Re: Sun's distance

[Gary Karshner <karshner@STMARYTX.EDU>]

> During the time between the first and fourth volumes of Lalande's
treatise,  >the problem was solved as follows:
> Mercury's orbit is quite eccentric, and also inclined a few degrees from
> the Earth's orbital plane, the ecliptic.  Thus it is a rare event that
> Mercury actually passes across the solar disk as seen from Earth.  Such
> "transits" of Mercury happen in pairs, about two years apart,  with an
> interval of about 125 years between pairs of transits.

Peter,
        You are making me feel rather old - I have seen three transits of mercury!
They occur about a minimum of seven but typically 13 years between
transits. Transits of Venus on the other hand are the order of 125 years
between pairs of transits. The pairs are eight years apart. The last pair
were 1874-1882 and the next ones in June 2004 and 2012.
        An unanticipated problem with the early transit timings was that as
Mercury or Venus passes over the limb of sun they appear to stick to edge
and break away like a drop of water. The effect is caused by the
diffraction of sun light around the disk of the planet. The transiting
planet appearing smaller because of the light bent around the edge of the
disk. When the actual edge contacts the edge of the disk there is no longer
a source of light to be refracted and the hidden portion of the disk
appears. Since the eighteenth century astronomers had not anticipated this
effect it played havoc with their timings.
        The thing that gets me about aging is I remember the stories but forget
the facts. Luckily in this case I could look them up!

Gary
Gary Karshner

St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU