Tonight – January 16, 2016 – the moon reaches its half-illuminated first quarter phase. The exact phase is reached at 23:26 Universal Time on this date. Although this first quarter moon happens at the same instant worldwide, it occurs at different times by the clock, depending on one’s time zone. Here, in the mainland United States, the first quarter moon comes on January 16 at 6:26 p.m. EST, 5:26 p.m. CST, 4:26 p.m. MST and 3:26 p.m. PST.
From around the world tonight, people will see the moon in their evening sky. It’ll appear half-illuminated.
One half of the lunar disk is always illuminated by sunlight, while the nighttime half is submerged in the moon’s own shadow. At first quarter moon, we see half the moon’s day side, and half its night side.
The lunar terminator – the shadow line dividing day from night – shows you where it’s sunrise on the first quarter moon as it waxes toward full moon. It’s along the terminator that you have your best three-dimensional views of the lunar terrain through binoculars or the telescope. Try looking in evening twilight, when the sky isn’t quite dark yet, to eliminate glare from the moon itself.
If you were on the moon at its first quarter phase, and looking back at Earth, you’d see the Earth at its last quarter phase, as displayed on the image below.
As seen from the moon, the terminator on the last quarter Earth depicts sunset, as the last quarter Earth wanes toward its new phase.
However, when the last quarter moon comes on February 1, the lunar terminator will show you where it’s sunset on the last quarter moon.
On that future date – February 1, 2016 – from the vantage point of the last quarter moon, the earthly terminator will show you where it’s sunrise on the first quarter Earth, as illustrated below.
After the January 16, 2016, first quarter moon, the moon will wax toward full moon.
Meanwhile, as seen from the moon, the January 16 last quarter Earth will wane toward Earth at its new phase.
Approximately one week after today’s first quarter moon, it’ll be a full moon in Earth’s sky but a new Earth in the moon’s sky.
Bottom line: Enjoy the January 16, 2016 first quarter moon! And know that the Earth and moon are like mirrors to each other in that – when we see a first quarter moon in our sky – those on the moon would see a last quarter Earth.