SKYLIGHTS
Astronomy news for the week starting Friday, June 18, 1999.
The Moon grows toward first quarter the beginning of the week, the
phase reached on Sunday the 20th just about the time of moonrise in
North America, the Moon midway between Leo and Virgo. The night of
Tuesday, the 22nd, the Moon will be found several degrees to the
northeast of Mars.
The red planet is now moving to the east, the motion easily noted
relative to the bright nearby star Spica. In the west, Venus,
having passed elongation, is now slowly being overtaken by the Sun,
though it is still moving easterly relative to the stars and still
brightening.
The week belongs not to the Moon or planets, however, but to the
Earth and Sun, as on Monday the 21st at 2:49 PM Central Daylight
Time, the Sun will pass the Summer Solstice in Gemini, beginning
northern summer. At that moment, the Sun will reach its maximum
angle of 23 1/2 degrees (23 degrees 26 minutes 21 seconds) north of
the celestial equator, and the Earth's axis will be tilted as much
as possible toward the Sun. The Sun will rise for us as far north
of east as possible and set as far north of west, and the duration
of daylight will be at a maximum, though earliest sunrise and
latest sunset do not quite coincide in part because of the slight
eccentricity of the Earth's orbit. On that special day Sun will
pass overhead at the Tropic of Cancer (at 23 1/2 degrees north
latitude) and will be circumpolar (yielding a midnight Sun) as far
south as possible, technically at the Arctic Circle, but actually
a bit to the south of it because of the extended solar diameter and
lofting by refraction in the Earth's atmosphere. From that point
on the Sun will move to the south until it passes the Winter
Solstice in Sagittarius on December 22.
With the Sun at the Summer Solstice, the Winter Solstice and
Sagittarius (now opposite the Sun) will pass to the south at local
midnight (1 AM daylight time). Sagittarius contains the center of
the Galaxy and the brightest part of the Milky Way, which
unfortunately for most northerners is so far south that the Earth's
atmosphere gets in the way, dimming its grandeur, the sight best
seen from the southern hemisphere. At the center of the Galaxy,
coincidentally pointed to by Sagittarius's arrow, lies a brilliant
point called "Sagittarius A-star." Visible with radio telescopes,
but obscured in the optical by interstellar dust, Sagittarius A-
star is produced by hot matter that orbits a black hole over a
million times as massive as the Sun.
