Ancient eclipse records quietly measure how fast Earth's spin is slowing
Astronomers can say with confidence that a total eclipse will happen on a given date centuries from now, but not exactly which longitudes the shadow will cross. The reason is that Earth's rotation is slowing unevenly. Run the sums backwards, though, and old written eclipse sightings reveal how long past days actually lasted.
Predictors track the slowdown with a correction called delta T, which grows as the planet decelerates. The geometry of the Moon's shadow can be computed precisely, but where it lands depends on how far Earth has turned, so an old chronicle noting a total eclipse over a particular city pins down the rotation at that moment. Forecasting began early; Chinese astronomers were making eclipse predictions by the 4th century BC.
That we get total eclipses at all rests on a coincidence. The Sun is about 400 times wider than the Moon and also about 400 times farther away, so both span roughly half a degree of sky. The match is imperfect, however. On average the Moon looks about 2.1 percent smaller than the Sun, so around 60 percent of central eclipses are annular, leaving a blazing ring, from the Latin for ring rather than year. Totality needs the Moon near its closest approach.
Timing is equally fussy. The Moon's orbit tilts just over 5 degrees from Earth's, so its shadow usually passes above or below us. Eclipses happen only when a new moon falls near one of the two crossing points, windows that come around about every 173 days. Every year brings at least two solar eclipses and as many as five, no more than two of them total.
The shadow races eastward because the Moon moves at about 61 kilometres a minute while Earth's equator turns at about 28. A totality track can be as wide as 267 kilometres, and although one occurs somewhere every 18 months or so, any particular spot waits roughly 360 to 410 years between them. The longest calculated so far falls on 16 July 2186 over northern Guyana, lasting 7 minutes 29 seconds.
Source: Solar eclipse
