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Brad,

Good point about transits! Again, you can work out the speed with a really simple plane trigonometry diagram "looking down" on the Solar System. Or in the case of a transit, the triangle collapses, so consider this: Mercury is advancing at a Sun-centered rate of 4 degrees per day while the Earth is moving at a Sun-centered rate of 1 degree per day (we've known these heliocentric rates since the first heliocentric models of the Solar System). Relative to the position of the Earth in the sky as seen from the Sun, Mercury is advancing 3 degrees per day. Now let's flip it around and view this motion from the Earth. The Earth is further from Mercury than the Sun is. Since Mercury orbits at a distance of 0.4 AU and the Earth orbits at 1.0 AU, the Earth when Mercury is in transit is 0.6 AU from Mercury. So it's 1.5x further from Mercury than the Sun, and therefore the angular rate of Mercury across the sky relative to the Sun is two-thirds of the heliocentric rate or 2 degrees per day. There's some rounding down here, so let's call it 2.4 degrees per day. Note that the rate can be somewhat higher or lower depending on whether Mercury is near perihelion or aphelion since it has a relatively eccentric orbit, but that averages out so we can ignore it for now. That rate of 2.4 degrees per day is 0.1 degrees per hour. Divide both by sixty and we get 0.1 minutes of arc in one minute of time. Lunar distances change at a rate of about 0.1 minutes of arc in 12 seconds of time, so a rare Mercury transit is five times slower than a common lunar observation and therefore not as useful for determining longitude by a long shot. It would work, just not very well. Also, historically, the almanac data for Mercury was not particularly accurate until the early 19th century.

There is, however, an important special case regarding Mercury transits and longitude determinations. You can do it without a high-quality sextant by timing contact events. So suppose you're an explorer in Australia in 1820. You have a decent telescope, and you have a rather low-grade octant and an artifical horizon, but you cannot afford a quality sextant. By good luck, the day of a Mercury transit arrives, and the sky is sparkling clear. You observe multiple altitudes of the Sun to get local time, before, during, and after the transit, and you look through the telescope and mark the instants when Mercury's tiny disk touches the limb of the Sun, perhaps counting off seconds to the next Sun altitude observation. Then you write up your observations and send them off to an astronomer with some skill who observed the same transit from a known longitude. Calculate... calculate... calculate. Wait for the mail. And then a letter arrives (on a Saturday!) informing you of your longitude based on your observations. Unfortunately, there was a typo in your first letter, and your longitude, as calculated, is in the middle of the South Atlantic. Bug detection is an iterative process. More letters are exchanged and eventually, weeks later, you know your longitude. Civilization has been advanced, and you move on to the next watering hole. :)

-FER

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