Tonight – September 27, 2017 – as darkness falls around the world, the moon will be at or near its first quarter phase. The terminator line – or line between light and dark on the moon – will appear straight. What’s more, the illuminated side of this September moon points toward two bright “stars.” Only one is a true star, Antares in the constellation Scorpius. The other is the planet Saturn.
First quarter moon happens on September 28 at 2:54 UTC. At U.S. time zones, that places the date and time of the first quarter moon on September 27 at 10:54 p.m. EDT, 9:54 p.m. CDT, 8:54 p.m. MDT and 7:54 p.m. PDT.
By definition, and in the language of astronomy, the moon at its first quarter phase is at east quadrature – 90o east of the sun in geocentric ecliptic longitude. Technically speaking, the first quarter moon is not exactly 50% illuminated at east quadrature, although the lunar disk certainly looks half lit to the eye. Depending on the month, the illuminated portion of first quarter moon varies from 50.117% to 50.138%.
To be less ambiguous, we could say the moon at the instant that it lies 90o east of the sun is at east quadrature, rather than at first quarter. However, the term first quarter is synonymous with east quadrature, and the term last quarter moon is synonymous with west quadrature.
The moon is exactly half-illuminated at dichotomy, yet a tiny bit more than half-illuminated at quadrature (quarter moon). The moon always reaches dichotomy (50% illumination) a short while before its first quarter phase; and the moon always reaches its last quarter phase shortly before dichotomy. Depending on the month, the time period between dichotomy and quadrature can vary anywhere from about 15 to 21 minutes.
When the moon is at east quadrature (first quarter) in Earth’s sky, then it’s the Earth that’s at dichotomy in the moon’s sky – and vice versa.
Bottom line: Tonight – September 27, 2017 – as darkness falls, enjoy the moon at or near its first quarter phase.
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.