We begin our week with the Moon in its waning gibbous
phase just shy of third
quarter, that phase reached on Sunday, June 18. Bracketed
between Uranus and
Venus, the Moon passes Uranus on Saturday the 17th. After the
quarter, it then wanes in its crescent
phase through the end of the week (Friday the 23rd), when it
will be seen to the northeast of Venus (which rises as dawn
begins), the duo presenting us with a pretty sight.
Two big events mark the week. The first involves us and the Sun, as the latter crosses the Summer Solstice near the Taurus-Gemini border at 7:26 AM CDT (8:26 EDT, 6:26 MDT, 5:26
PDT, earlier in Hawaii and Alaska) on Wednesday, June 21,
marking the beginning of
astronomical summer in the northern hemisphere. At that time,
the northern axis of the Earth will lean directly toward the Sun (tilted through its
23.4 degree angle relative to the orbital perpendicular), the Sun
will be overhead at the Tropic of
Cancer (23.4 degrees north latitude), and will be circumpolar
(not setting) at and above the Arctic Circle (66.6 degrees north,
though because of atmospheric refraction and finite angular solar
diameter, a bit to the south of the Circle).
The other event is a lovely conjunction between Mars and Saturn. Mars
has been moving up on Saturn for weeks now, and on Saturday the
17th will pass just 0.6 degrees to the north of the ringed planet.
Look low to the west just after dark, as the pair will set around
11 PM. Then watch in subsequent nights as Mars, moving quickly
toward the east, leaves Saturn behind. Though the two will look to
be physically close, the pairing is only a line-of-sight effect, as
Saturn will be 4.3 times farther away (Mars 2.3 times farther from
the Sun than we are, Saturn 9.8). In spite of being farther,
Saturn (at zeroth magnitude) is 3.6 times the brighter (Mars,
reddish and at second magnitude) because it is so much larger and
has bright reflective rings. At the same time, the Beehive Cluster in Cancer will be but half a degree away
to the north, the "trio" making a unique and wonderful sight in
binoculars.
In lesser news, Mercury
hits its greatest eastern elongation on Tuesday the 20th, making it
nicely visible in northwestern evening twilight. That same day, it
passes six degrees south of Pollux
in Gemini. With Uranus beginning
retrograde (westerly) motion on Monday the 19th, only
Jupiter is left to go, the giant planet crossing the meridian
to the south in mid-twilight, then setting around 3 AM, about an
hour before Venus rises.
We are beautifully into Bootes
season, when the Herdsman rides high and nearly overhead around 9
PM along with its shining luminary, Arcturus, the fourth brightest star
in the sky and the brightest of the northern hemisphere (just
edging out Lyra's Vega). Down below is Spica in Virgo, and far below Spica lies vast Centaurus (whose southern reaches
hold the closest and third brightest star, Alpha Centauri, which is out of reach for all in the
US except those in the very far south).
