The moon has been in the news a lot this month, after the successful lunar flyby during the Artemis II mission. And while the full moon usually takes center stage as the big lunar event of any given month, unless there’s a lunar eclipse, like there was back in March, there are a few times a year when the new moon is equally interesting, and one of those events is Earthshine.
Earthshine — a phenomenon you may know from the Rush song on the band’s 2002 album Vapor Trails — is a real thing that you can experience during April, May and June. You know you’re looking at Earthshine when you look up at the moon, and you can see all of it, but not as bright as a regular full moon.
During a full moon, the sun’s light is reflected off the moon’s surface and sent to Earth, making it bright and easily visible. In Earthshine, sunlight bounces from Earth to the moon, then reflects back to Earth. These multiple reflections give the moon a much dimmer, more ghostly appearance.
It’s visible for only a few days a year, usually the few days before and after a new moon in spring. According to NASA, this is because the respective orbits of the moon and Earth line up in such a way that light from the sun can bounce off Earth, and then the moon, and then back to Earth again. The Rush song sums it up nicely when the band sings the phrase: «On certain nights, when the angles are right, and the moon is a slender crescent.»
NASA also says it’s more prevalent in spring due to winter’s latent snow. This makes the Northern Hemisphere brighter in the spring than at any other time of the year, making Earthshine more common. The polar caps receive less sunlight during winter, so there is less ice and snow to reflect light during summer and autumn, which makes Earthshine less likely to occur during those seasons.
When is the best time to see Earthshine?
The actual dates change every year since the moon’s phases don’t perfectly line up year to year. Earthshine can happen at any time there is a crescent moon in the spring.
This happens twice per moon cycle, during the waning crescent and waxing crescent phases. Those phases occur just before and just after a new moon, respectively. The new moon is on April 17, which makes the best days April 12-14 and April 19-22.
You won’t need any special equipment to see it, but you do need timing. Earthshine is only visible within about 45 minutes of sunrise and sunset, giving you about a 90-minute window twice a day to see it. According to When the Curves Line Up, the effect should be strongest on April 14 and 22, so space photographers looking to get a good shot of it should be ready on those days.
If you miss it in April, you’ll have four more time frames to view it, including May 12-14, May 18-20, June 11-13 and June 17-19.
