Topline
This weekend ends May with a rare occurrence — a second full moon in a single calendar month, a calendrical phenomenon commonly known as a Blue Moon. While the term has nothing to do with color, it will have many people looking for the full moon who usually ignore it, and many of them will miss it. There are two reasons for that. Non-experts are saying that the Blue Moon is on Sunday, May 31, but as any moon gazer knows, by far the best moment to see a full moon is not when it’s high overhead, but when it appears low on the horizon just after sunset. That happens during dusk on Saturday, May 30. Here’s everything you need to know about this weekend’s Blue Moon.
Timeline
The 98%-lit nearly full moon is already high in the eastern sky by sunset, appearing large and bright. As it rises, look below it for Antares, the brightest star in the summer constellation Scorpius.
It’s the best evening to watch moonrise for the full Blue Moon effect. Find an unobstructed southeastern horizon at the time of moonrise where you are, and you’ll see a large, orange full moon appear. You’ll soon see Antares just below it. Much closer than on Friday. Why tonight? The moon rises as the sun sets, which creates a dramatic effect.
During the night, at 4:45 a.m. EDT, the Blue Moon will become official as the lunar surface becomes 100% lit by the sun. However, tonight it will rise around an hour after the sun has set, as seen from North America.
What Makes A ‘blue Moon’
A blue moon is simply the second full moon in a calendar month, a consequence of the moon’s 29.5-day cycle not aligning perfectly with our calendar. It happens every two to three years, making it relatively uncommon — but not rare. The next time this kind of “monthly” blue moon will take place is Dec. 31, 2028, according to TimeAndDate.com. That will also be a total lunar eclipse.
What matters is timing. Because this full moon rises close to sunset, Saturday, May 30, offers the perfect opportunity to watch it emerge against a landscape, giving it scale and color that are lost when it climbs higher. For urban observers, this is one of the easiest and most reliable sky events of the year. No dark skies required — just timing and a clear view to the east.
Why The Blue Moon Is Orange
The term Blue Moon relates to the rarity of having two full moons in one calendar month, and is not related to its color. Just as the sun at sunrise and sunset appears a deep orange, so too does the full moon at moonrise and moonset. Why? Visible light is a form of electromagnetic radiation, which is made up of photons — particles of light — that travel in different wavelengths. Earth’s atmosphere scatters light, deflecting photons off molecules of nitrogen and oxygen. Blue has the shortest wavelengths of light, which more easily strike molecules and scatter more readily. Red has the longest, so more of its travels through Earth’s atmosphere unimpeded. The phenomenon is called Rayleigh scattering and is named after Lord Rayleigh (John William Strutt), a British physicist, who wrote a paper about it in 1871.
What’s Next In The Night Sky
Between June 9-11, Venus and Jupiter come together in a striking conjunction low in the western sky after sunset — one of the standout naked-eye events of the year. The new moon on June 15 brings a brief period of dark skies before the June solstice on June 21 ushers in the shortest nights of the year for the northern hemisphere.
The times and dates given apply to mid-northern latitudes. For the most accurate location-specific information, consult online planetariums like Stellarium.
