The not so super Supermoon

Each 29 days the moon rotates around Earth. Moon’s orbit’s not circular, but egg-shaped. So there’s a point at which it’s closest to Earth. This point’s called the lunar perigee. It’s not a fixed distance. On average it’s 364,397 kilometres. But two nights ago it was 356,577 kilometres. This is the closest the moon’s been to Earth since 1993. When one lunar perigee’s in the lowest 10% of the range of all lunar perigees, the moon’s called a Supermoon [1, 2, 3]. When the moon’s at perigee it exerts more gravitational pull, creating higher tides (and a bigger difference between high and low tides). So a Supermoon causes high tides that are slightly higher than normal. And it has some impact on seismic activity due to the stronger gravitational pull between the sun, the moon and Earth. But even so, there’s no clear evidence the Supermoon was a causal factor in the earthquate and tsunami that hit Japan on 11 March – which was eight days before the Supermoon. On 11 March the moon was at an average distance from Earth. The 19 March Supermoon was about 20% brighter and 15% bigger than an average full moon. These increases were insufficient to be noticeable to the casual observer. But that didn’t prevent media hype [4]. I was tempted to look. But the entire time between moonrise at 7:28 p.m. on 19 March and moonset at 6:55 a.m. on 20 March, the Mole Creek sky was in full cloud. Ho hum.

P.S. Still on matters astronomical, happy southern hemisphere autumnal equinox to you.

farmdoc's blog post number 620

The last day of each year’s never a happy day for me. Because on this date, my darling Mum died. Fourteen years later, I still miss her dreadfully. To my shame, I didn’t mention her in last year’s 31 December blog post. I resolved not to do that again. So far, so good. Also each 31 December I’m conscious, increasingly so each year, that my own end nears. I don’t know how many years I have left. But I’m always glad to make it to the end of another one. The big year-end topic that’s fascinated me this year, is whether or not tonight’s moon is a blue moon. Wikipedia mentions several definitions of a blue moon. Colour has nothing to do with any of them. One definition is two full moons in the same calendar month. The previous full moon was on 2 December. The question is whether there’s another one tonight. Last Monday’s Age said yes. But Tuesday’s SMH said no. The SMH is correct: Geoscience Australia’s website tells us the full moon’s on 31 December at 7:13 p.m. UTC (i.e. Greenwich Mean Time) which equates to 6:13 a.m. on 1 January eastern Australia daylight saving time. Anyway that’s for the purists. I can tell you two things: first, when you look at the New Year’s Eve sky tonight you’ll see a full moon; and second, only once in a blue moon is someone lucky enough to have a Mum as wonderful as mine.

farmdoc's blog post number 457

It is 384,403 km from Earth – about 30 times Earth’s diameter. Its diameter is 3,474 km – about 27% of Earth’s. It’s almost spherical – its equatorial radius is 1,738 km and its polar radius is 1,736 km. Its volume is 2% of Earth’s. About 59% of its surface can be seen from Earth, but only 50% at any one time. Its force of gravity is about 17% of Earth’s. It orbits Earth each 27.3 days. Its average orbital speed is 1.02 km/second. Its phases repeat each 29.5 days. Its average surface temperature is 107°C during the lunar day and -153°C during the lunar night. It’s responsible for Earth’s tides. Its larger appearance when closer to Earth’s horizon’s an optical illusion. It has no magnetic field, no wind and no water. It’s the inspiration for many songs including these [1, 2, 3]; for the word ‘lunatic’ stemming from the Latin word ‘luna’ denoting the traditional folklore link between madness and its phases; and for a walk made famous by Michael Jackson. The first spacecraft that touched it and photographed its far side in 1959, were Russian. Twelve people – all American men – have walked on it. The last was Eugene Cernan on 13 December 1972. The first was Neil Armstrong (pictured) at 0256 GMT on 21 July 1969 which in Eastern Australia was 12:56 p.m. That was 40 years ago today. And seven days after my father died.