Mercury to put in a rare appearance
12th Nov 1999
Solar maximum is just around the corner, which means that the Sun is
peppered with sunspots. Just today there were 5 groups and at least
50 individual spots visible on the surface of our star. Sunspots --
cool areas created by twisted magnetic field lines poking through the
sun's surface -- move rather slowly. They're usually visible for about
two weeks as they move from east to west with the solar rotation. On
Monday, November 15, observers in the Pacific hemisphere can catch a
glimpse of a different kind of sunspot -- a black dot that zips across
the sun in little more than an hour. It's not really a sunspot; it's
Mercury, the nearest planet to the sun!
ABOVE : This white light image of the sun shows several sunspot groups
on November 12, 1999. On Monday, November 15, another tiny dark spot
will appear briefly near the Sun's northeastern limb when the planet
Mercury crosses in front of the Sun.
The transit or passage of a planet across the disk of the Sun is a relatively
unusual occurrence. As seen from Earth, only transits of Mercury and
Venus are possible. On average, there are 13 transits of Mercury each
century. Transits of Venus are even more rare. They occur in pairs with
more than a century separating each pair. On 1999 November 15, Mercury
will cross the visible disk of the Sun for the first time since 1993.
At approximately 2115 UT (4:15 p.m. EST) the black disk of the planet
will appear at the Sun's northern limb, about a third of the way around
from North to East. These cardinal directions are easy to figure by
simply nudging an equatorial mounted telescope back and forth on both
axes. The black disk of the planet will be small -- 9.9 arcseconds across
-- and blacker than any normal sunspot.
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