Everyone notices how the Moon changes shape, and generally knows the names of all the phases: Full. Half. Crescent. Gibbous. Whoa, wait – gibbous is not universally known. That’s the football-shaped Moon, fatter than half but smaller than full. It’s what we’ve seen all this past week.
But Saturday night and Sunday morning (April 11/12) we’ll get my favorite: The Third Quarter Moon. Let me tell you why I’m in love with it, and see if I can convert you, too.
Way back in 1840, the first-ever celestial photo was taken: a picture of the Moon. Today, in our age of highly sensitive CCD-sensors built into nearly every handheld camera, anyone can repeat that feat. If you haven’t tried a maximum “zoom” of the Moon, you’re in for a treat. You’ll capture craters and lunar mountains whenever the shadowing is optimal, as it is right now. If you can include a foreground earthly object, you’ll be amazed.
This weekend the Moon rises after midnight and is highest up at dawn. It’s best seen in daylight. The Moon actually spends as much time in the day sky as in the night, but gets noticed mostly when it hovers against the deepest blue portion of the heavens. That’s where it will be Sunday morning – and even Saturday and Monday mornings, too, making this exercise pretty cloudproof.
If you can, go out Sunday morning between daybreak and 9:30 a.m. You’ll see the Last Quarter Moon – a Half Moon – against the deepest blue sky possible. It’s the region of sky most highly polarized, so using polarizing sunglasses makes it “pop” even more.
While a Half Moon may look superficially like any other Half Moon, it isn’t so. The First Quarter (the Half Moon lit up on its right side) is the one that everybody notices: It’s always highest at nightfall, prominent at dinnertime and sets around midnight. Since that’s the normal still-awake span for most of us, the First Quarter gets reliably viewed by the world’s 7.2 billion people each and every month.
By contrast, this weekend’s Last Quarter or Third Quarter Moon doesn’t typically clear horizon obstructions until 1 to 3 in the morning. It’s illuminated on the left, imparting an odd lighting to its features. This is the Half Moon that you’ve seen all your life in the morning, on your way to work or school. Another oddity: The Half Moon is not half as bright as a Full Moon, but just one-eleventh as bright.
Yet another: The Half Moons have different brightnesses. The First Quarter is about ten percent more luminous than this weekend’s Third Quarter, because its region has fewer dark “seas.”
Once a month, as the Moon orbits our world, it momentarily sits in the spot directly ahead of us in space: the place toward which Earth is traveling. It happens this Saturday and Sunday mornings – and with just a small near-miss, on Monday morning too. When you look at that Half Moon against the daytime morning sky, you’re gazing at the direction that Earth travels, at 18 ½ miles per second: the look-ahead direction.
You will be right there – at the very place occupied by this weekend’s Half Moon – three-and-a-half hours later. Just check it out. If you see it at 9 a.m. …well, that’s the spot in space you’ll be for lunch.
Join the family! 
