Tonight – July 19, 2018 – as the setting sun closes the curtains on the day, and the darkening skies bring out a myriad of far-off suns, let the moon introduce you to a very special star. The bright star to the west tonight’s first quarter moon is none other than Spica, the sole 1st-magnitude star in the constellation Virgo the Maiden.
The much brighter object to the east of the moon on July 19 is the giant planet Jupiter. Jupiter, the fifth planet outward from the sun, will remain in front of the constellation Libra until November 2018. In fact, if you aim binoculars at Jupiter, you can see Libra’s alpha star – Zubenelgenubi – taking stage with Jupiter in the same binocular field. Through binoculars, you can view Zubenelgenubi as a double star – two stars in one!
The above sky chart is for North America. If you live in the Earth’s Eastern Hemisphere, the moon will still appear in between Jupiter and Spica, but offset somewhat toward Jupiter. If you live in Hawaii, the moon will be offset somewhat in the direction of Jupiter. The moon on the sky chart appears larger than it does in the real sky.
But on matter where you live, the moon continually moves eastward in front of the backdrop stars of the zodiac at the rate of about one-half degree per hour. For a convenient measuring stick, the moon’s angular diameter approximates one-half degree of sky. So the moon moves its own diameter eastward per hour away from Spica and in the direction of Jupiter. Look for the moon to snunggle up more closely with Jupiter tomorrow, as darkness falls on July 20.
When the moon is no longer close to Spica, you might find it helpful to “star-hop” to Spica instead, as shown on the sky chart below:
If you live in the Northern Hemisphere, and you’re familiar with the Big Dipper, you can count on this famous pattern of stars to guide you to Spica. Simply extend the Big Dipper handle to arc to the brilliant yellow-orange star Arcturus and then to spike Spica, a blue-white gem of a star. (If you have difficulty discerning stellar color with the eye alone, try your luck with binoculars.)
Bottom line: Let the moon guide you to Spica on July 19, 2018, and then use the Big Dipper to locate Virgo’s brightest star, after the moon’s flirtation with Spica ends.