Hello Art, and List, As far as I understand it, the method works on simultaneous observation of the planet's transit from different places. Because of the effect of parallax and the earth's diurnal rotation, the durations of the transit, observed from two widely separated places, will differ by a small amount of time. The actual working is beyond me, I'm afraid. The beauty is that you don't need the exact moment in time, just the duration.
For this to work, the planet used has to cross in front of the sun (only Mercury and Venus do) and, I suppose, has to be not too close to the sun because the duration difference would get small, making the method too sensitive to errors. The Mercury transit _was_ used, I believe, at least by Delisle of France in 1753 (he also proposed some improvements over Halley's method). Not that it would present a problem here, but Mercury is so close to the Sun that relativistic effects change the orbit's attitude all the time, i.e. perihelion and aphelion (if that is how they are spelled) are not fixed in space, but walk round and round. I gather this is where the expression "to Defy the Laws of Gravity" comes from, said of stark misbehaviour - not much of Relativity being known at the time Newton had this particular problem. Wishing you all a good view; Rudolf ________________________________ Van: [EMAIL PROTECTED] Onderwerp: Transit of Venus: and another thing I don't understand ... Why was the transit of Venus - really - so important in the determination of the size of the solar system? Although a transit of Mercury has a less favorable geometry, i.e., all other things being equal the accuracy is less, it happens all the time. Wasn't it used? -
