SKYLIGHTS
Astronomy news for the week starting Friday, March 19, 1999.
The Moon will be in its waxing crescent phase the early part of the
week as it grows through first quarter on Wednesday the 24th, that
night rising just slightly gibbous. It, Venus, and Saturn play tag
with each other, the outcome quite predictable. As the two planets
draw closer, the Moon will pass just below the planetary pair the
night of Friday, the 19th, the three making a glorious triangle in
western twilight. Venus and Saturn then make their closest pass to
each other, with brilliant Venus standing three degrees to the
north of much more distant Saturn on Saturday the 20th. They will
be closest during mid-afternoon in North America, but there will be
little difference by the time they are visible, that night the Moon
well up and to the left of the planets. The Moon also plays tag
with Aldebaran, occulting (passing over) it on Monday the 22nd, the
event visible in Europe and the Middle East.
Jupiter is too deep into twilight to be seen, setting only half an
hour after the Sun, and Mercury is completely out of the action, as
it passes inferior conjunction with the Sun (between us and the
Sun) on Friday, the 19th. At such inferior conjunctions, Mercury
and Venus almost always pass above or below the Sun. This year is
special however, as on November 15 inferior conjunction occurs just
as Mercury is crossing the ecliptic path, allowing us to witness a
"transit" of Mercury and to watch it actually crawl slowly across
the solar disk (for safety, observed by projection with the
telescope only!). Transits of Mercury take place in May and
November in 7 and 14 year cycles. Transits of Venus are much
rarer, the last taking place in 1882, the next not until 2004, the
twentieth century devoid of them.
The week's biggest event is the passage of the Sun across the
vernal equinox in Pisces at 7:46 PM Central Time on Saturday, March
20 (the event celebrated by the conjunction of Venus and Saturn!),
or 01 hours 46 minutes Greenwich Time, Sunday, March 21, marking
the beginning of astronomical spring in the northern hemisphere and
of astronomical autumn in the southern. At that moment, the Sun
crosses the celestial equator and passes into the northern
celestial hemisphere. The Earth's axis will be perpendicular to
the direction to the Sun, the Sun will rise due east and set due
west, will pass overhead at the Earth's equator, and the days and
nights will be of approximately equal length. In fact, the day
will be a few minutes longer than the night because the Sun is a
disk rather than a point and because refraction by the Earth's
atmosphere at sunrise and sunset raises the Sun about half a degree
above its actual position.
