The planet Mercury crossed the disc of the Sun, as viewed from Earth, on 11 November 2019. The event lasted for five and a half hours and could be seen from most parts of the world. Transit of a planet occurs when the planet moves directly through the path between the Earth and Sun.

This, naturally, can happen only with planets whose orbits are within the orbit of the Earth. Hence, in the Solar System, it can happen only with the planets, Venus and Mercury.

In the case of Mercury, the planet goes around the Sun once every 88 days. As the Earth also moves by the time Mercury comes round, it passes between the Earth and the Sun once every 116 days.

However, as the plane of the orbit of Mercury is tilted, compared to that of Earth, the path of Mercury does not come between the Earth and Sun every time it goes past. The transit is thus a relatively rare event, just 13 or 14 in a century — the last was June 2016, and the next is expected in 2032.

Transits also occur with planets in orbit around distant stars. Even with our own Moon, a solar eclipse is a transit event. These events are useful because they give us a different view of celestial objects. We normally see these objects by the reflection of light that their mother star shines on them.

During a transit, what we see is the shadow of the object. In the case of distant planets, their reflected light is hardly visible in the glare of the mother star. It is hence only by the transit that they can be made out. This has now helped detect and estimate the orbit of thousands of planets around stars in remote parts of space.

Another use of a transit event, particularly within the Solar System, is that at just beyond the edges of the shadow, we can see light from the Sun that comes through the atmosphere of the planet.

Analysis of the light that comes through reveals things about the planet that cannot be made out by other means. But the first, and important use, which was made of transit events, however, was to determine the distance of Earth from the Sun.

The wonderful thing about the Solar System is that the distances of the planets from the Sun and the time they take to go around it are related by a simple mathematical relation — the closer the planet, the shorter the time of its year. It was with the help of this idea that it was confirmed, for instance, that Mercury and Venus are closer to the Sun than Earth, unlike the remaining planets, which are farther away. It was hence possible, once any one distance was known, to tell the distance of other planets from the Sun.

The quest was hence to find at least one distance — the earliest methods to discover anyone distance was by using the same method our pair of eyes tell the distance of daily things that we see around us.

As our eyes are a few inches apart, what one eye sees is not the same as what the other one sees. The brain is thus able to put together the images from the two eyes and work out the distances.

The first estimates of planetary distances were made in the same way, by sighting the planets from places as far apart as possible.

In 1672, Cassini and his partner, Richer, measured the angle of a sighting of the planet, Mars, to from Paris at one end of the Earth and the Cayenne Island in South America, at the other. A year later, Flamsteed made similar measurements but from the same place and one day apart, using the motion of the Earth for the separation.

From these sightings thousands of kilometres apart, they could hence work out the distance to Mars, and from this, the distance of Earth from the Sun. The distance they got was 87 million miles, which is quite close to the accepted figure of 93 million miles, or 150 million kilometres.

Transit of Venus: A century later, in June 1769, the transit of Venus gave astronomers the opportunity for a more accurate measurement.

In the case of a transit, the planet that is being viewed enters and exits the disk of the Sun a good few hours apart. The timing of transit depends on the parts of the Earth where the points of observation lie and the difference permits accurate calculation of the distance of the planet from Earth.

The quality of chronometers and telescopes had improved and the scientific community was more organised.

Teams were stationed at different parts of the globe and the observations brought together. It took till 1771 for the data to be analysed and the distance worked out was 95 million miles, a lot closer to the correct value than before.

We now have more sophisticated means of measuring distances, specifically Radar, which bounces a radio signal off the distant object and receives the echo. Unlike differences in millions of miles, we can now be correct to three parts in a billion — over 93 million miles, this works out to be something like 400 metres.

Now that we are sure of the distance to such a fine level of accuracy, it has become possible to make out changes in the distance. And it is found that the distance is gradually increasing. The increase is because the Sun is constantly losing energy and hence it’s mass, which allows the planets to move to larger orbits.

But the start was the first sighting of a transit, 250 years ago and every time transits happen, there are new experiments to test and refine what was seen the last time around.