The Earth's reflectance - or 'albedo' - is closely related to cloud cover and the concentration of airborne particles. But it fluctuates daily and annually because land, sea, polar ice and vegetation all reflect different amounts of light. Earthshine becomes brighter as the Sun rises over Asia, for example, because this huge landmass is more reflective than the Pacific Ocean. "This makes it necessary to average lots of data for an accurate indication of a changing albedo", says Goode.

Goode's team used charge-coupled devices at the Big Bear Solar Observatory in California to measure earthshine for two years around 1998. The measurements were taken during periods when the Moon was a thin crescent and the dim portion was large. At these times, an observer on the Moon would see an almost 'full' Earth - that is, nearly all of the Earth's disk would be visible, and reflected light from almost half of the Earth's surface reaches the Moon. This gives a far more complete picture of the albedo than the limited scope of current satellite-based measurements.

After compensating for scattering in our atmosphere and the elevation of the Moon, Goode's team puts Earth's albedo at 0.297. This means that nearly a third of the sunlight that impinges on Earth is reflected into space, a value that closely matches the team's earlier computer simulations. "But we found a surprisingly large seasonal variation - up to 20% - in the Earth's reflectance", explains Goode.

The result of a comparison with similar observations in the mid-1990s was also unexpected. "We found a hint of a 2.5% decrease in the albedo in the last five years", says Goode. A proven drop in the Earth's reflectivity over that time - during which the Sun's activity has climbed from a minimum to a peak - would support the theory that the 11-year solar cycle directly affects the Earth's climate. Scientists also believe a drop of just 1% could play a role in global warming.