First Day Of Fall 2013 On Sunday: Harvest Moon Heralds Autumn Equinox

 @rpalmerscience
on September 17 2013 11:06 AM

Several celestial events are scheduled to light up the sky this week in the leadup to the equinox on Sunday.

From Wednesday night to Thursday morning, skywatchers will get a glimpse of a Harvest Moon lighting up the night sky. While popular lore sometimes refers to any moon that looks bigger than normal as a “harvest moon,” the true Harvest Moon is merely the full moon that falls closest to the autumnal equinox. Since this is the fourth full moon in a season, the previous one grabbed the title of Blue Moon.

The Harvest Moon is not necessarily any bigger or brighter than a regular moon, but can sometimes seem more spectacular thanks to the “moon illusion,” a phenomenon where the moon appears to be bigger near the horizon than when it hangs high in the sky. Scientists still aren’t quite sure what causes this moon optical illusion, but there are several theories:

On Thursday night, if you look to the eastern side of the sky, you might catch a glimpse of Uranus near the moon (no jokes, please). The planet will be a bit washed-out thanks to the light of the nearly full moon, but those with binoculars should be able to pick it out of the celestial crowd.

“Just look for a tiny greenish-blue disk in the field of view,” National Geographic writer Andrew Fazekas advises. “By the way, the absorption of red light by methane in the atmosphere is what gives Uranus its cool cyan coloring.”

Meanwhile, if you’re out star-watching close to sunset this week, you may get a glimpse of a close encounter of the planetary kind. Venus and Saturn are dancing practically cheek to cheek this week, and will appear to come within four degrees of each other in the sky, according to National Geographic.

The September equinox -- and first day of autumn in the Northern Hemisphere and spring in the southern -- will fall on Sunday, at 4:44 p.m. EDT. At that moment, the sun will appear to cross the equator, going from north to south. Earth will be positioned such that its axis is perfectly perpendicular to the plane of its orbit, meaning that the sun’s rays are hitting the daylight side of the planet dead-on, bathing the Northern and Southern Hemispheres in equal amounts of light and creating days and nights of near-equal length. “Equinox” is Latin for “equal night,” after all. But in reality, the date with equal day and night lengths comes a few days before the equinox -- plus, day length varies depending on how close you are located to the equator.  In New York City, for instance, equal day and night lengths actually will occur on Sept. 25 this year.

Why don’t the day lengths and the equinox date match up? It turns out to be the fault of both our atmosphere and a slight technical discrepancy.

Earth's atmosphere bends the sun's light, which has the effect of making the sun appear higher in the sky than it actually is. Also, astronomers calculate the date of the equinox based on the position of the center of the solar disc. But when astronomers calculate day length, it’s the upper edge of the solar disc that matters in recording sunrises and sunsets.

With shorter days (and Daylight Savings Time) on the horizon, the arrival of the equinox may spur you to make the most of the little light you have left. And after a nice day of leaf-peeping, you might want to try a bit of sky-peeping as well.

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