快猫短视频

The Transit of Venus by David Sellers, MagaVelda Press, Leeds, 拢12.95,
ISBN 0954191308

ASTRONOMERS need to know exactly how far away the Sun is. This
distance鈥攖he Astronomical Unit (AU)鈥攊s the baseline from which all
planetary, stellar and galactic distances are estimated. But accurately
measuring that staggering 150 million kilometres or so is no mean feat. The
Greeks, who were right about so many things astronomical and mathematical, got
this crucial measurement completely wrong 2200 years ago.

So in the absence of an exceedingly long tape measure, how do you gauge the
Astronomical Unit? The stately voyage of Venus across the face of the Sun proved
to be the clue. From the mid-18th century, detailed timings of the rare passages
of Venus across the solar disc gave the distance from the Earth to the Sun,
aided by measurement of the slight nightly movements of Mars and nearby
asteroids against the stellar background, and, more recently, how long it takes
radar signals to be reflected back from the Sun.

David Sellers has produced a thoroughly engaging and well illustrated tale of
the hunt for the accurate AU, concentrating on the transits of Venus. We read of
early examples of government science funding鈥攂eginning in the 18th
century鈥攁nd of the interplay of international cooperation and rivalry.
Enticing details are given of strange observing practices at the Greenwich
Observatory, Californian astronomical expeditions being decimated by typhoid,
accusations of cheating in Lapland, and even a ship in the Pacific being made
unseaworthy due to the nails holding it together being exchanged for sexual
favours.

Accuracy was everything, and Sellers rightly emphasises the continual quest
for improvement in both the instruments and the techniques, as well as the care
that was needed when analysing the data.

Sellers eagerly anticipates the morning of 8 June 2004, when Venus will cross
the face of the Sun for the first time since 1882, taking about five and a half
hours. You can check the measurement for yourself.