I am embarrassed to say that I am confused about what must be a very simple aspect of the moon - the timing of its phases. More specifically, I would have “intuited” that the time between a new moon and its first quarter is the same as first quarter to full moon. Likewise, for successive cycles, I would have guessed that time from new to first quarter is the same.
In fact, the intervals vary. Just look here and do some simple arithmetic.
What am I missing? The time (i.e. distance) that the earth has moved while the phases were changing? I’m not even sure, frankly, that I understand what that means!
I don’t think this really addresses your question, but I created this animation* a few weeks ago to help illustrate how the phases of the moon work for another doper… I just wanted to share again
You’re probably missing the bit about the moon’s orbit not being circular.
At apogee it’s about 406500 km away, while at perigee it’s only 356500 km away (see).
Kepler’s laws reqire different speeds at those different orbital extremes, while the phase you see is dependent solely on the angle between earth moon and sun.
Some months the orbital velocity will be speeding up as the moon moves toward first quarter, and some months it’ll be slowing down. Those changes affect the rate at which the earth/moon /sun angle changes.
As well as the Earth’s orbit around the Sun not being circular as well, albeit it has a much smaller effect on the timing of the phases of the Moon compared to the effect created by the Moon’s orbit around Earth.
People don’t think about it, but the moon doesn’t return to the same spot in the sky from phase to like-phase (i.e.: one full moon to the next, for example). Since the earth has moved in its orbit, the apparent background stars have changed, Hence, one full moon will not have the same “backdrop” as the next.
This is known as the sidereal period (i.e.: approx. like celestial position to the next) vs. synodic period (i.e.: like phase to the next). Confusing, isn’t it?
