Moon sweeps through Leo May 11 to 13

On May 11 to 13, 2019, the moon is moving through the prominent constellation Leo the Lion. Watch for the moon on May 11, when it’ll be near the bright star Regulus, representing the Lion’s Heart. Also on the night of May 11, the moon will be at or near its half-illuminated first quarter phase, and its dark (or nighttime) side will be pointing right at Regulus.

The dark side of a waxing moon always points in the moon’s direction of travel relative to the backdrop stars of the zodiac. Although – on any given night – the moon will move westward because of the Earth’s rotation, the moon’s orbital motion will actually cause the moon to travel eastward. Thus, from the night of May 11 to the following night, May 12, the moon moves toward Regulus. The moon moves one-half degree (its own diameter) eastward per hour in front of the constellations of the zodiac – or about 13 degrees eastward per day.

Note where the moon appears relative to Regulus at nightfall May 11 and then again as darkness falls on May 12, and the change in the moon’s position will be obvious.

By May 13, the moon will have moved again on your starry dome, as it pursues its endless orbit around Earth. On May 13, the moon will be near a fainter star in Leo, called Denebola. The word “deneb” in star names typically means “tail.” The name Denebola indicates that this star represents the Lion’s Tail.

Back to the first quarter moon on May 11 … our sun travels nearly one degree (two sun-diameters) per day in front of the constellations of the zodiac. At first quarter phase, the moon is 90 degrees east of the sun, as measured along the ecliptic – the sun’s annual pathway in front of the backdrop stars. Because the sun takes about three months to move 90 degrees eastward along the ecliptic, we can figure the sun will be in approximately the same spot relative to Regulus as the May 11 first quarter moon.

Sky chart of the constellation Leo the Lion with black dots on white background.

Chart of the constellation Leo via the IAU. The ecliptic depicts the annual pathway of the sun in front of the constellations of the zodiac. The sun passes in front of the constellation Leo each year from around August 10 to September 17, and has its yearly conjunction with the star Regulus on or near August 23.

Bottom line: On the evenings of May 11 and 12, 2019, use the moon to find Regulus, the constellation Leo the Lion’s one and only 1st-magnitude star. By May 13, the moon is closer to Denebola, in the Tail of Leo.

Bruce McClure