Why some planets rotate “the wrong way”

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

23 January 2011

Mwero Mataza asks whether Venus is the only planet that rotates clockwise and if so, whether there is a scientific explanation for this. Now that question is not exactly accurate. I am compelled to wonder: “clockwise when viewed from which side?

The proper way of describing the rotation of Venus is that it is in “retrograde” with respect to the other planets. That is, if you viewed the whole solar system, you would see all the planets rotating in the same manner, except Venus which spins in the opposite direction.

Now, if you were to go high above the Earth’s surface at the North Pole and looked down at our planet, you would see it rotating in the anticlockwise direction. Do the same observation from high above the South Pole, and the spin will be clockwise.

And why does Venus behave in this strange manner? The most probable explanation is that the planet was struck by a large meteorite that changed its direction of rotation.

On a related matter; Joseph Nderitu asks why the sun in Kenya sets earlier during some months of the year yet we are at the equator. The reason is that the axes of rotation and revolution are not parallel. They lie 23.5 degrees from each other.

Now depending on the position of the Earth along its orbit, we shall see the sun rising early and setting late or vice versa. However, the length of daytime remains a constant 12 hours.

As a resident of Mombasa, it is understandable why Justin Osey would wonder about floating. He asks: “Why do some people float on water and some sink - the same happens to some metals?  Secondly; is it true that what floats on sea water will sink in fresh water, even humans?  Finally; for an object to float on water, is it the shape or it's lightness on the weight or the air around it or inside it [that matters]?

I will answer them in reverse: objects float on a fluid if they have less mass per unit volume than the fluid. This is why a hollow metal container floats while a solid block of the same material sinks.

No. It is not true that whatever floats on sea water will sink in fresh water. If that were the case, then boats used in the sea would not be used in rivers!

However, sea water has a lot of minerals dissolved in it – that is why it is salty. For that reason it has more mass per unit volume (about 1.03kg per litre) than fresh water (about 1.00kg per litre). Therefore, it is slightly easier to float in the sea than in a river, for example.

And now the answer to Justin’s first question will make sense. The reason some people can float while others can’t is that they have different masses per unit volume.

Note that two people might weigh the same but have different volumes! In that case, the fatter one will float easier!