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First quarter moon at east quadrature

Tonight – as darkness falls on September 27, 2017 – the moon will be at or near its first quarter phase. What’s really cool is that lit side of tonight’s moon points at the planet Saturn and the star Antares.

If you’ve been watching the moon at nightfall lately, you may have noticed that its position has changed relative to the star Antares and 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, 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%.

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

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

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.

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, 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.

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

Bottom line: Tonight – September 27, 2017 – as darkness falls, enjoy the moon at or near its first quarter phase.

Bruce McClure

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