position & time from a picture of the sky.

[michel123456]

Stars "move" quickly with Earth's rotation, but that won't tell the time AND the position.

To get both, you need different movements that don't result from Earth's rotation. For that, I claim that planets move too slowly against the stars or against another, so the date or time obtained that way is inaccurate, and this results in a very wrong position.

Hence my preference for the Moon or for geosynchronous satellites.

In 100 BC you have no geosynchronous satellite.

But you have the planets and the moon.

I don't know what precision that gives, but if you are not on a ship but on the ground, and if you can repeat your observation for several consecutive nights I guess you can get an acceptable idea of your location. Enough to put an entire island, or a city, or even a region on the world map.

[Enthalpy]

There's one better method to know the longitude (latitude already results from Polaris' height). With a deep well, you know when it's Solar noon. Better tools than a well were feasible then.

 

It just needs an instantaneous signal (shadowed fire?) between two locations that observe their local Solar noon, and you have the difference of longitude between the locations, from the time interval. Which limits the distance between the locations, so you have to iterate for longer distances.

 

One ancient Greek measured the radius of Earth that way, which was necessary to convert a longitude angle in a distance. Feasible, if one distance between two locations had been measured previously by other means.

 

Surprisingly, he compared the longitude angle, not a latitude angle, to the distance. This suggests that measuring a short time lapse was easier to them than an angle - or rather, that the latitude-to-distance conversion was already well known, but they ignored if the elliptic Earth was a sphere.

 

This gives relative positions of port cities in latitude, longitude, and through a conversion, distance. It needs a significant effort, but worth it for a people of seamen and scientists.

 

Many documented clock designs date back to that period, which is consistent with the needs of sea navigation. From Piraeus to Alexandria you don't want to follow the coast, to Creta you can't.

[michel123456]

Eratosthenes

(Opening the parenthesis:
As much as I know, Eratosthenes measured the angle between Alexandria and a town south of Alexandria upon the tropic. That is not longitude.
http://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_the_Earth.27s_circumference

 

cleaning the parenthesis)

 

----------------------------------------------------

 

I was thinking about this mechanism

The Antikythera mechanism



Here a computer generated image of the front panel, as assumed

 

 

 

 

And the back panel

 

 

There was only one input, from a handle on the side of the mechanism.

IOW, turning the handle, you obtained a configuration of the planets on the front panel and a set of 2 indications* on the back panel.

The most intriguing feature is that the back panel is mainly composed of 2 spirals (made up of half circles with different centers). And as everybody knows, a mechanism with spiral has a beginning and an end, unlike a circle. IOW you wouldn't be able to turn the handle as many turns you wanted, at some time the mechanism would reach the end.

Note: upon on each pointer of the spirals had a pointer that slide into the gap of the spiral.

 

That all makes me think that the spirals were intended to give a set of coordinates. A position.

 

*not exactly 2, but 5 since there are 3 other small dials on the back panel.

Edited November 15, 2013 by michel123456

[michel123456]

The method relies on the relatively quick movement of the moon across the background sky, completing a circuit of 360 degrees in 27.3 days. In an hour then, it will move about half a degree,^[1] roughly its own diameter, with respect to the background stars and the Sun.

 

This quote is taken from the wiki article about defining position with the Lunar Distance method.

 

The method uses a set of published tabulated values.

Using a sextant, the navigator precisely measures the angle between the moon and another body.^[1] That could be the Sun or one of a selected group of bright stars lying close to the Moon's path, near the ecliptic. At that moment, anyone on the surface of the earth who can see the same two bodies will observe the same angle (after correcting for parallax error). The navigator then consults a prepared table of lunar distances and the times at which they will occur.^[1][6] By comparing the corrected lunar distance with the tabulated values, the navigator finds the Greenwich time for that observation. Knowing Greenwich time and local time, the navigator can work out longitude.^[1] Local time can be determined from a sextant observation of the altitude of the Sun or a star.^[7][8] Then the longitude (relative to Greenwich) is readily calculated from the difference between local time and Greenwich Time, at 15 degrees per hour.

 

Instead of tabulated values, one could imagine having at hand a mechanism. Both do the same job.

 

Which reinforces my suggestion that the Antikythera Mechanism was a positioning device. Probably used in the purpose of making maps.

Edited January 26, 2014 by michel123456