Astronomy news for the week starting Friday, November 19, 1999.
With "Leonid fever" at an end, at least for this year (there is
still a possibility of activity next year), we look again at the
more-quiet sky and at the Moon, which at the beginning of the week
waxes toward full. As it goes, it passes four degrees south of
Jupiter the evening of Saturday the 20th, and three degrees south
of Saturn the night of Sunday the 21st. Full phase will be reached
just about midnight the night of Monday the 22nd with the Moon
close to the meridian to the south and set against the stars of
Taurus, the Moon to the south of the Pleiades -- the "Seven
Sisters" -- star cluster.
As the full Moon rises, look for the "Moon illusion," which makes
the Moon look much larger on the horizon than when it is high in
the sky. It is strictly an optical illusion, as the Moon has the
same angular diameter at midnight, a half a degree, as it does at
its rising or setting. (Constellations also look larger as they
rise). In fact the Moon is angularly quite small (as anyone who
has tried to take a picture of it will attest). As it passes the
Pleiades, note that it could easily fit within the cluster's
confines, the cluster double the angular lunar size (binoculars
By the end of the week the Moon will be well within its waning
gibbous phase as it rises progressively later after midnight. Less
than one day after full, the Moon passes perigee, its closest point
to the Earth. The coincidence will raise particularly high tides
at ocean beaches throughout the world.
On Tuesday the 23rd, the Sun enters the confines of the constell
ation Scorpius from Libra for the briefest of stays. Though
Scorpius is famed as one of the great classical constellations of
the zodiac, the Sun inhabits it for less time than any other,
passing through it in a mere week. The most southerly point of the
ecliptic (the winter solstice) is in Sagittarius, but Scorpius dips
considerably farther to the south, and only a piece of its modern
boundaries come up to meet the solar path. From Scorpius the Sun
will cross over the modern boundaries of the non-zodiacal
constellation Ophiuchus, in which it tarries much longer.
Directly north of the full Moon, find the brilliant autumn
constellation Perseus, whose main figure consists of branching
streams of stars. With the Moon out of the way later in the week,
we can appreciate the density of stars at Perseus's center, the
collection yet another naked-eye cluster.