Here’s a challenge for you this holiday season. When night falls on Christmas Day, the Moon will be only two days past New. The splinter of a crescent, 7 percent illuminated, will lie low in the west after sunset and hug the horizon just after the official start of nightfall. The challenge is to observe the descending Moon through a telescope when it is only a couple of degrees above the horizon, and to follow it until it sets — for only during that time will the colors of the Christmas crescent appear.
This column was inspired by Peter Michaud of Hilo, Hawaii, who earlier this year told me he once saw, through his 12.5-inch reflector, a phenomenon on the Moon similar to the green rim that appears on the Sun when it is near the horizon. “Normally I wouldn’t look at the Moon when so close to the horizon,” Michaud said, “but I was looking to see if I could see a green flash at moonset. As I was watching, the bright edge of each crater rim flickered with a green ‘frosting’ and stole the show!” Michaud was curious if others had ever witnessed the phenomenon.
While I was not aware of any, his query came at an opportune time: I was about to travel to the Makgadikgadi salt pans in northern Botswana — one of the flattest places on Earth. On the night of June 3, 2022, a 14 percent crescent sank into a largely featureless horizon, yielding a chance to search for Michaud’s phenomenon under exceptional conditions.
I was not disappointed. Using a 3-inch Tele Vue refractor and a Canon 90D camera, I observed and photographed the Moon as it neared the horizon. As expected, the Moon’s upper limb near the terminator turned emerald, while its lower limb shone blood red. Both phenomena are common and related to the dispersion of the Moon’s light as it sinks toward the horizon.
Also present, however, was Michaud’s “frosting” of green crater rims. As excited as I was to see that phenomenon, I also noticed the appearance of patches of red splashed across some crater floors near the terminator.
Out of curiosity, I checked to see the phase of Moon this December; I was pleased to find a similar situation taking place on Christmas night and Boxing Day. Seen as a whole, the pointy tips along the Moon’s green terminator, combined with its interspersed beads of red, could easily bring to mind a wreath of holly. During their Yule celebration at the winter solstice, the ancient Celts decorated their homes with holly. Druid priests believed it had mystical restorative powers, especially as their waxy green leaves and bright red berries thrived during the long cold nights of Northern Hemisphere winter while other plants withered away.
This supernatural aspect may be related to the green flash. Cicely M. Botley wrote in the 1938 edition of her The Air and Its Mysteries that the green flash may be found in Celtic folklore. Folklore from the Isle of Man, for instance, “contains much about the soilshey-bo, or ‘living light,’ a mysterious emanation from the sun which, if it fell upon on certain herbs, gave them almost miraculous healing powers.” This light, she writes, is “almost certainly a reference to what sailors call more prosaically ‘the sun putting out his sidelights’ ” — a “brilliant green like the starboard light of a ship,” which we now call the green flash.
As the Moon will be near perigee come Christmas, it will appear nearly 14 percent bigger and 30 percent brighter than when it is at apogee, so the effect may also be magnified.
Either way, as Michaud writes, “It’s pretty easy to see under the right conditions with a clear horizon.”
If you succeed in seeing it, send your observations to me at sjomeara31@gmail.com.
