By Bruce McClure and Deborah Byrd
Thursday, May 28, 2009
The moon is still a waxing crescent in the evening sky. It’s moving toward the first quarter phase, which will come on May 31.
At tonight’s phase of the moon, we see a nearly half illuminated lunar disk. Slightly more than half the moon appears engulfed in shadow. Yet the moon has a “day side” and a “night side” just as Earth does. We on Earth say that the moon waxes and wanes, but this is, in a way, a fiction — albeit a charming one! The reality is that the moon is always half illuminated, just as Earth is. From Earth, we see various fractions of the moon’s day side, as our vantage point on the moon shifts slightly throughout each month — while the moon orbits around Earth.
Now maybe you can see why the far side of the moon is not the same thing as the dark side of the moon.
The near side of the moon — the side that we see from Earth — is sometimes illuminated, and sometimes mostly dark. The far side of the moon — the part that we don’t see from Earth — is also sometimes illuminated, and sometimes dark. A full half of the moon is always illuminated, but this day side shifts around and around the entire globe of the moon — just as Earth’s day side shifts around and around our globe, endlessly. The far side of the moon is only the dark side of the moon at full moon.
The terminator — the shadow line on the moon that divides day from night — shows you where it’s sunrise on the moon when the moon is waxing from new moon to full moon.