In 1889. G.V.Schiaparelli announced that his extensive observations of Mercury had shown that the planet rotated on its axis in the same time. 88 days, that it took to revolve around the Sun and that as a result it always kept the same face turned towards the Sun. This result was confirmed by several other astronomers between 1889 and 1965. It therefore came as a great surprise when radar observations in 1965 by G.H.Pettergill and R.B.Dyce showed conclusively that the rotation period was in fact 59 days, with an uncertainty of about five days. More recent and accurate measurements have shown that the period is 58.65 days, precisely two-thirds of the orbital period. This has been confirmed by observations from the spacecraft Mariner 10 in 1974 and 1975. The earlier, erroneous value, based on visual observations, arose from two effects. Firstly, Mercury is always close to the Sun in the sky and is a small planet, so that it is difficult to see surface features with certainty. Secondly, because of the eccentricity of Mercury’s orbit only some elongations are favourable for visual observations. In general, several successive favourable elongations occur at such an interval that Mercury turns more or less the same face to the Earth at each to give the impression of synchronous rotation.

Because of the extensive cloud cover, the surface of Venus can¬not be seen and it was only the use of radar that enabled the rotation period, of 243 days, to be measured. The rotation is retrograde (opposite to the orbital motion), unlike that of all the other planets except Uranus. If the period is 243.16 days, then the same face of Venus will be turned towards the Earth at each inferior conjunction.

The rotation periods of the other inner planets can be easily measured by direct observation. The Moon is in synchronous rotation so that it turns the same face towards the Earth, and Mars rotates in just 41 minutes more than does the Earth.

Mercury, Venus and the Moon therefore have their rotation locked into an orbital motion to give what is known as RESONANCE. Astronomers believe that all the inner planets were formed with periods of several hours or a day and that in some cases tidal friction was sufficient to slow down the rotation until it became locked into the resonance observed today. The most obvious tidal effect is the regular rise and fall of the Earth’s seas and oceans twice a day . Tides are also raised in the solid bodies of the planets. Because of the rigidity of solid matter, these tides are much smaller than those in the oceans. The actual sizes of these tides are not known and it is likely that there are substantial variations between the planets. Tidal friction causes the rotation of the Earth to slow down. To conserve angular momentum the orbit of the Moon must be simultaneously increasing in size. The Moon’s gravity acts on the elongated shape produced by the tidal bulges and causes a torque which tries to pull the bulges back in line with the Moon and this has the effect of slowing down the Earth’s rotation. The same effects apply on the Moon although, because there are no oceans, there are only tides in the solid body The radius of the Moon towards the Earth is about four kilometers greater than it is in directions at right angles. This is in fact much greater than any tidal effect and the Moon’s elongated shape must have been formed when the Moon was younger and hotter and been frozen in as it cooled and formed its extensive lithosphere. The effects of tides in the solid body of the Moon slowed down its rotation until it locked into synchronism with its orbital motion. Today there is no net torque on the Moon but were it to spin up or spin down, the torque on its elongated shape would in each case act to return the rotation to synchronism; the situation is stable.

The resonance of Mercury’s rotation with its orbital motion depends on the planet’s elongated shape but the two to three ratio of periods is only possible because of the high orbital eccentricity. The long axis points towards the Sun at perihelion but is at right angles at aphelion. The orientation at perihelion tends to give stability and outweighs the destabilising effect of the orientation at aphelion because of the 52 per cent greater distance at aphelion .

The rotation of Venus appears to be controlled by the Earth. For this to be possible, Venus must be elongated with its long axis towards the Earth at each inferior conjunction. Because of the great distance to Venus, this asymmetry must be rather large and it is not certain that Venus is strong enough to maintain the re¬quired shape. From the Earth, measurements of the actual shape cannot be made sufficiently accurately to detect the expected departures from a spherical shape and the question of their existence is unlikely to be resolved until suitable measurements are made from a spacecraft in orbit around Venus.

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