posted: 135 days ago, on Monday, 2016 Nov 14 at 00:37
tags: astronomy, outreach, Sun, Moon, Earth, astrophotography.

So it's Supermoon tonight. How can you tell? Because of the cape, silly!

"Supermoon" is apparently a word coined by an astrologer, showing that these ridiculous people can, after all, do something useful - make a contribution to the English language.

The idea of the supermoon is very simple: it's the more-or-less coincidence of a Full Moon (when the Moon is opposite the Sun in the sky) with the Moon at perigee (near the Earth on it's elliptical course). It's a more-or-less coincidence only, because it's super-unlikely that both events occur simultaneously. The table below shows the date of Full Moon (from 2016 until 2030) and the nearest date of perigee. The third column shows the difference, in round hours, between these two times. The Full Moon tonight is about 2 hours after perigee.

Here in Cape Town, we won't be able to see the Moon until around 19:30 when it rises, so where's missing the moment by about 6 hours.

By the way, the best views of a Full-ish Moon is the evening BEFORE the published date of Full Moon. There'll be enough light from the Sun to illuminate foreground objects (buildings, trees, cows) making them great photographic subjects.

Full Moon and lunar perigee

Full Moon (SAST)perigee (SAST)difference (hours)diameter (arcmin)
2016 Nov 14, 15:542016 Nov 14, 13:24233.13
2018 Jan 02, 04:252018 Jan 01, 23:56433.66
2019 Feb 19, 17:542019 Feb 19, 11:07633.26
2020 Apr 08, 04:362020 Apr 07, 20:10833.77
2022 Jul 13, 20:382022 Jul 13, 11:09933.77
2025 Nov 05, 15:202025 Nov 06, 00:30933.03
2026 Dec 24, 03:302026 Dec 24, 10:31733.67
2027 Jan 22, 14:192027 Jan 21, 23:511432.87
2028 Feb 10, 17:052028 Feb 10, 21:55433.19
2029 Mar 30, 04:282029 Mar 30, 07:41333.82

Generally speaking, the Earth has one moon. In all likelihood we have thousands. Google "mini-moons" if you don't believe me - or Temporarily Captured Objects (TCOs) if you prefer. During 2006-2007, we had two moons. A car-sized asteroid was captured by Earth's gravity and it orbited us four times before it was tossed out again. The moons of Mars, for example, are also tiny captured asteroids. In total, there are 181 known moons in our solar system. But still just the one Moon. Luna has been with us for about 4 billion years, and is our "sister planet", or rather more like your quiet aunt who's always there when you need her.

Without the Moon, we'd be screwed. For starters, we'd have only tiny ocean tides. Generally, the sea would be a chill place, because the Moon's gravity game churns the oceans about, circulating nutrients. Without this, a zillion ocean species go extinct. Before that happens, however, the climate goes to heck. The Moon's gravity stabilizes the Earth's movement in space; without it, our planet would experience turbulent seasons and we'd be going to hell in a wobbly hand basket as we twirl chaotically around the Sun.

Without the Moon, we'd have no werewolves, and patients in lunatic asylums wouldn't go extra-nuts once a month (or twice on a Blue Moon month). Oh wait, neither of these is true now. There really aren't shape-shifting lycanthropes, and behaviour of the mentally ill is not influenced by the lunar phase.

So what happens on the Supermoon? Well, first of all, the Moon is closer. Today, the Moon will be about 356,500 km from Earth; the average distance is 384,400 km. So that's like, numbers, closer. I wouldn't want to walk that far - about 28 kilokilometres - and it's also about the distance that your mouse moves in 145 years.

A closer Moon means a bigger Moon. It also means that seen from the Moon, the Earth will be bigger. Tonight, the Earth will be just over 2° across on the sky. From the Earth, however, the Moon will be 33.1-arcmin across, or just over half a degree. On average the Moon is about 31-arcmin across, so this is not a screamingly large difference. You certainly won't notice the difference. The easiest way to prove that the Moon really looks bigger, is to take a photo. It lasts longer, too. Then take another photo with the same camera settings, and you'll see the difference.

The Moon will also appear brighter. The Moon reflects sunlight, although not a lot of it. Only about 7% of light falling on the Moon is actually reflected, making the Moon as dark as a lump of coal. But at its closest, the Moon shines some 30% brighter. That's a noticeable difference. However, since you probably can't remember what you were wearing last Wednesday, you won't be able to compare the brightness from a month ago either. Taking a photograph and doing some fancy image processing may prove the 30% claim, but your eye/brain simply isn't a very good absolute photometer, so if you claim it was brighter this evening, you're fibbing.

Times for Moon rise can be found in the Sky Guide, an excellent publication that I can heartily recommend. On page 9 is a table showing the size of the Full Moon for the year, whilst on page 33 is a graph of the distance to the Moon at apogee and perigee for the year. And in the 2017 edition, you can win a telescope to help with your moongazing.

Meanwhile, grab your free copy of the "Virtual Moon Atlas" which gives lots of other information. For some calculatory fun, visit John Walker's " Lunar Perigee and Apogee Calculator" page at "Earth and Moon Viewer".

Looking for tips on photographing the Moon? Carol Botha's article is just the thing: "Moon photography tips, from me to you". For more, see "Photographing the Full Moon with a Digital Camera". Don't have film in your camera? Then find out about "Observing the Moon with the naked eye". Can't see the Moon to enjoy the lovely craters? Then "Make your own impact craters".

nothing more to see. please move along.