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Moon and star Spica on July 11

Tonight – as darkness falls on July 11, 2016 – the moon will be shining close to Spica, brightest star in the constellation Virgo the Maiden. As for the moon itself, we’re seeing half its lighted face now, or a quarter of the whole moon. In the (usually) unerring logic of astronomy we’ll call it a first quarter moon.

The moon reaches first quarter on July 12 at 0:52 UTC. At U.S. time zones, that means the first quarter moon occurs on July 11 at 8:52 p.m. EDT, 7:52 p.m. CDT, 6:52 p.m. MDT and 5:52 p.m. PDT.

At quarter moon, the moon’s disk appears half-illuminated in sunshine and half-engulfed in the moon’s own shadow.

The lunar terminator – the shadow line dividing the lunar day from the lunar night – shows you where it’s sunrise on the moon as it waxes from new moon to full moon.

Karl Diefenderfer caught a first quarter moon at sunset on June 12, 2016.

Karl Diefenderfer caught last month’s first quarter moon at sunset on June 12, 2016. “Such a lucky shot,” he wrote. Thanks, Karl!

By definition, the moon at its first quarter phase is 90o ahead of the sun in geocentric ecliptic longitude. Technically speaking, the first quarter moon is not exactly 50% illuminated, but always a tiny bit more than 50% covered over by sunlight.

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 last quarter with west quadrature.

Not to scale! The illustration shows the moon at dichotomy as seen from Earth, and Earth at quadrature as seen from the moon. The moon resides  at the vertex of the right angle. However, when it's the Earth that resides at the vertex of the right angle, then it's moon that's at quadrature as viewed from the Earth, and the Earth that's at dichotomy as seen from the moon.

Not to scale! The illustration shows the moon at dichotomy as seen from Earth, and Earth at quadrature as seen from the moon. The moon resides at the vertex of the right angle. However, when it’s the Earth that resides at the vertex of the right angle, then it’s moon that’s at quadrature as viewed from the Earth, and the Earth that’s at dichotomy as seen from the moon.

The moon is exactly half-illuminated at dichotomy, but a tiny bit more than half-illuminated at quadrature (quarter moon). The moon always reaches dichotomy (50% illumination) a short while before reaching 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.

Want more? See this cool diagram of dichotomy vs. quadrature via GeoGebra!

Bottom line: Tonight – July 11, 2016 – as darkness falls, enjoy the first quarter moon and the Spica, the brightest star in the constellation Virgo the Maiden.

Bruce McClure

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