That means that its apparent size as seen from Earth varies from 33' 31" to 29' 22". That's a difference of 30% in apparent area, or that it's almost 205 square arcminutes bigger when at extreme perigee than at extreme apogee. And of course the brightness will increase by even more than that since the brightness varies with the inverse square of the distance.
The experts will know that there are actually five astronomical definitions of a month (as well as the civil ones). The two of interest to us are the -
1. Anomalistic: (apse to apse, or perigee to perigee): 27d.55455. This can vary in length by several days.
2. Synodic: (New Moon to New Moon) = 29d.53059. This can currently vary by up to 7 hours.
If the anomalistic and synodic months were the same length, then we would have perigee at the same phase of the month each time, at least on average. But because of the average difference of 1.97604 days, the date and time of perigee moves backward through successive months at an average of just under 2 days per synodic month. E.g. forthcoming perigees are: Aug 10d 17h 42m; Sep 8d 03h 00m; Oct 6d 09h 38m, Nov 3d 00h 28m.
So what gives us our Supermoons? Remember, we're looking for a perigee to occur as close as possible to the date and time of Full Moon. There's no standard definition, but the popular websites, magazines, etc, seem to agree that if they occur on the same date then that's a Supermoon. But you could have perigee at 00h 00m, and Full Moon on 23h 59m, giving a gap of almost a full day. Alternatively, you could have Perigee on, say, May 31d 23h 59m, and Full Moon on June 01d 00h 01m, giving a Supermoon even though the two events occur on different days, indeed in different months, although the time difference is only 2 minutes! So I would use a working definition of a separation in time by no more than 12h, irrespective of the dates.
But of course, not all perigees are equal. For example, in 2014, the least close perigee occurs on Nov 27 at a distance of 369,827km. That's 12,932km more distant than the perigee on 10 August. So for a really super Supermoon, we want a really close perigee, and for it to occur as close as possible to Full Moon.
And for us to see it at its best, it also needs to happen when the Moon is above our horizon, otherwise the distance will have increased a bit (and the Moon will no longer be exactly 'Full') by the time the Moon rises.
Ideally, it should occur at the time the Moon is highest in our sky, i.e. transiting the S meridian. And for the absolute best circumstances, it should occur when the Moon is overhead for the observer. If the Moon is overhead, you are about an Earth radius closer to it than when it is on your horizon: that's about 6370km.
BEST IRISH SUPERMOON?
The one on August 10 this year is quite good: it's the closest perigee of the year, at 356,895km. And it occurs only 27 minutes before Full Moon. But FM occurs at 19.09 BST, which is 1h 22m before Moonrise in Belfast, at 20.31 BST. That is not too bad, but it means that by the time the Moon transits here at 01.39 next morning, it will be a little further away, at 356,977km from the centre of the Earth. But the reduced distance from us here in Ireland as it transits, will more than make up for that. However, it will also be even more past Full.
Still, it will give us near perfect conditions to see a Supermoon combined with the Moon Illusion, which makes the Moon appear much larger when it's close to the horizon. It's only a psychological effect, but a very powerful one. So the rising moon on the evening of 10 August should appear really big.
I've done an article for the next issue of STARDUST going into this all in more detail, but to summarise, the one on 10 August will be roughly equalled by one on 2016 Nov 15, but it won't be bettered until 2018 Jan 1.
NB: This is a professional level event (apart from the public lecture), so be prepared from some fairly advanced maths and physics! T.M.
A second speaker to be confirmed.
The recent discovery that the cosmic microwave background also bears the scars of gravitational waves - the squeezing and stretching of space time itself - is enabling us to build an increasingly detailed picture of the birth of the universe.
(A very limited number of tickets are available for this lecture) If you are going to be in London then, this might be worth attending
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