Photo of the Week. The Sun hides behind wind-blown
clouds.
Astronomy news for the week starting Friday, June 27, 2008.
The Moon runs through the latter part of
its
phases this week, starting just after third quarter
(which took place last Thursday, June 26), so that we get to watch
the crescent
wane to new, that phase taking place
on Wednesday, July 2, the waxing crescent
not readily visible until the evening of Friday, the Fourth of
July. As it travels near the ecliptic, the Moon passes well
south of little Mercury the morning of Tuesday, July 1, but
otherwise visits no other planets. That same day, Mercury reaches greatest
western elongation, making it modestly visible in eastern morning
twilight.
The first of the weeks's two "big events" is the passage of Mars less than a degree
to the north of Regulus in Leo the night of Monday, June 30, the
sight as good the following evening just as the sky is turning
dark. With Saturn to the
east of the pair, the trio will make a fine sight. Then watch over
the next few evenings as Mars pulls in between the ringed planet
and Regulus, the motion quite obvious (Mars and Saturn passing
their own conjunction the night of July 10th). Look early, as the
two now set in the west just after 11 PM Daylight Time, less than
an hour after the end of evening twilight.
The second of the two events is the passage of the Earth through its aphelion point the morning of
Friday, July 4 (helping celebrate the day), when the Earth is
farthest from the Sun, at a distance of
94,513,144 miles (152,104,160 km), 1.7 percent farther than
average. Given the obvious heat of the day (or the usual heat at
least), the distance between the Earth and the Sun has little to do
with the origin of the seasons, which are caused instead by the
23.4 degree tilt of the terrestrial rotation axis relative to the
orbital perpendicular. A bit over two days before aphelion
passage, the Moon goes through its perigee
, where it is closest to the Earth.
Among the ancient planets, that pretty much leaves us with Jupiter. Beautifully set just to the northwest of
classic Sagittarius, the giant
planet is nearing its opposition to the Sun on July 9. Now rising
in bright twilight, by the time the sky is dark, Jupiter dominates
the southeast, crossing the meridian
to the south a bit after local midnight (1 AM Daylight). Among the
outer planets, Uranus
begins retrograde motion on Friday, June 27.
Tracing the sky from Jupiter and Sagittarius to the west, first find wonderful Scorpius, and then a bit to the
northwest much fainter Libra, the
Scales, only two of its stars (Zubenelgenubi, the southern one, and
Zubeneschamali) readily visible.
Libra transits the meridian around 9 PM Daylight shortly before the
sky is fully dark. Farther to the northwest lies Virgo (made notable by the bright
star Spica), which some 2500 years
ago took the Autumnal Equinox
away from Libra as a result of the 26,000 year precession -- wobble -- in the
Earth's rotation axis.
