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Venus’ rotation

Venus rotates around the sun in an elliptic orbit that is the most circular of any of the planets. The difference between its aphelion and its perihelion is only 1.5 million kilometres, which means its orbital eccentricity is only 0.007.

For comparison, that of the Earth is 0.0167. The inclination of Venus’ orbit to the ecliptic plane is about 3° 24'. Venus orbits the sun in the same direction as all the other planets and the time for one full tour is 224.70096 Earth days.

Rotation  Vénus

The rotational period of Venus on its own axis was unknown for a long time. Astronomers perceived small details in Venus’ atmosphere that implied that the clouds rotated in about 4 days, in the opposite direction to Venus’ orbital direction.

Finally, in 1962, radar penetrated the clouds and allowed us to measure the rotational period of the planet. Venus’ rotational speed is very slow : it turns once in 243.0185 Earth days, whereas the Earth turns in one. The planet rotates clockwise if viewed from the north pole. The sun rises in the west. We call this retrograde rotation (backwards compared to the Earth and most other planets).

 

Click for the difference between rapid and slow rotation.

(credit: Pearson Education)

So, the planet takes 243 days to turn on its axis and 224.7 days to orbit the Sun : a year on Venus is therefore shorter than a sidereal day (0.924 days to be exact).

The cause of this retrograde rotation is still poorly understood. The most likely explanation if a giant collision with another large body during the planet formation phase. The venusian atmosphere could also have played a role: heated by solar radiation, it would thicken; the tidal forces acting on Venus could then have slowed its rotation to the point of stopping the planet and even make it turn in the other direction.

This slow retrograde rotation is the origin of the solar day being shorter than the sidereal day. The latter are longer for planets characterized by anterograde rotation. The Earth, for example, has a solar day average of 24 hours and a sidereal day of 23h 56min 4.09s.

On Venus the solar day is a little less than half of the sidereal day i.e. 116 and ¾ Earth days (116d 18h). This means a little more than 2 complete solar days in one sidereal day! Venusian days and nights last almost 2 terrestrial months (58d 9h). However the thick atmosphere probably produces very gradual dusks and dawns.

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