SKYLIGHTS
Astronomy news for the week starting Friday, January 12, 2001.
The week begins with the Moon in its waning gibbous phase. Passing
third quarter the morning of Tuesday the 16th about the time of
sunrise, it finishes the week as a waning crescent. The morning of
Wednesday the 17th finds the Moon to the northwest of Mars (now in
Libra), the morning of Thursday the 18th to the northeast of the
red planet.
The week belongs not to Mars or the Moon, however, but to Venus,
which reaches greatest eastern elongation from the Sun shortly
after it sets the night of Tuesday the 16th. "Elongation" is the
angle that any planet makes with the Sun. Venus, closer to the Sun
than we are, can never get more than 47 degrees from the Sun, its
maximum angle. The planet thus achieves its greatest visibility
this week. Since it is now at the tangent point in its orbit as
seen from Earth, it will (through the telescope) also appear as the
quarter moon does in the sky, one half seen in sunlight, the other
half in darkness. From here on to conjunction with the Sun on
March 29, the planet will appear as an increasingly thinner
crescent. At the same time, it is approaching the Earth and
appearing ever-larger, and thus brighter, maximum brilliancy to be
reached on February 21. Though the angle between the Sun and Venus
will now decrease, since the Sun is moving northward along the
ecliptic and is setting later, Venus will continue to set later as
well until early February, the "evening star" quite dominating the
early night sky.
At the other end of the planetary spectrum, Mercury passes
conjunction with Neptune (Mercury two degrees south) on Saturday,
the 13th, the solar glare causing the event to go unseen. In
between, the early evening eastern sky is dominated by Saturn and
Jupiter, both still retrograding (but not for long) in Taurus. The
three planets, Venus, Saturn, and Jupiter, act like a dotted line
in the sky that to a good approximation shows the ecliptic and the
path the Sun will soon follow as northern winter wanes and turns to
spring.
Look to the southeast around 8 PM to see Orion climb the sky. To
the right, on the meridian to the south, winds dim Eridanus, the
celestial River that represents the "River Ocean" of classical
times. From modest Cursa, just northwest of Rigel in Orion,
Eridanus flows west, and then curves south below the horizon of
northern latitudes, ending in brilliant Achernar, which can be seen
only south of 32 degrees north latitude. High above, the Pleiades,
the Seven Sisters star cluster, crosses the meridian as well. Most
eyes see 6 stars, but those with sharper vision might catch 8 or
even more, the view richly enhanced by binoculars.
