The chart above shows the path of the moon on August 7, 8 and 9, 2019, in front of the rather faint constellation Libra the Scales and then on to brighter Scorpius the Scorpion. First quarter moon comes on August 7, when you’ll see half the moon’s day side, what some call a half moon. First quarter moon falls on August 7 at 17:31 UTC; translate UTC to your time. At U.S. time zones, that is 1:31 p.m. EDT, 12:31 p.m. CDT, 11:31 a.m. MDT and 10:31 a.m. PDT. A first quarter moon rises around midday and sets around midnight for all of us, everywhere on the globe. For us in the mainland U.S., the moon will be somewhat past first quarter at nightfall August 7. Along the Eastern Seaboard of the northeastern United States, moonrise and the first quarter moon happen at nearly the same time, roughly around midday on August 7.
You might – or might not – see Libra’s two brightest stars, Zubenelgenubi and Zubeneschamali, in the moon’s glare on August 7. These two stars are modestly bright, and pretty easy to see on a dark night, yet may fade from view in a sky beset by moonlight or light pollution. After all, these Libra stars are about five times fainter than the red supergiant star Antares, which shines at 1st-magnitude brightness.
On August 7 and 8, the dark side of the waxing moon points in the direction of the star Antares and the king planet Jupiter. Although Antares and Jupiter shine fairly close to one another on the sky’s dome, you won’t have any trouble distinguishing Antares from Jupiter. The king planet outshines Antares by over 20 times. Of course, it’s only Jupiter’s nearness to Earth that causes it to look so bright. Jupiter is a planet, shining by reflected sunlight, while Antares is a star, shining by light made in its own interior.
The moon always travels in orbit around Earth eastward relative to the backdrop stars and planets of the zodiac. Antares and Jupiter reside to the east of the moon on both August 7 and 8, yet the moon will be much closer to Antares and Jupiter on August 8 than on August 7, for the whole Earth. By August 9, look for the moon to pair up quite closely with Jupiter.
The moon is now waxing (increasing) toward full moon. The dark side of a waxing moon always points eastward, in the moon’s direction of travel through the constellations of the zodiac. Even though the moon goes westward throughout the night because of the Earth’s rotation, the moon always journeys eastward through the zodiacal constellations because of its orbital motion. This orbital motion causes the moon to travel its own angular diameter of about 1/2 degree eastward – about the width of a pencil at arm’s length – per hour. That means you can notice the moon’s orbital motion tonight, just by noticing its distance from, say, Jupiter in early evening and again in the hour before midnight, when the moon is about to set.
In addition to Earth’s spin on its axis, and the moon’s motion in orbit around Earth, there’s another motion that causes a shift in heavenly objects. That is the motion of Earth’s orbit around the sun.
The Earth in its orbit circles some 30 degrees around the sun in one calendar month. Because of Earth’s orbital motion, next month’s first quarter moon will shine in a different spot on the zodiac than it does this month. The first quarter moon on August 7, 2019, shines in front of the constellation Libra. Yet, next month’s first quarter moon on September 6, 2019, will shine some 30 degrees east of where it does this month, nearer the king planet Jupiter and the star Antares.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.