A daytime view of Mercury and the stars
5 August 1999During the total eclipse of the Sun on 11 August, the sky will be darkand some bright stars should be easy to see. Avert your eyes for amoment from the glories of the solar atmosphere, and you can glimpse theplanet Mercury, a newly fashionable target for space exploration.
Eclipse watchers in Europe and Asia will have an advantage over Copernicus. According to legend the great Polish astronomer's chief regret, on his death bed in 1543, was that he had never set eyes on Mercury. As the planet closest to the Sun, it is normally seen with difficulty only at dawn or dusk. During the forthcoming eclipse Mercury will be located upwards and to the right from the Moon, with the twin stars of Gemini (Castor and Pollux) pointing helpfully towards it. Space explorers have neglected Mercury, except for brief passes 25 years ago by the US spacecraft Mariner 10. These revealed a hot and bruised little planet, with a mysterious magnetic field. Now NASA is planning to return and European scientists are contemplating a possible ESA Cornerstone mission to Mercury, in the belief that it contains important clues to the violent origins of the Earth itself. On 11 August, Mercury will be almost at the western extremity of its orbit, as seen from the Earth. As for the planet Venus, it is on the nearest part of its orbit, passing between the Earth and the Sun. In a telescope Venus would look like a crescent Moon, but even so it will be much brighter than Mercury. The stars to the left of the eclipsed Sun make up the head of the imagined lion in the constellation Leo, which is usually seen in the evening early in the year. The brightest star in that group is Regulus. The accompanying sky chart uses the positions of stars as determined by ESA's Hipparcos spacecraft during its historic star-plotting mission 1989-93. Only naked-eye stars (magnitude 5.0 and brighter) are shown here, and in the viewing conditions the faintest of these are unlikely to be visible.