Transition’ is a technical, actually astronomical word, simply meaning the passage of a planet in front of the Sun; in other words, across the visible disc of the Sun.

Actually, it is the passage of a smaller celestial body (any body) in front of the other, usually larger body. In the case of a bigger body passing in front of a smaller celestial body it would be a sort of an eclipse.

In physics, a transition is the movement of an electron from one atomic orbit to another. But that does not concern us for now. Well, in the course of its orbit, every planet is bound to pass in front of the Sun. So that it is always in transition. Then why is transition such an event? And why, if it is always in front of the sun, is it being said that planet Venus will be in transition on so and so date.

The matter of transition will have to be understood or else we might confuse matters and give up on an issue which does not occur often.

First of all, only two planets can transit the Sun; only the “inner” planets. That is, those planets that are near the Sun and orbiting “inside” the orbit of the Earth. These are Mercury and Venus. The outer planets (Mars, Jupiter, Saturn, Uranus and Neptune) do not transit, for from our position in the Solar System, we cannot observe the outer planets making a transit.

But if there were any “people” on these planets, they could watch these planets playing transit, transit!

Of course there are no people on any of the planets or moons of the Solar System (except for the Earth). Then again, every time a planet (say Venus) makes a transit (that is, it goes across the disc of the Sun) it is not always possible for it to make a transit.

For a transition, the position of the Earth in its orbit around the Sun has to be favorable, as also that of Venus (or Mercury), from which position Venus can be seen to be in transition. On other occasions it will appear to pass from above or below the disc of the Sun, and so no transit will take place.

Because of the movement of Earth, it is only possible for a transition to be seen during the daytime. Naturally! So, half of the world cannot see this remarkable natural phenomenon because the planet will complete its movement across the face (disc) of the Sun in quick time. Venus moves at high speed, just over 35km per second. It is quite easy for our young friends to work out the speed (orbital velocity) of Venus by dividing the distance from the Sun (108.2 million kilometers) by the time it takes to go around the Sun once (224.7 days). Now work it out from days to seconds. That will give you its orbital speed (velocity) which is always expressed as kilometers per second.

Another thing it will give you is a great deal of pleasure in working out a secret of nature all by yourself! Suddenly, you will become a young astronomer of whom parents, siblings and friends will be proud! And remember another very important thing: never ever look directly at the Sun. Not even with sunglasses, neither with a filter. Not with anything. It can destroy the cornea and consequently the eye permanently. Only use very special filters that carry a written guarantee against any damage to the eye. So, while you know that transition of Venus has happened, you possibly missed viewing it. So far so good!

Another important thing that we missed (and clever boys and girls must have noticed our doing so) is not the mention of Pluto as a planet. It is unfortunate that as a result of many astronomers’ dislike to call it a full-fledged planet, the poor fella was reclassified as a planetoid by the IAU (International Astronomers’ Union) in the year 2008. However, some astronomers, like me, still insist that it is a true planet. After all, it goes around the Sun (once in 248 Earth years) and has a moon of its own (called Charon), though it is small and cuddly! I love it! More about Pluto in due course!

The writer is a professional astronomer and a former head of PIA Planetaria. He can be reached at

astronomerpreone@hotmail.com