[sci.astro] StarDate: November 12 More Transit of Mercury

dipper@utastro.UUCP (Debbie Byrd) (11/12/86)

What happens when Mercury passes in front of the sun -- after this.

November 12  More Transit of Mercury

Today -- actually, tonight for most of us -- the planet Mercury will
transit the sun.  That means it will cross in front of the the face of
the sun as seen from Earth.

The sun won't be up for most of us.  But the entire transit will be
seen from India, Southeast Asia, China, Japan, Indonesia, Australia and
New Zealand.  Observers in those locations who are equipped with
telescopes and safe solar filters will witness the tiny black dot of
Mercury moving across the brilliant face of the sun.  The whole event
will take approximately five hours.

An interesting phenomena occurs during transits as the little disk of
the planet comes wholly inside the disk of the sun.  Then the so-called
black drop effect is seen, when the planet will seem to be attached to
the edge of the sun by a thin column or thread.  That illusion ends
when the thread appears to break, and the planet is entirely surrounded
by sunlight.

Since Mercury and Venus are the only planets orbiting inward from
Earth, they are the only ones we ever see transiting the sun.  While
transits of Mercury occur 13 times each century, transits of Venus are
much more rare.  The last one was on December 6, 1882.  The next one
will be on June 8, 2004.

If you were on Mars, however, you'd occasionally have the pleasure of
seeing Earth transit the sun!  The last transit of Earth as seen from
Mars in this century took place on May 11, 1984.  If there had been
astronomers on Mars on that date, and if they'd had good weather,
telescopes, and filters to dim the blinding sunlight, they could have
seen Earth as a black dot moving across the sun's disk.

Script by Deborah Byrd.
(c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin