London - Be prepared for something extraordinary on the night of 27 to 28 September: both the brightest full Moon of the year - and the dimmest full Moon.
First, we are due a supermoon. Our celestial companion travels round the Earth in an orbit that's distinctively oval. Every month, the Moon swings from a distance of 360 000km out to 405 000km. When it's nearest, the Moon naturally looks bigger.
Most months, we're not really aware of the Moon's changing diameter. But it's different when the Moon is closest at the same time as it's full because its proximity makes the illuminated Moon even more brilliant. Everybody's now calling this phenomenon a Supermoon - a phrase dreamt up not by astronomers but an astrologer.
On the night of 27 to 28 September, the Moon is closest to us at 2.46am, only an hour before it's full. As a result, this supermoon will appear 14 percent bigger in the sky than the Moon at its most distant and smallest, and it should be 30 percent brighter. The Moon will certainly look unusually big and brilliant around 2am. But at 2.07am you'll see a small chunk being nibbled out of its brilliant disc by the Earth's shadow. Sinking deeper and deeper into the darkness, the Moon is totally eclipsed by 3.11am. It remains completely in the shadow of the Earth until 4.23am, when the full Moon gradually begins to emerge.
During totality, the Earth blocks sunlight from falling on the Moon so you might expect it to go completely black. In fact, some sunlight is bent around by the Earth's atmosphere and lights up the Moon feebly. The light is reddened by passing through our atmosphere -the same reason why sunsets look red - so the Moon usually shines with a faint coppery glow even in mid-eclipse. Catch it if you can, as it's the last total lunar eclipse we'll see for over two years.
The illumination on the Moon depends how cloudy the Earth's atmosphere happens to be, so we can't predict how visible the eclipsed Moon will be - but it will certainly be the darkest full Moon of the year.
And that's not all. As the full Moon nearest to the Autumn equinox, this is the Harvest Moon, whose radiance used to help farmers bring in the ripened corn - in the days before combine harvesters fitted with brilliant lights.
At the equinox, the Moon's gravitational pull on the Earth is exactly in line with the Sun's, and that means they pull in unison on our oceans. As a result, the ''equinoctial tide'' is the highest of the year. Having the Moon closest to the Earth this month will pile up the water even more.
Astrologers and doom-mongers are sure to latch on to this combination - along with the baleful lunar eclipse - to fill the media with dire prognostications of flood and devastation (some pundits wrongly say that tidal forces also cause earthquakes and volcanic eruptions).
But don't panic. According to Britain's National Tidal and Sea Level Facility, around most of Britain's coast the sea-level will be no more than an inch or two above the highest tides of the past 20 years.
WHAT'S UP
Saturn is the only planet visible in the evening sky. It's low down in the south-west in the constellation of Libra, setting at around 9.45pm - that's why the ringworld isn't marked on the charts, which are drawn up for 11pm.
If you're a lark, watch out for brilliant Venus, which rises nearly four hours before the Sun by the end of the month. Mars lies to its left, but the Red Planet can't hold a candle in the brightness stakes to its dazzling sister. And giant Jupiter joins the trio of planets in the morning sky, rising at 4.30am.
Also for larks: on the morning of 5 September the Moon (at last quarter), passes through the Hyades star cluster. It occults the bright red giant star Aldebaran between 5.40am and 5.55am - depending on your location. And if you're lucky enough to be in southern Africa, Madagascar (or even Antarctica) on 13 September, you'll be able to catch a partial eclipse of the Sun.
DIARY (WHAT TO LOOK FOR)
5 September, 10.54am: Moon at last quarter
13 September, 7.41am: new Moon; partial eclipse of the Sun
21 September, 9.59am: Moon at first quarter
23 September, 9.20am: autumn equinox
28 September, 3.50am: full Moon; total eclipse of the Moon
The Independent