Transit of Venus and the changing nature of discovery

TWO hundred and forty-three years ago, astronomers around the world - from the Arctic Circle to the South Pacific to the Indian Ocean - turned their instruments towards the sun. Many had braved perils ranging from pirates to parasites to reach their far-flung observing posts. But they considered the hardships worth enduring for their final chance to observe the transit of Venus across the blazing solar disc.

Under a master plan proposed by British astronomer Edmond Halley (whose eponymous comet is another once-in-a-lifetime attraction for most sky-gazers), this scattered fraternity aimed to measure the transit of Venus - the last that would occur for more than a century - in order to establish the distance from the Earth to the sun. That distance, the "astronomical unit", is the first in a system of yardsticks that now extends to the farthest reaches of the cosmos.

This week scientists will once again trek to remote corners of the globe to make last-chance observations of a Venusian transit - although they're not risking life and limb to do so, merely lost luggage, missed connections and inclement weather. Their objectives, too, are very different to those of their predecessors. NASA's Kepler space telescope - itself named after the astronomer who first predicted a transit of Venus - detects exoplanets by looking for dips in their parent stars' light output when they make transits of their own. The hope is that this year's Venusian transit will provide a benchmark for further efforts to uncover the properties of these distant worlds (see "Transit of a lifetime: Why all eyes are on Venus now").

The 18th-century astronomers could scarcely have envisaged any of this, although they clearly anticipated that their 21st-century successors would follow in their footsteps. Equally, we can scarcely imagine what uses astronomers will make of Venus's next transit. By 2117, space-borne emissaries will very likely have rendered Earthly observations redundant.

Sentiment rarely makes for good science, but it's nonetheless pleasing that researchers have found good reasons to continue one of astronomy's most venerable traditions. One might hope they'll find fresh grounds to do so in a century's time.

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