Tonight – January 24, 2017 – the moon reaches its first quarter phase. From around the world tonight, people will see the moon in their evening sky. It’ll appear half-illuminated.
Did you know that the Earth and moon are like mirrors to each other in that – when we see a 1st quarter moon in our sky – those on the moon would see a last quarter Earth?
As seen from the moon, Earth’s terminator line – the line between light and dark on the last quarter Earth as seen from the moon – is the line of sunset. As seen from the moon, the last quarter Earth will be waning toward its new phase in the week ahead. This new Earth – seen from the moon – will block the sun on January 31. For us on Earth, the Earth’s shadow will fall onto the moon’s face, giving us a total lunar eclipse.
And on that future date – February 7, 2018 – from the vantage point of the last quarter moon, Earth’s terminator will show you where it’s sunrise on a first quarter Earth, as illustrated below.
One half of the Earth – and one half of the moon – are always illuminated by sunlight. Meanwhile, on both the Earth and the moon, half of each world is submerged, continuously, in its own shadow. Day and night on Earth. Day and night on the moon. At first quarter moon, we see half the moon’s day side, and half its night side.
The lunar terminator – line dividing day from night – can give you your best three-dimensional views of the lunar terrain through binoculars or a telescope. Try looking in evening twilight, when the sky isn’t quite dark yet, to eliminate glare from the moon itself.
The exact first quarter moon for January 2018 is reached at 22:20 Universal Time (UTC) on January 24. 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 exact first quarter moon occurs on January 24 at 5:20 p.m. EST, 4:20 p.m. CST, 3:20 p.m. MST and 2:20 p.m. PST.
Bottom line: Enjoy the January 24, 2018, 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.
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.