The moon looks so sad in Shawna’s painting and so it should. Tomorrow’s Blue Moon will be the last until Halloween 2020. We shouldn’t feel too brokenhearted though. 2018 has been rich in Blue Moons, enough to tide us over for a couple years. In popular parlance, a Blue Moon is the second of two full moons to fall within a calendar month. Because the interval from one full moon to the next is 29.5 days, this can only happen when the first full moon of the pair falls on the 1st or 2nd of the month. That allows enough time for the moon to wane to new and then wax back to full for a second time around at the end of the month.
This year has (had) two Blue Moons: January 31st and March 31. This a bit unusual and the reason February, the shortest month, had no full moon at all. The next two-Blue-Moon year will be 2037 with extra full moons in January and March just like this year. Normally, a Blue Moon happens about every 2.5 years. True to average, we’ll see our next Blue Moon on October 31, 2020. Here’s a table of Blue Moon dates from 1900 to 2100.
Of course the rising moon is anything but blue. The miles and miles of atmosphere we look through when the moon is low on the horizon scatter away the blues and greens in moonlight, leaving only the warm colors to reach our retinas. Sometimes the air stacks in layers according to temperature and resembles a Viennese torte. Because air is transparent we can’t see the layers except when the moon or sun is near the horizon, when their distorted, layered appearance dramatically reveals this amazing fact.
The squashed appearance of a rising moon is caused by atmospheric refraction. The dense, lower atmosphere acts like a lens or prism and bends the moon’s rays. Light rays coming from the bottom of the moon, where the air is thickest, are bent more than those from the top. This “lifts” the moon’s bottom up and squishes it into the top. Instead of a perfectly circular full moon, which we’d see if there were no atmosphere, it’s an oval. So you don’t miss this upcoming moonrise, click here to get your local rising time.
Saturday’s Blue Moon sits squarely in the constellation Virgo the Virgin, a somewhat faint pattern of stars that looks like an enormous cursive letter Y. You probably won’t see much of the constellation except for its brightest star Spica, a little more than a fist below the moon. Spica is an extremely hot, giant star 1,900 times more luminous than the sun and about 250 light years away. While the moon may lack blue, Spica’s high temperature gives it a hint of that color.
By the way, there’s an easy way to determine the date of full moon months from now. Remember that 29.5 days elapse between successive full moons. So if there’s a full moon on the 31st (as there will be this month), then the next will be 29.5 days after or late on April 29. By June, full moon occurs on the 27th and in November on the 22nd. Notice that the date is moving “backwards” at the rate of about a day a month. 2.5 years or 30 months — the average time between Blue Moons — sees the date of full moon move from the very end of the month to the very beginning. And that allows time for a second full moon to come round at month’s end.
What happens with the full moon also happens with all the other phases, so the waxing crescent, first quarter and all the rest move up about a day a month as you look ahead on the calendar. This little number games can help remember what the moon’s up to anytime.