SCI-TECH & AGRI

Retrograde Venus

QUESTION : Why does venus rotate backwards and slowly?

ANSWER : All the planets go around the Sun in the same direction, but they do not all spin in the same direction. Earth's spin axis is tipped 23 degrees relative to its orbit axis. Uranus is tipped 98 degrees and Venus is tipped 177 degrees, so astronomers say that Venus spins `backwards'. Mercury and Venus both spin slowly relative to other planets. One `day' on Venus is equivalent to 117 Earth days. However, on Mars one day is equivalent to 24.6 Earth hours, and on Jupiter one day is only 9.9 Earth hours. Asteroids spin even faster relative to planets. For asteroids one day is usually less than 10 hours.

Astronomers theorise that the planets' direction of spin was set during planetary formation. As small objects collided to form larger objects in the solar system, the final collisions involved planets the size of the Moon and Mars crashing together to make planets such as Earth and Venus. Depending on the relative motion before the collision, the spin of the resulting object could be in any direction.

The spin periods evolved after the objects formed. The direction of spin for Venus and Mercury was probably set by gravitational forces (tides) from the Sun. Tides are greatest when two objects are close together, so the Sun was able to slow the spin of Mercury and Venus but not any of the other planets. Tides from Earth have slowed the Moon down to the point that it does not spin at all relative to the Earth, which is why we always see the same side of the Moon. Why Mercury and Venus do not always show the same side to the Sun is subtle, and involves the non-circular orbit of Mercury and the massive atmosphere of Venus.

— The Science Teacher

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