How long does the third quarter moon last?

How Long Does That Third Quarter Moon Hang Around?

Ever looked up at the night sky and wondered about the moon’s phases? It’s a fascinating dance up there, and one of the most recognizable steps is the third quarter moon. But how long does that particular “half-moon” stick around? Well, let’s dive in, because it’s not quite as straightforward as you might think.

First off, the entire lunar cycle, from one new moon to the next, takes about 29 and a half days. During that time, the moon goes through all its familiar phases – new, crescent, quarter, gibbous, and full. Now, when we talk about the third quarter moon, things get a little tricky.

Technically, the third quarter moon – also called the last quarter moon – is a specific instant in time. Think of it like this: it’s the exact moment when the moon has completed three-quarters of its orbit around the Earth, relative to the sun. At that precise moment, it looks like a perfect half-circle, with the left side lit up (if you’re in the Northern Hemisphere, that is).

But here’s the thing: we don’t usually think of moon phases as just a split-second event, do we? What we really mean when we say “third quarter moon” is the period when the moon looks roughly like that half-illuminated shape. And that period lasts quite a bit longer.

Each of the main moon phases – new, first quarter, full, and third quarter – are separated by those in-between phases like the waxing crescent or waning gibbous. Each of these phases takes about a week to transition into the next. So, when someone says “third quarter moon phase,” they’re usually talking about that whole week-ish period when the moon is hanging out near that half-lit appearance.

On average, that “third quarter moon phase” – from the waning gibbous to the waning crescent – lasts about 7.38 days. But here’s a fun fact: that duration can actually vary by about ±19 hours! That means it could be as short as 6 and a half days or as long as 8 and a quarter days. Why the wobble?

Well, the moon’s orbit isn’t a perfect circle; it’s an ellipse. That means the moon’s speed changes as it goes around the Earth. When it’s closer to us (at a point called perigee), it zips along faster. When it’s farther away (at apogee), it slows down. And that speed difference affects how long each phase appears to last.

If you’re hoping to spot the third quarter moon, keep in mind that it rises around midnight and sets around noon. So, you’ll typically see it shining in the morning sky. It’s the opposite of the first quarter moon, which you’ll find in the evening sky.

So, to sum it up: while the exact moment of the third quarter moon is just a fleeting instant, the period when the moon looks like a half-disk lasts for about a week. And that duration can wiggle a bit depending on where the moon is in its elliptical orbit. Keep an eye out about three weeks after a new moon, and you should catch that beautiful half-lit orb gracing the morning sky!