Looking for a heavenly rendezvous? The moon's available 9/21
"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.
Why does the moon look bigger on first appearance each month? It is an old lunar mystery known as the "moon illusion." As the moon peeks over the horizon, we see it swell to enormous size and then, in just a few hours after it climbs in the sky, (as Earth rotates on its axis through the night) appear to melt like a giant snowball.
Timothy Ferris, offers a preliminary explanation in his new book, "Seeing in the Dark." This theory holds that when confronted with a phenomenon beyond its sensory experience, the human mind creates its own impression and an object near the horizon is perceived as larger than something high in the sky.
When considering the moon, there's always more than meets the eye. Some 900 million years ago, a day on Earth was 18 hours. Thanks to the gravitational "braking" force of the moon, we now have a 24-hour day. The reciprocal force of Earth's gravity on the moon long ago caused the moon's rotation to synchronize exactly with its orbit around Earth every 29 1/2 days. That's why, from Earth, we always see the same side of the moon.
And if this gravitational handshake isn't enough to make you stop and look anew at the moon, consider that the "inconstant" moon of literature is still, literally, drifting away from Earth as if gently ending its relationship with its suitor.
Laser beams shot from Earth and bounced off reflectors placed on the moon by Apollo astronauts confirm that the moon separates itself from Earth at a rate of 1.5 inches a year. At this point in galactic time, total solar eclipses occur because the moon is close enough to earth for them to occur. In a few million years, the moon will be too far away to cover the entire solar disk and only partial eclipses will be visible.
Better "come home" with the moon Sept. 21.
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