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Looking for a heavenly rendezvous? The moon's available 9/21

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"Dad, the moon is coming home with us."

Songwriter Greg Brown puts these words in the mouth of a young girl riding with her father in his pickup truck as a full moon rises over the Iowa plains. The song taps the celestial magic of a moon at the horizon's edge. The orange orb appears closer, even as it actually distances itself from Earth.

If it has been awhile since the moon came "home with you," next month's harvest moon on Sept. 21, is your best bet for a heavenly rendez-vous.

Hardworking New England farmers in the 17th century were the first Americans to "moonlight" on the job. They gave the adjective "harvest" to September's moon and thanked nature's God for providing them with extra light to bring in the crop.

Normally, the moon rises 50 minutes later each night. But there are key variations in its travels across the sky. In the northern latitudes, September is when these variations are most visible.

The reason for this is that the harvest moon is the full moon closest to the autumnal equinox. On the 21st, the full moon rises at sunset per the norm, but then, for several nights, it will appear just 25 minutes or so later. This stems from the tilt of Earth at this time of year. And the effect increases if the full moon occurs on or about the date of the equinox, as it does this year.

The harvest moon will also be more pronounced this year because it occurs at the same time as the low point of the moon's own orbital tilt, which traverses an 18.6-year cycle. This accentuates (fast-forwards from our vantage point on Earth) the angle its orbit makes with the eastern horizon at moonrise.

Full moons always rise at or near sunset. That's because, in order to appear full to us, the moon's fully illuminated hemisphere – its "day" side – must be facing us. And, for the dayside to face entirely in our direction, the moon has to be opposite the sun. Hence, all full moons rise in the east as the sun is setting in the west. And all full moons are highest in the sky around midnight, when the sun is below our feet.


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