This is Dudley Observatory’s Skywatch Line for Friday, September 4, through Sunday, September 6, written by Alan French.
The Moon reaches last quarter Saturday morning, so moonrise over the weekend is late, leaving the evening skies nice and dark. Moonrise is at 11:41 Friday night, 12:30 am Sunday, and 1:22 am Monday.
If you look high overhead at 9:00 pm you’ll see the bright star, Vega, luminary of Lyra, the lyre. Like the majority of the brighter stars in the night sky, Vega is bright because it is one of our nearer stellar neighbors. It is 25 light away and ranks as the fifth brightest star. The only brighter star in the sky at 9:00 pm now is Arcturus, the reddish star in the western sky, which on outshines Vega slightly,.
Lyra is one of the smaller constellations in the sky and its pattern of stars is easy to recognize. If you are facing south and looking high in the sky at Vega, look for a parallelogram of stars to Vega’s lower left. A fifth star to the upper left of Vega completes the pattern of the Lyre. The star to the upper left is quite famous – it is known as the double-double, and is actually four stars. If you have sharp eyes you might be able to see that there are really two stars there, close together. If you can’t see the two by eye, binoculars will nicely reveal them. A telescope and higher power reveals that each of the two is itself a double star, and they are a pretty sight. (If the air is unsteady, they can be hard to discern. Try another night.)
You have probably heard that there is a total eclipse of the Moon on Sunday, September 27, and we’ll be writing about it on that weekend’s Skywatch Line. You may also have heard it called a “Supermoon Eclipse.”
The Moon’s orbit is not quite circular, so its distance from the Earth varies. When it is closest to Earth it appears slightly larger than when it is farthest away. Sometimes the extremes occur close to full Moon, and in recent years the media has taken to sensationalizing the extremes, calling the slightly larger full Moon a “supermoon” and the smaller a “micromoon.”
Exactly how much difference is there between a supermoon and a micromoon? Fortunately, this is easy to show using photography, and has been done numerous times. You can see one example here. In such photographs the difference is quite obvious, but the photos do not reflect two realities of looking at the Moon in the night sky, so they can be misleading.
First, the Moon’s apparent size in the night sky is much smaller than it looks in the photo. To get some idea of how small the Moon actually looks, hold an aspirin at arm’s length. It would completely cover the Moon. When objects are so small it is harder to discern small differences in size. More importantly, we can never do direct comparisons – supermoons and micromoons can only be seen well separated in time.
Here’s an easy experiment you can do if you’re really curious. Draw two circles on a white sheet of paper. Make the first one inch in diameter and the other one and one-eighth inches in diameter. These represent the micromoon and the supermoon. Now place them 115 inches away from your eyes, a distance where they accurately represent the apparent sizes of the micro and supermoons. You should be able to tell which is larger, but it is not as obvious as it was with the larger photos. Now imagine that you look at one circle and then the other, separated in time by many months. (The next micro-full Moon is on April 22, 2016.) The “extremes” of the Moon’s size at full are perhaps not as newsworthy as they seem.