Here’s a scintillating celestial sight that’s visible to everyone on Earth this weekend—“planetshine.”
For anyone getting into stargazing and moon-gazing while at home during the lockdown, it’s a sight to behold, and one that you’re sure to seek-out again.
April, May and June are the best months to see “planetshine” at its finest, according to NASA.
The phenomenon of the Moon’s darkened side being lit-up only happens in the last and first few days of the Moon’s orbit of Earth. Since there was a New Moon on Wednesday, this weekend there will be three chances to see a beautiful crescent Moon gradually waxing.
On Friday it will be slim and low on the horizon at sunset. On Saturday, a little higher and between two stunning clusters of stars. On Friday, the Moon will be brighter and much higher, close to the sparkling planet Venus.
On all three occasions it will be possible, in clear skies, to see “planetshine.”
Here’s everything you need to know about “planetshine” and how, and when, to see it this weekend.
What is the ‘dark side of the moon’?
There is no “dark side of the Moon.” The Moon is always 50% illuminated by the Sun, but because the Moon orbits Earth every 29 days we see its lit-up portion wax towards full and wane towards “New,” when it gets lost in the Sun’s glare.
This week, as the Moon emerges from that glare, its eastern limb will appear to be ever-so-slightly illuminated. However, the rest of the Moon’s surface, though darkened, will appear to be lit-up to our eyes. Why?
What is ‘planetshine’?
It’s the Earth’s albedo—sunlight being reflected from Earth and on to the Moon. It’s always happening, but it’s only visible to the human eye when only a slither of the Moon itself is being lit by the Sun. At the time, Earth is at its brightest as seen from the Moon—it’s almost 100% illuminated by the Sun. So you have the perfect recipe for reflected light.
You can see “planetshine” by casting your eyes to the left-hand side of the Moon, its darker part. You will be able to perceive some detail there for a few nights until the sheer brightness of the lit-up crescent waxes. As the nights pass the Moon gets brighter and the Earth gets dimmer), so your eyes will become overwhelmed with light as the Earth’s albedo dims—so you’ll no longer be able to see “planetshine.”
Other names for ‘planetshine’
It’s also called “Earthshine” and “Earthlight.” On a waxing crescent Moon it’s sometimes called “the old Moon in the new Moon’s arms” and on a waning crescent “the new Moon in the old Moon’s arms.”
Best times to see ‘planetshine’ on the Moon
It’s at its best either side of a New Moon in the northern hemisphere’s spring—April, May and June—when the Arctic ice and snow reflects the most light (there’s less landmass and less snow and ice in the southern hemisphere, so the effect is less during the south’s spring).
The phenomenon is easily visible for a few days before (so, in the very early morning) and after (post-sunset) New Moon. The latter scenario applies this weekend, when “planetshine” will be detectable on Friday, Saturday and Sunday—and with a few bonus celestial sights, too:
Friday, April 24, 2020:
With the Moon 2% illuminated, this is a great evening for seeing a slim crescent Moon displaying “planetshine” in twilight close to the western horizon. You may need to get up high to see it, but it’s visible (though quickly sinking) for about an hour after sunset.
Saturday, April 25:
A 6% illuminated Moon will tonight be positioned between two splendid open clusters of stars in the constellation of Taurus—the Hyades and the Pleiades (the latter is also called the “Seven Sisters”).
Sunday, April 26, 2020:
With the Moon 12% illuminated and Earth reflecting less light, “planetshine” becomes harder to see, but you should still be able to make it out on the Moon as it appears to be close to Venus. The planet is now almost as bright as it ever gets.
Will you see Earth’s reflected light on the Moon? “Planetshine” will be back 12 nights after May 7’s “Super Flower Moon,” as our satellite wanes, to become “the new Moon in the old Moon’s arms” once again.
Wishing you clear skies and wide eyes.