Tonight – February 20, 2018 – the western limb, or edge, of the moon is most exposed to Earth for this month. In other words, today marks the time of this month when you can see a bit of the moon’s far side.
Why? It’s common knowledge that our moon has a nearside and a farside. One half of the moon always faces Earth, and one half always points away. Does this mean we can only see 50% of the moon’s surface from Earth? No.
In fact – although we can see only about 50% of the moon at any one time, diligent observers can view up to 59% of the lunar terrain, given enough viewing time. That’s because of a slight north-south rocking and east-west wobbling of the moon known as lunar libration.
Due to libration, in the course of every month, there are certain dates on which the moon’s western, eastern, northern and southern limbs are most exposed toward Earth. February 20, 2018, is the date on which the moon’s western limb, or edge, is most exposed (-6 degrees). February 22, 2018, is the date on which the moon’s northern limb will be most exposed (+6.8 degrees).
Want to know which is the moon’s west limb? Then take a look at tonight’s waxing crescent moon, in the west shortly after sunset. The outer edge of the moon’s illuminated side outlines the moon’s east limb. The opposite edge of the lunar disk is the moon’s west limb. Confusingly, west on the moon is east in Earth’s sky (toward the sunrise direction), while east on the moon is west in Earth’s sky (toward the sunset direction).
Confused? Maybe the map of the moon below can help to clarify.
The lunar terminator – the shadow line dividing day from night on the moon – shows you where it’s sunrise on a waxing moon, like the one you’ll see tonight. So maybe you can see that – while the lit side of tonight’s evening crescent points west (sunset direction) for us Earthlings – it points in the direction of the rising sun (east) on the moon.
This east-west reversal between the moon and Earth’s sky is a bit mind-boggling. But, whether it makes sense to you or not, suffice it to say that the nighttime edge of any evening crescent outlines the west limb of the moon.
On February 20, 2018, the west limb of the moon is covered over by the lunar nighttime. But you might – or might not – discern the earthshine softly illuminating the lunar night on this date, with either the unaided eye or an optical aid. This earthshine, really sunlight reflected from Earth, can help you see the moon’s west limb.
By the way, the east-west rocking of the moon is called libration in longitude and the north-south nodding is called libration in latitude. Almanacs list northern and eastern librations in positive numbers, yet western and southern librations in negative numbers. Click here if you’d like to know the moon’s present libration in longitude and latitude (scroll down to Appearance) or click here if you’d like to know the moon’s libration in longitude and latitude for every day of the year.
Tonight – February 20, 2018 – enjoy the waxing crescent moon as its western side maximally shifts a tiny portion of the moon’s back side toward Earth.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.