Wednesday, September 26, 2007

Harvest Moon

It's a full moon tonight, the full moon closest to the autumn equinox, known as the harvest moon. Full moons always rise very close to sunset (the moon is full because it's opposite the sun, you see). For these few nights there is no long period of darkness between sunset and moonrise. The moonlight sort of extends the day, granting farmers extra time to bring in their crops, hence the name.

The harvest moon also appears larger and yellower than usual. This is because you tend to view it when it is close to the horizon. Because you are looking at it through a great thickness of atmosphere, differential scattering makes it look yellower or redder than normal. If you take a photograph of it, the change in colour will show up on the photo so this is a real effect even though the actual moon has not changed colour.

The apparent increase in size is just an illusion, though. If you measure the size of the moon, it is no different than at other times. But our minds perceive objects close to the horizon as being larger than objects higher in the sky. Even though I know it's just an illusion, it's still a very convincing one, and the harvest moon is a stunning sight. I hope we get clear skies tonight so I can go out from about 7pm and look at it.

3 comments:

Nik said...

We have a gorgeous clear night here tonight and the moon is amazingly large and bright. I wonder if the moon closest to spring equinox has a special name for us southern hemispherians?

Melanie Rimmer said...

In the north, the moon closest to our spring equinox is sometimes known as the Storm Moon (it tends to be stormy in March in the North - is the same true of September in the South?) or the Seed Moon (spring is a good time of year to plant seeds, that's true everywhere). So perhaps either of these names could apply to the full moon you're experiencing now.

The first full moon after the March equinox is also the Paschal full moon and is used to calculate the date of Easter. But obviously that doesn't reverse for the southern hemisphere, and you celebrate Easter in March or April like the rest of the globe, even though that's your autumn.

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