By Peter Becker
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Perhaps you have noticed the "Big Triangle" in the southern sky on spring evenings this month.
As evening twilight deepens, far up in the south (as seen from mid-northern latitudes) is the bright orange-yellow star Arcturus, one of the first stars you will see.
This season Arcturus is making a huge nearly isocscles triangle (each side of nearly equal length) with the bright reddish planet Mars to the right and bright yellow planet Saturn to the left. Not quite in the middle but closer to Mars is a bright white star, Spica.
It looks something like a huge odometer, with an imagined line from Arctuus to Spica being the needle. It may also remind us of a celestial metronome, keeping time to the "Music of the Spheres."
Although constellations would never have been made with moveable planets, the planets move slow enough from our perspective, to allow us to use our imagination and make temporary patterns. Unofficial constellations, made up of stars, are known as asterisms. The most familiar in the Northern Hemisphere is the Big Dipper, which is only part of a larger constellation, Ursa Major the Big Bear.
Another well-known asterism is the "Summer Triangle," made by connecting three prominent stars of summer evenings, Vega, Deneb and Altair. On June evenings you can see these stars in the east. Vega is the most brilliant, and highest; Deneb is to to left rising in the east-northeast' Altair is farther down and to the right, rising due east.
Be sure to admire lovely Saturn in the southeast, as darkness arrives. A small telescope is all that is needed to show its glorious ring system, open wide to us this year (its perspective shifts year to year).
Currently, Saturn happens to be within (in front of) the constellation Libra the Scales. The four brightest stars of Libra form a quadrangle, like a diamond standing on one tip.
Three of Libra's principal stars have unusual names. Saturn happens to be almost between the two brightest of these, the one on top and the one on the right in the quadrangle.
The star to the lower right of Saturn is Zebenelgenubi. The star on top, to the upper left of Saturn, is Zebeneschamali.
Zebenelgenubi is a very wide double star; look with binoculars to see the brighter, blue white star (magnitude 2.7) and its dimmer, white companion star, magnitude 5.2.
Zebeneschamali is the brightest star in Libra, magnitude 2.6, and often described as greenish. See what you think; try binoculars. Not many stars have this distinction.
The dimmer star at far left in the quadrangle is named Zubenelakrab. This star is an orange giant, magnitude 3.9.
Last-quarter moon is on July 19.
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Keep looking up!
Looking Up: Triangles, quadrangles in the sky
By Peter Becker