What Is A Blue Moon? May 31 Antares Occultation
What is a blue moon becomes clear on May 31, when May's second full moon reaches peak illumination and, in parts of the southern hemisphere, moves in front of Antares. Stargazers in southwestern Australia, New Zealand, Fiji, Argentina, Chile, and other southern hemisphere nations will see the moon rise at sunset and briefly hide the red supergiant star's light over the following hours.
A blue moon is the second full moon in a calendar month, and it occurs roughly once every 2.5 years. The term also applies to the third full moon in a season with four full moons, but the May 31 event uses the monthly definition.
May 30 in North America
For North American observers on the evening of May 30, the Blue Moon will rise in the east as the sun dips below the horizon. Antares will sit roughly 3 degrees to the lower left of the moon, close enough to make the pairing visible before the lunar disk sets at sunrise on May 31.
That setup gives observers a short window to track the moon's position before the occultation begins for viewers farther south. The sequence starts with the moon visible at sunset, then continues into the hours when the lunar disk passes directly in front of Antares.
Antares Over the Southern Hemisphere
On May 31, the second full moon of May will slip in front of Antares in the constellation Scorpius. Astronomers call that passage an occultation, and it will briefly hide the star's light for observers in southwestern Australia, New Zealand, Fiji, Argentina, Chile, and other southern hemisphere nations.
The event combines a blue moon and an occultation in the same night, which is the part worth catching for skywatchers in those locations. Antares is a red supergiant star, so the moon's path across it gives viewers a precise, time-sensitive target rather than a broad full-moon display.
May 31 Viewing Window
People in the listed southern hemisphere locations should look for the moon at sunset on May 31, then follow its movement over the next hours as it crosses Antares. The moon will rise above the eastern horizon first, and the occultation will unfold after that as the lunar disk moves directly across the star.
For everyone else, the practical difference is geography: the blue moon is visible more widely, but the Antares occultation is tied to those southern hemisphere viewing locations. The moon's brief pass in front of the star is the rare part of the night, and it is the part observers in those countries will have to time carefully.
