Before dawn on October 5, 6 and 7, 2018, watch as the moon slides in front of the constellation Leo the Lion. Leo is identifiable for the prominent backwards question mark pattern within it; this pattern is a well-known asterism, called The Sickle. The illuminated portion of the waning moon always points eastward. That’s also the moon’s direction of travel in front of the background stars of the zodiac. This motion is, of course, due to the moon’s movement in orbit around Earth.
So – on the mornings of October 5, 6 and 7 – you can easily see the moon’s change of position relative to Regulus, the constellation Leo’s only 1st-magnitude star, located at the bottom of the backwards question mark pattern. Regulus depicts the Lion’s Heart and is sometimes known as Cor Leonis.
On the morning of October 5, note that the lit side of the waning crescent moon points at Regulus.
By the the morning of October 6, the moon will have swept past Regulus, moving, as it always does, in its ceaseless orbit around Earth. It’ll be located in Leo’s mid-section.
By the morning of October 7, the very thin waning crescent moon will have moved closer to Leo’s hindquarters, represented by a triangle pattern. On the chart below, pick out the star Denebola in Leo. Many stars have deneb in their name. It means tail, in this case the tail of Leo the Lion.
The ecliptic is shown in green on the chart at the top of this post. It’s like a center line on the great big celestial highway. It divides the band of stars that we call the zodiac into its northern and southern sides. Regulus, the only 1st-magnitude star to align almost squarely with the ecliptic, resides a scant 1/2 degree north of it. For reference, the moon’s angular diameter equals about 1/2 degree.
The moon’s monthly path in front of the zodiac is inclined by about 5 degrees to the plane of the ecliptic. So, in the course of one month, the moon gets as far as 5 degrees (10 moon diameters) south of the ecliptic, and then about two weeks later, gets as far as 5 degrees north of the ecliptic. Midway between these extremes, the moon crosses the ecliptic at points called nodes. In fact, the moon recently crossed the ecliptic, going from south to north, on October 4, 2018, at 03:10 Universal Time (UTC); translate UTC to your time.
Every month, for years on end, the moon will be swinging to the north of Regulus as it parades in front of the constellation Leo the Lion. Finally, the moon will sweep directly in front of this star, to stage a lunar occultation of Regulus, starting on July 26, 2025. There will be a total of 20 occultations of Regulus that’ll end on December 27, 2026. (Of course, you have to be at the just right spot on Earth to witness any one of these occultations.)
Then, after that, the moon will swing to the south of Regulus every month, for years on end, until the next series of monthly Regulus occultations from June 11, 2035, to November 11, 2036.
Bottom line: Watch for the waning crescent moon to sail to the north of Regulus and the ecliptic as it passes in front of Leo from October 5 to 7, 2018.
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.