After darkness falls on May 28, 29 and 30, 2020, watch the moon as it travels in front of the constellation Leo the Lion. When the moon first enters Leo, it’ll display a rather wide waxing crescent phase. When the moon finally leaves Leo a few days later, it’ll show a waxing gibbous phase. Midway though its trek in Leo, the moon will exhibit its half-illuminated first quarter phase.
The moon reaches its first quarter phase on May 30, at 3:30 UTC. At United States time zones, that means the first quarter moon comes on May 29, at 11:30 p.m EDT, 10:30 p.m. CDT, 9:30 p.m. MDT and 8:30 p.m. PDT.
The moon’s passage through Leo can help you identify Regulus, Leo’s brightest star. This blue-white gem of a star is of 1st-magnitude brightness and is the 21st brightest star to light up the nighttime sky.
The name Regulus means “little king.”
The moon is close to Regulus for only a few days each month. So when the moon is no longer there to guide you, let the Big Dipper serve as your handy guide to this star. The two bowl stars on the handle side of the Big Dipper faithfully point to Regulus. See the chart below.
The color of Regulus – blue-white – reveals that this star has a high surface temperature. Considering that Regulus is nearly 80 light-years away, it must be quite luminous (intrinsically bright) to shine so brightly in Earth’s sky. Regulus is several hundred times more luminous than our sun. At Regulus’ distance, our sun would be not be visible to the unaided eye.
Normally, a star’s blue-white color indicates that the star is in the heyday of youth (only 50 to 100 million years old). But Regulus has a very close companion star which can’t be seen through a telescope; it can only be detected with a spectroscope. It’s thought that Regulus’ companion could be a white dwarf star, in which case Regulus and its companion star would have to be at least a billion years old. Possibly, mass transfer of material from one star to the other in this close-knit binary star system acts as a fountain of youth, keeping Regulus young in its old age.
Regulus is the only 1st-magnitude star to sit almost squarely on the ecliptic – the sun’s apparent yearly pathway in front of the constellations of the zodiac. Of course, the sun’s apparent motion in front of the stars is really a reflection of our planet Earth’s revolution around the sun.
Looking at the sky chart below, notice that Regulus dots a backwards question mark of stars, called The Sickle. The Sickle outlines the Lion’s head and mane, whereas the star Denebola (whose name means “tail of the lion” in Arabic) marks the Lion’s tail.
Bottom line: On May 28, 29 and 30, 2020, use the moon to locate the constellation Leo and its bright star Regulus. Once the moon leaves the evening sky, starting around mid-June 2020, try to piece together the starlit figure of the Lion in a dark sky.
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.