Orbit variation means Sunday's moon is more than just super
Posted November 12, 2016
Updated November 13, 2016
The "supermoon" rising on Monday will be the closest full moon since 1948 and until Nov. 25, 2034. We call any full moon that happens near perigee, or the point in the Moon’s orbit where it is closest to Earth, a supermoon.
When astrologer (that’s right, as fun as this is, this is not a scientific term) Richard Nolle coined the term 30 years ago, he included both new and full moons. We’ll stick to full moons since they are much more fun to observe. With that definition, November’s supermoon is the second of three supermoons finishing out 2016 (the next is Dec. 14).
But what’s so super about November’s supermoon? Why won't December’s supermoon be as super?
The moon is at its fullest when the moon, Earth and sun are lined up in that order. This happens with great regularity every 27.322 days (a sidereal month). The moon’s orbit around the Earth is slightly elliptical which varies how close each full moon is by more than 31,000 miles. So some full moons are super, some are not. Some supermoons, like November’s, are super-duper because of additional variance the shape of the orbit.
The orbit’s shape is measured in "eccentricity" which ranges from 0.0 (a perfect circle) to 1.0 (no longer an orbit but rather something leaving orbit). The moon’s eccentricity varies between 0.026 and 0.077. Lunar eccentricity is currently on the high side of that range, which brings the moon closer this month than in other months.
A supermoon occurs when a full moon occurs while the moon is at its closest point in its orbit. Monday's supermoon occurs when orbital variance brings the moon even closer.
December’s full moon occurs during a period of lower eccentricity, so the moon won't be as close at perigee, a difference of about 1,200 miles.
Sunday brings best time to view
Sunday night after sunset is looking like the best time to view the supermoon our area. The moon will appear about 14 percent larger (not really that noticeable) but also 30 percent brighter (very noticeable). The moon will be visible all night, but try to take a look before it rises too high in the sky, around 6 or 7 p.m. The moon will appear largest when it is close to the horizon thanks to the “moon illusion."
This is also when professional photographers recommend attempting to photograph the moon.
“Think of how to make the image creative – that means tying it into some land-based object. It can be a local landmark or anything to give your photo a sense of place,” recommends Bill Ingalls, NASA’s senior photographer.
While it may be tempting to wait until the moon is at its closest Monday morning at 6:15 a.m and reaches its fullest 2 hours 37 minutes later, mostly cloudy conditions are expected then.
If you don't get a chance to observe the supermoon Sunday night, try again Monday after sunset. Clouds are forecast to begin decreasing about noon as the chance for showers moves out of the area.
Try this at home
The "moon illusion" is one of the oldest optical illusions known. It happens when our brains use familiar objects in the foreground (trees, houses, etc.) to trick us into thinking the moon is further away than it is when it is directly overhead.
You can demonstrate this illusion with a sheet of paper and a little tape. Roll the paper into a narrow tube. While viewing the moon near the horizon, adjust the tube's size until it just fits around the moon’s edge, then tape the tube to ensure the size stays the same. This is the moon's apparent diameter. Observe the moon again through the tube in a few hours when it's higher in the sky. Though it looks smaller in the sky, you’ll see that it fills the same space in your paper tube and the apparent diameter is unchanged.
Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.