Using a telescope to spot Charon, Pluto's moon, passing in front of a star is like trying to catch a fly with chopsticks. The satellite is tiny and rarely passes across a star bright enough to yield useful measurements.

But Bruno Sicardy, of the Paris Observatory, decided to attempt such a feat this year, in the hope of determining the satellite's radius. Before Sicardy's work, the best estimate of Charon's radius, calculated from models, was 600–650 km. “This translates into big errors on density,” Sicardy says. The mass of Pluto and Charon are well known, he adds, but measurements of radii are needed to determine their density, and therefore composition.

Also, Sicardy wanted to see whether Charon has an atmosphere. As the satellite is about half the size of Pluto — which does have an atmosphere — Charon provides an opportunity to ask an interesting question, says Sicardy: “What is the lower size limit for a body to keep an atmosphere?”

The problem is that, for Earth-bound observers to measure Charon's radius, the satellite must pass directly between us and a star; such ‘occultations’ are rare. One, however, was scheduled for 11 July 2005. To make sure he seized the moment, Sicardy worked for a year forming alliances with both government and amateur astronomers, all of whom could, based on calculations of where Charon ought to be, point their instruments in its path.

And even making these predictions was tricky, as Pluto and Charon's orbit wobbles through the sky, creating minute changes in position that could defeat astronomers' efforts to take their measurements. Calculations from NASA's Jet Propulsion Laboratory helped to improve the odds. Charon was discovered in 1978; the most recent occultation occurred in 1980, and was observed from South Africa.

Sicardy eventually lined up about 15 telescopes of various sizes across six Latin American countries. “The idea was to catch Charon at different places, and then eventually reconstruct its size and shape,” he says.

Organizing this mix of astronomers was half the fun, Sicardy says: “We've created a club. It's a very long process. It takes years.” But visiting the members was enjoyable. “It's like eighteenth-century astronomy,” he says. Just as Captain James Cook travelled halfway round the world to observe a transit of Venus, “we go and travel somewhere far away, where you can observe something hard to get elsewhere”.

But the most important part was successfully seeing the star disappear behind Charon. There were problems: “Some of the observations were lost because of stupid things, such as data not being recorded on the hard disk,” Sicardy says. But despite the glitches, the team got four solid occultations. “All four of these chopsticks were aligned with the trajectory of the fly.”

The result, published on page 52, shows that Charon has a radius of 603.6±1.4 km. This agrees well with measurements by a team at the Massachusetts Institute of Technology, made during the same occultation and published on page 48, of 606±8 km.