Historical Notes: Total eclipses old hat for dinosaurs

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"ECLIPSES ARE brilliant events," claim enthusiasts, boldly coining an oxymoron. They are certainly not dull, notwithstanding the lack of light. Apart from the astronomy, all manner of intriguing zoological phenomena result, the birds and the bees returning to their roosts, dogs howling, and people migrating. Into the eclipse path, that is.

Whilst solar eclipses are striking, the path of totality is thin, a strip typically 60 miles wide. Random locations experience totality just once every four centuries, on average, and only for a few minutes, giving them rarity value. Lunar eclipses - when the Moon passes into the shadow of the Earth - are visible from the whole of the planet's night-side, and last for several hours. Thus maybe 60 per cent of humankind could witness a specific lunar eclipse (plan now to view that next 21 January, visible from throughout the UK). Being more widely seen, lunar eclipses are noted often in the records of ancient civilisations.

When was the Crucifixion, for example? Many modern versions of the Bible state that the Sun entered eclipse after Jesus died on the Cross, but the original texts indicate a lunar eclipse, the Moon being the dark colour of blood when it rose that evening. We know the Crucifixion was at Passover, meaning near full moon, when only a lunar eclipse can occur: solar eclipses happen when the Moon passes between Earth and Sun, a fortnight away in the lunar month. Calculations based on the precise orbits of Moon and Earth indicate that as the former rose as seen from Jerusalem on 3 April in AD 33, it would have been in eclipse. So, modern astronomy indicates the date.

Eclipses may be back- calculated with considerable precision, the main uncertainty being the spin phase of the Earth: over centuries the planet is gradually slowing because of the drag force imposed through the tides raised by the Moon. A day in 2000 is about 0.0017 seconds longer than a day in 1900, which is why leap seconds must be inserted every 19 or 20 months. Stepping back two millennia, the accumulated milliseconds add up to about three hours, meaning that the Earth's total rotation over that interval is about one- eighth of a turn less than it would have been without tidal drag. This shifts the calculated tracks of totality by a couple of thousand miles from those derived assuming the day-length to be unchanging. Knowing that totality was observed in some year in Athens, say, allows astronomers to define the spin phase at that time with high precision, because the track is so narrow.

Fossil evidence informs us about how the day-length has changed over geological time. When the dinosaurs first walked the Earth there were about 375 days in a year, 10 more than at present, each day more than 30 minutes shorter than now. The month was shorter in absolute terms because the Moon was closer, making it appear larger in the sky, and that means that it was easier for it to blot out the Sun. The dinosaurs saw total eclipses rather more frequently than we do. That's not to say that they were better astronomers than us. They didn't anticipate the asteroid which did them in. Then again, maybe we won't either.

In more recent history eclipses have been regarded with suspicion rather than being auspicious events. In King Lear, Shakespeare wrote, "These late Eclipses in the Sun and Moon / Portend no good to us." Similarly, John Milton in Paradise Lost: "In dim eclipse disastrous twilight sheds / On half the nations, and with fear of change / Perplexes monarchs." Perhaps Prince Charles will be perplexed by the eclipse track of 11 August 11 crossing his Duchy of Cornwall. When the Sun goes out, we should recall how the ancients regarded these events,but without foreboding ourselves. It's all perfectly natural. Brilliant, actually.

Duncan Steel is the author of `Eclipse: the celestial phenomenon which has changed the course of history' (Headline, pounds 14.99)