How to tell if the moon is waxing or waning
A waxing moon--that is to say a moon which is between new moon and full moon--will apparently grow larger night by night until it reaches full moon.
A waning moon--between full and new--will seem to decline (or wane) each night until it reaches the new moon.
But how can you tell whether the half or quarter moon is waxing or waning?
Cultural imperialism alert: This only works in the Northern hemisphere. Anyone south of the equator needs to swap hands. But what happens on the equator?
Answer: At the poles the straight bit of the half moon appears vertical, whereas at the equator, the straight edge is horizontal--or parallel with the horizon. The moon kinda seems to tilt over more and more as you travel closer to the equator. The bright bit of the moon is always near the horizon. So on the equator, the waxing and waning moons appear the same, the only way to tell the difference is that the waxing moon is visible in the early part of the night, while the waning moon is visible toward morning. This works in all parts of the world, incidentally.
Form a 'C' shape with the thumb and forefinger of the left hand. Hold it up to the moon. Do the shapes match? Then it's waning. If the moon matches your right hand when making the same shape, it is a waxing moon.
To remember which way around it is, think of a line of moons at different phases, starting at the left with a new moon, progressing through the waxing quarter and half moon to the full moon in the centre, and back to new moon. The waxing moons to the left match your right hand, while the waning moons to the right match the left hand.
Now, hold up your hands
and we will try
to make the shapes of the moon
changing in the sky
Into the curve of the right
fits the moon when it's new
Make a circle with both
and the full moon's in view
Then curve to the left,
to fit the moon when it's old.
And that is what I was always told
Extracted from What is the Moon? by Caroline Dunant and Liz Loveless