Northern Sky: Sept 15 - 28

Norther Sky – by Deane Morrison  -  Sept. 15-28 2018

During the last two weeks of September, we have bright planets in the evening and a couple of regularly scheduled astronomical events.
 
If you face south 40 to 45 minutes after sunset, you’ll see Mars fairly low and still quite bright. Turning a bit westward, you’ll see the Teapot of Sagittarius, with Saturn above it. Next, even lower than Mars, there’s Antares, the red heart of Scorpius. Finally, low in the southwest, we have Jupiter. You probably won’t see Venus, though, especially later in the month, because it’s dropping into the sunset on its next trip between Earth and the sun. That trip also takes it, officially, from the evening to the morning sky.
 
As for stars, the Summer Triangle of bright stars is high in the south after nightfall. And in the west, the brilliant star Arcturus is still pulling its kite-shaped constellation, Bootes the herdsman, down toward the horizon. You might want to compare Arcturus with Vega, the brightest star in the Triangle and see if you can tell that Arcturus is ever so slightly brighter.
 
Now, about those scheduled events. First is the fall equinox. It arrives at 8:54 p.m. on Saturday the 22nd. At that point an observer from space would see the sun poised over the equator and Earth lighted from pole to pole. The equinox is also a crossover time of sorts. In spring and summer, the Northern Hemisphere tilts toward the sun, and the farther north you go, the longer the day length. But after the fall equinox, it tilts away from the sun and the days get shorter as you go north.
 
And, this is the time of year when the Northern Hemisphere is most rapidly shifting its tilt away from the sun. Which brings us to the second scheduled event: the harvest moon. Some say the harvest moon is the full moon closest to the fall equinox, others say it’s the first full moon after the equinox. This one qualifies on both counts. The full harvest moon, on Monday, September 24, rises over Grand Marais at 7:12 p.m., which is just two hours and 40 minutes before perfect fullness, so it’ll be nice and round.
 
But the harvest moon is more than just a name. Here’s how it goes. The full moon occupies a position on the other side of Earth from the sun—so it’s opposite the sun in the sky. Therefore, when we’re tilting most rapidly away from the sun, making it move south and rise later each day, we’re tilting most rapidly toward the full and nearly full moons and making them move north and rise relatively sooner each day.
 
Note that’s relatively sooner. The moon’s orbit makes it rise later from one day to the next; on average, around 50 minutes later. But around the time of the fall equinox, that interval gets slashed because of the moon’s rapid movement up through the northern sky. This year, for a few days centered on September 24, moonrise comes only 24 minutes later each night. That’s the harvest moon effect.
 
It’s fortunate for farmers, because it means that near full moon time, farmers harvesting crops don’t have to wait as long for a bright moon to come up and light their fields.
 
Finally, the rapid change in Earth’s tilt is sapping the day length faster than ever at this time of year, which I’m sure comes as no surprise. We’re losing around three minutes of daylight every day. But, I keep reminding myself, people in places like Alaska and Iceland have to put up with losses of six minutes a day.