Astronomy news for the week starting Friday, July 7, 2000.
One wishes for some celestial action this week, but there is
precious little. Will the Galaxy (seen by us as the Milky Way)
perhaps cooperate and produce an exploding star? Likely not.
Instead we are left with the Moon going through its first quarter
on Saturday, July 8th, and that is about it. The event that will
take place about the time North and South Americans rise for the
day, before the quarter rises around noon.
After sundown, admire the quarter set within the stars of Virgo,
the Moon just up from the bright star Spica. Of the five first
magnitude stars of the Zodiac (Aldebaran, Pollux, Regulus, Spica,
and Antares), Spica is number two in proximity to the ecliptic, the
apparent path of the Sun. (Regulus, only a half a degree north of
the ecliptic, the angular diameter of the Moon itself, is the
winner. Spica is two degrees to the south of the ecliptic). As
such, Spica is regularly covered, or occulted, by the Moon on its
monthly path around the Earth. Note the position of the quarter
Moon now. Because of the orbital motion of the Earth around the
Sun, the next first quarter will take place a constellation over to
the east, smack within the stars of dim Libra. All the phases so
move around the Zodiac over the course of the year.
The lunar orbit is tilted by about 5 degrees to the ecliptic, so it
can be found as much as 5 degrees north of the path and as much as
5 degrees south of it. Moreover, the orbit "wobbles" with an 18.6
year period (The effect is called the "regression of the nodes, as
the "nodes," the points where the Moon crosses the ecliptic, move
backward around the ecliptic with the same 18.6 year period.)
Though the Moon is now passing north of Spica, in 10 years, it will
pass to the south. Over the nearly two decades of the regression,
the lunar disk will occult all stars within 5 degrees of the solar
path. Astronomers love such occultations, as they can determine
the size of a star from how long it takes it to disappear behind
the lunar disk, and can also check for close companions.
Of course the sky does not have to DO anything to be wonderful to
watch. Simply admire stars and planets for what they are, admire
their passage across the celestial vault. Watch Jupiter and Saturn
rise against the dark eastern sky, look at great Scorpius on the
meridian at 11 PM, see the Milky Way, the combined light of the
billions of stars of our Galaxy, pass overhead near midnight, the
center of the Galaxy set within the confines of Sagittarius.