Regulus and the 1st quarter moon on May 19

From around the world this evening – May 19, 2021 – you’ll see the moon at or close to its half-illuminated 1st quarter phase in the vicinity of Regulus, the brightest star in the constellation Leo the Lion. Because a first quarter moon rises around noon and sets around midnight, you’ll find the moon and Regulus at their highest in the sky when darkness falls, no matter where you are on the globe.

The moon reaches the crest of the 1st quarter phase on May 19 at 19:12 UTC. At mainland United States’ time zones, that converts to 3:12 p.m. Eastern Time, 2:12 p.m. Central Time, 1:12 p.m. Mountain Time and 12:12 p.m. Pacific Time. Thus the moon will be somewhat past its 1st quarter phase as darkness falls on May 19 in the U.S. Even so, the eastern half of the United States can view the 1st quarter moon in a daytime sky, because, again, a 1st quarter moon rises around noon and so will rise in the east on May 19 before it assumes its 1st quarter phase.

Want to find out when the moon rises into your sky and reaches 1st quarter phase? Click on Sunrise Sunset Calendars, and remember to check the Moon phases and Moonrise and moonset boxes.

Regulus, a blue-white gem of a star, is the only 1st-magnitude star to sit almost squarely upon the ecliptic, the apparent path of the sun in front of the constellations of the zodiac. Whereas the sun annually has its conjunction with this star once a year (on or near August 23), the moon and Regulus have their conjunction every month (or rarely: twice in one calendar month).

Vertical egg-shaped object with much smaller spherical object beside it.
A computer generated model of Regulus, created in 2005 by Georgia State University’s Center for High Angular Resolution Astronomy (CHARA). A model of the sun is shown next to it for scale. The high rotation rate of Regulus creates pronounced equatorial bulging such that its diameter across its equator is one-third longer than its north-south diameter. Regulus’ equator is almost vertical in this image. Image via Wenjin Huang / Georgia State University / NSF.

The moon reaches its 1st quarter phase and passes to the north of Regulus almost simultaneously at their conjunction on May 19, 2021. Next month, the moon will sweep north of Regulus for the following conjunction on June 15, 2021. Yet, the moon won’t return to its 1st quarter phase until a few days later, on June 18, 2021. That’s because the moon laps a given star of the zodiac in a mean period of 27.3217 days, yet returns to the same phase in a mean period of 29.5306 days.

Although the sun meets up with Regulus on or near the same date every year, the same can’t be said for the 1st quarter moon. One lunar year consists of 12 lunar months of about 354 days. Twelve lunar months (12 returns to 1st quarter moon) later – on May 9, 2022 – the May 2022 1st quarter moon will recur noticeably west of Regulus. On the other hand, 13 lunar months later – on June 7, 2022 – will find the 1st quarter moon appearing considerably east of Regulus. There will be no similarly good coincidence of the 1st quarter moon with Regulus in the year 2022.

Lunar cycles are not necessarily easy to commensurate. For instance, the sidereal month – the duration of time between successive returns of the moon to the same star – represents a mean period of 27.3217 days. On the other hand, the lunar month – the span of time between successive lunar phases – depicts a mean period of 29.5306 days. (Incidentally, the lunar month is also called a lunation or the synodic month.)

Over the centuries, numerous attempts have been made to correlate sidereal and lunar months, with varying degrees of success. At some indefinite time in human history, it was discovered that 67 sidereal months are nearly equal to 62 lunar months:

27.3217 x 67 sidereal months = 1830.55 days

29.5306 x 62 lunar months = 1830.90 days

Earth, with moon in 8 positions around it showing how much is visible from Earth.
Moon’s orbit around Earth, seen from above. The moon’s dark side faces Earth at new moon, and its illuminated side faces Earth at full moon. At the quarter moons, we see 1/2 of the moon’s daytime side and 1/2 of its nighttime side. Read more: Understanding moon phases

This approximate 1830-day period is the equivalent of five calendar years plus a few to several days. On May 23, 2026, the moon makes its 67th return to Regulus slightly before the moon’s 62nd return to 1st quarter phase. However, the coincidence of 1st quarter moon with the moon-Regulus conjunction is actually a bit closer timewise on May 19, 2021, than it will be on May 23, 2026.

In other respects, though, the 1st quarter moon will align much more closely with Regulus on May 23, 2026. Depending on the year, the wayward moon can swing anywhere from 5 degrees north to 5 degrees south of the ecliptic whenever the moon and Regulus stage their monthly conjunction.

For instance, on May 19, 2021, the 1st quarter moon swings a whopping 5 degrees (10 moon-diameters) north of the ecliptic. Five degrees away from the ecliptic represents a northern extreme for the moon’s travels.

Some five years later on May 23, 2026 the moon will display its 1st quarter phase and – almost concurrently – will cross the ecliptic at its descending node. At its node, the moon coincides with the ecliptic.

Thus, the moon will be only a hair’s breadth shy of its 1st quarter phase as it occults – covers over – Regulus fon May 23, 2026. For a map of the occultation viewing area, click In-the-Sky and scroll to the bottom of the page.

Bottom line: This evening – May 19, 2021 – enjoy the moon as it showcases its first quarter phase and shines to the north of Regulus, the constellation Leo the Lion’s brightest star.

May 19, 2021

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Bruce McClure

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