Early views about Mercury

Mercury has been known since the time of the Sumerians (3rd millennium BC). The planet was given two names by the ancient Greeks: Apollo, for its apparition as a morning star, and Hermes as an evening star. Greek astronomers knew, however, that the two names referred to the same body. To the Greeks, Hermes was the messenger of the gods. In Roman mythology, Mercury was the god of commerce, travel and thievery. The planet probably received this name because it moves so quickly across the sky.

The first map of Mercury was made in the 1880s by Giovanni Schiaparelli, using a simple telescope. The map showed only areas of dark and light. A more detailed map was produced by Eugenios Antoniadi between 1924 and 1933, but has since been proved inaccurate. Both these astronomers believed Mercury rotated once on its axis in 88 Earth days, with one hemisphere permanently facing the Sun. This meant that it was thought that Mercury's day was the same length as its year. However, radar measurements carried out in the early 1960s showed that the true axial rotation period was 58.65 days. Thus it is now known that Mercury rotates three times during two orbits of the Sun. The result of this is that the same hemisphere is pointed towards Earth every time the planet is best placed for observation. This effect also means that the Mercurian day (sunrise to sunset) is 176 Earth-days long, or two Mercurian years.

Antoniadi also believed that Mercury had an atmosphere because he thought he

Figure 4.1 Mercury as seen by the Mariner 10 space probe in 1974 from 200 000 km away. (Photo: NASA)
Table 4.1 Details of Mercury

Distance from Sun

57 910 000 km (0.38 AU)


4880 km


3.3 x 1023 kg (0.055 times Earth's mass)


5.43 g/cm3 or 5430 kg/m3

Orbital eccentricity

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