Gordon Uber <[EMAIL PROTECTED]> writes: > The length of the tropical year was determined with a gnomen between > successive solar solstices. The length of the sidereal year was determined > from successive heliacal risings. > > From Time in History by G. J. Whitrow.
I have long wondered how to make "accurate observations of the sun relative to the stars" (as John Sheperd put it). Given the key word "heliacal rising", I have been able to find the definition and some discussions on the Net. I find it surprising that this could be, as John Sheperd said, "pinned down to a single day". Wouldn't this depend on the brightness of the star and the viewing conditions and God knows what? On the other hand, the position of a given star at sunrise will change by 1 degree from one day to the next, which seems like a manageable distance. And I suppose what counts (for present purposes) is not what the actual relationship between the sun and the star is, but just the reproducibility of the phenomenon. Still, you would need to take years where the meteorological conditions were comparable. As for determining the "length of the tropical year ... with a gnomon between successive solar solstices", I don't believe this is a good method. One can determine the exact date/time of an equinox much more accurately than that of a solstice (although the solstice is conceptually a bit easier to deal with). --Art Carlson
