Northern Sky: April 28 - May 11, 2018

Northern Sky by Deane Morrison for April 28 - May 10 2018

April's weather has been forgettable, but on the evening of the 29th it gives us a gorgeous full moon. Perfect fullness happens at 7:58. That's just minutes after moonrise in Grand Marais, so when it first clears the eastern horizon it'll be one of the roundest moons ever. April's full moon is called the full pink moon, after wild ground phlox, also known as grass pink, a small pink flower that carpets the land this time of year. This full moon won't be especially large or small, but if you can get outside just before eight o'clock on Sunday, the 29th, it will be worth it.
 
The next night is a treat for anybody who follows the Celtic holidays. It's April 30, the eve of May Day. According to old Celtic tradition, sundown on April 30 was the beginning of the holiday known as Beltane. It signified the end of the dark half of the year, which began at sundown on Halloween. The critical events that defined the dark and light halves of the old Celtic year had to do with evil spirits. At sundown on Halloween, they came rushing out of exile and started causing illness and otherwise making trouble, but at sunrise on May 1 they disappeared into exile again, and the light half of the year began. Beltane, or May Day, and Halloween are astronomically based holidays. They're what's known as cross-quarter days and they fall more or less halfway between an equinox and a solstice. Groundhog Day is another one, but the fourth cross-quarter day doesn’t get much press. That’s Lughnasa, a harvest holiday at the beginning of August.
 
As we get into May, the moon will be waning and spending less time in the evening sky. Be sure to have a look at Venus; it's the bright object that comes out in the afterglow of the setting sun. In the east, Jupiter is a rival beacon. These are the two brightest planets, and when darkness falls you can compare them not just in brightness but in color. On the night of the full moon, Jupiter rises below the moon and follows it across the sky.
 
Jupiter is about as bright as it gets now because Earth laps it in the orbital race early on the night of May 8th. That event is called opposition because it puts Jupiter opposite the sun in the sky. At opposition an outer planet rises around sunset and sets around sunrise. On the 10th, Jupiter and Earth come within about 409 million miles. That's their closest approach, and it comes after opposition because Earth is moving farther from the sun while Jupiter is moving in, and that brings them closer before speedier Earth pulls away.
 
For early risers, we still have, in this order, Mars, Saturn and Jupiter strung from left to right across the predawn sky in the south. This string of planets is lengthening as Mars moves eastward against the background of stars while Saturn and Jupiter move westward. Saturn and Jupiter appear to move westward against the stars, in what's called retrograde motion, now because they’re at or nearing opposition. When Earth is lapping one of the outer planets, the act of passing it by makes it appear to move backwards, or westward, even though the planet’s own motion carries it eastward. It's the same when a runner in an inside lane laps a runner in an outer lane. The outer runner will seem to move in the opposite direction from how he or she is actually moving. Saturn reaches opposition in late June, Mars not till late July.