Astronomy news for the week starting Friday, January 21, 2000.
Having been eclipsed last Friday, the Moon once again survives to
wane through its gibbous phase, heading toward third quarter, that
phase to be reached in the early morning hours of Friday the 28th,
just about moonrise in North America. Eclipses come in pairs. If
the conditions are right for a lunar eclipse, then the preceding or
following new Moon must eclipse the Sun, and sure enough there will
be a partial solar eclipse on Saturday, February 5. It needs
advance notice, as it is visible only from Antarctica, and you need
time to make arrangements to go! Since the Sun is now up for 24
hours at the south pole, the scientists there will have a fine
Jupiter and Saturn still reign in the evening sky, both planets
high to the west of south at sundown. Though moving easterly
against the stars, the Sun is catching up to them, causing them to
slip a bit more to the west each evening. Jupiter now sets around
midnight, and Saturn follows an hour later. And though Venus still
dominates the morning, it is moving so quickly east against the
stars that it is catching up with the Sun, thereby getting lower in
the southeast each day. In between, Neptune passes conjunction
with the Sun on Monday the 24th; Uranus, not far to the east of
Neptune, follows suit in early February.
Winter somehow seems "clearer" than summer, the stars brighter.
Depending on your location, the skies can in fact be clearer, as
summer can be so humid. But in fact we are in part fooled because
Orion (now high to the south in early evening) and his companions
are loaded with bright stars that twinkle madly in the colder
crisper air. The champion of all twinklers is Sirius, the
brightest star in the sky, seen down and to the left of Orion.
Twinkling is caused by erratic refraction (the bending of light) in
the Earth's turbulent atmosphere. Along with refraction goes
dispersion (the spreading of colors), since each color bends
through a different angle, the reason that diamonds sparkle so.
The diamond of Sirius does just the same, as the dispersion of the
light seems the greatest in the brightest star. As a result,
Sirius is occasionally taken as some sort of "UFO." But watch the
even-brighter planets, which tend to stare down serenely at you,
their twinkling not very prominent. The reason is that to the eye,
even through the telescope, stars are effectively points, whereas
planets are extended disks. Each point on a planet's surface
twinkles just as much as Sirius, but the twinkling of the different
points averages out, and the planetary image to the naked eye
quiets down. Go watch!