Moon Standing Still
If you’ve been outside after dark the past few days, you’ve likely noticed the full-appearing moon. While Thursday the 19th marks the true full phase, September’s Harvest Moon fills the sky for several days at a time. The full moon closest to Autumnal Equinox, the Harvest Moon gets its name from the role it played historically in providing light for farmers to bring in the last of the season’s crops.
It’s not that this moon shines any longer than other moons. In fact, the full moons of December and January shine more than 15 hours compared to the 12 hours of Harvest Moon. All full moons rise as the sun sets and set the next morning as the sun rises. But for several days this moon rises and sets little more than 30 minutes later from day to day compared to the usual hour difference. Farther to the north the shift is even less, around 20 minutes near the U.S.-Canada border and a mere 10 minutes at the Arctic Circle.
The reason this moon stands still is its place along the ecliptic, the apparent path followed by the sun, moon, planets and constellations as seen from Earth. This time of year, the nighttime ecliptic is a low, shallow arc through our southern skies, with the moon slowly inching eastward from night to night.
While the moon lingers in our skies, the sun makes its fastest exit at the time of Autumnal Equinox, September 22. The ecliptic is at its most shallow in the night, but at its steepest in the day. With the sun setting along a near-vertical line, the time from the first edge of its disc sinking beneath the horizon until the whole sphere is gone is less than four minutes. Juxtaposed to the moon inching from east to west day by day, the sun moves in leaps and bounds, each day setting more than two minutes earlier and rising more than two minutes later than the day before.