SKYLIGHTS
Astronomy news for the week starting Friday, April 6, 2001.
The Moon passes through its full phase this week the night of
Saturday, April 7, somewhat after Moonrise in North America. The
Moon will therefore rise a bit before sunset, but since the full
phase is passed while the Moon is visible, it will also set a bit
after sunrise. The night of Thursday the 12th, the waning gibbous
Moon will pass to the north of Mars and make a handy reference with
which to find the red planet.
Mars now officially moves into the evening sky, rising just before
midnight (1 AM daylight time). It is also very close to bottoming
out in its current journey along the ecliptic path, having recently
entered Sagittarius. Nearly 23 degrees south of the celestial
equator, the planet rises well into the southwest. During most of
this month, as Mars crosses -- transits -- the celestial meridian
to the south, Venus rises in the northwest, but after the beginning
of twilight, so the planet will appear low in the sky. On Friday
the 6th, Venus passes 10 degrees north of Mercury, the little
planet so close to the Sun that it is not possible to see from
North America.
The evening sky still provides the best planetary view, with
Jupiter and Saturn glorious to the west among the stars of Taurus.
Saturn now sets around 10:30 (Daylight Time), Jupiter an hour
later, the two slowly drawing apart. Slowly being overtaken by the
Sun, the pair will, one at a time, disappear into twilight during
the month of May.
Daylight time has just been launched over most of the US and
Canada. The time of day refers to the "hour angle" of the Sun
(with the effects of orbital eccentricity and axial tilt smoothed
out), the angle the Sun makes with the celestial meridian, each
hour past noon corresponding to another 15 degrees. Time is really
longitude dependent. Standard time is local time at specified
meridians. Technically, everyone in a band 15 degrees wide
centered on that meridian keeps the same time (though the bands are
wildly shifted for political and social reasons). If you are west
of your meridian, the Sun seems to set late, while if east it will
seem to set early. In daylight time, we shift to the time zone to
the east of us, and as a result, the Sun seems to set an hour later
than "normal."
Dim Cancer, flanked by Gemini to the west and Leo to the east,
crosses the meridian around 9 PM. Above it is even dimmer Lynx,
below it the circlet that makes the head of Hydra, the Water
Serpent. Much farther down is Pyxis, the Compass, all a challenge
and nearly impossible to see under the glare of full Moonlight.
