which had been specifically calculated for that task.
A little later, as Senor Molinero says, cyclical anomalies in the ephemeris for the Galiliean satellites showed up the finite speed of light.

Grant Hutchison

[mugaliens]

There are two ways, and both require an almanac (table of celestial events).

The first way is via lunar distance, which is the angle between the Moon and another celestial body, such as the sun, one of various stars, or a planet.

The second method does indeed involve measuring the positions of the moons of Jupiter.

A typical approach would be to take the measurements while simultaneous noting the time on a clock, then figure out the actual time, subtract the two, and apply the difference to a second clock. The second clock need not keep time all that well as the next action is to take your normal celestial measurements within a few minutes of putting in the accurate time.

[Veeger]

Interesting discussion, I think. I have been involved in a similar discusssion on another board and we have speculated greatly about whether ancients - say 2500BC - had the ability to determine longitude. One guy has speculated that certain ancients where able to determine precise bearing angles to sacred sites over 1000km distance through some method now unknown. I have long thought about the possibilities but have never really found a suitable solution. Could it have been possible?

[KaiYeves]

According to Longitude by (the all out amazing) Dava Sobel, that was the method used to find longitude on land until reliable clocks were made, but because Jupiter can't always be seen, it couldn't be trusted by sailors.

[Veeger]

You mean observing the moons of Jupiter in 2500BC?

[Hornblower]

You mean observing the moons of Jupiter in 2500BC?

No, after 1609, when Galileo first observed them with a telescope.

[mugaliens]

The earliest known use of lunar distance was by Amerigo Vespucci, in 1499, by means of comparing the Moon's position with the position of Mars.