Tonight – May 21, 2018 – the half-lit first quarter moon passes 1.5o (3 moon-diameters) north of Regulus, the brightest star in the constellation Leo the Lion. Although our sky chart at the top of this post is specifically designed for mid-northern latitudes in North America, nearly everyone worldwide can use the moon to find the star Regulus tonight. Just look for the moon in the evening sky. The nearby bright star will be Regulus.
There’s little chance of mistaking another star for Regulus, because Regulus is the only bright object within a stone’s throw of tonight’s moon. The much brighter “star” a long jump to the east (toward the sunrise direction) is a planet, giant Jupiter. You’ll easily see Jupiter. It’s the second-brightest starlike object in the May evening sky, after Venus. But you can’t mistake Jupiter for Venus because Venus sits rather low in the west at nightfall and early evening. Look for the moon in the vicinity of Jupiter on May 26 and 27.
The moon reaches its half-illuminated first quarter phase on May 22, at 3:49 Coordinated Universal Time (UTC). Although the first quarter moon happens at the same instant worldwide, the hour differs by time zone. Here, at United States time zones, the first quarter moon comes on May 21, at 11:49 p.m. EDT, 10:49 p.m. CDT, 9:49 p.m. MDT and 8:49 p.m. PDT.
No matter where you live worldwide, the moon always moves eastward relative to backdrop stars (and planets) of the zodiac. That’s in spite of the fact that the moon and Regulus move westward across the sky (like the sun in daytime) because of the Earth’s rotation. That eastward motion of the moon relative to the backdrop stars is a reflection of the moon’s orbit around Earth. Starting tonight and over the next several days, you can notice the orbital motion of the moon as it keep traveling eastward of the star Regulus.
Bottom line: Tonight – May 21, 2018 – the first quarter moon pairs up the star Regulus, the brightest star in the constellation Leo the Lion.