Dr N T Jiwaji

At the end of the first week of June this year, one of the rarest commonly observable astronomical events, the Transit of Venus, will be visible from Tanzania, on June 6. This event will not reoccur until after another 105 years - well beyond the lifetime of anybody alive today.

Historically, this event provided the first direct method of measuring the distance of the Earth from the Sun. This distance is the primary unit used for measuring distances and sizes in our solar system and beyond and is known as the Astronomical Unit (AU). Hence, it is not difficult to recognize the significance of the event that will take place in a few days’ time on June 6, 2012.

The Sun is completely out of human reach for any possibility of direct measurement of the Earth-Sun distance. However, in 1631, Johannes Kepler recognized that the extremely bright evening and morning star, Venus, being an inner planet, could provide a method of measuring this fundamental astronomical distance whenever Venus is directly between the Earth and the Sun. 

Note that if the Moon were to come between the Earth and the Sun, it would block the Sun completely and a solar eclipse would result. However, Venus is too far away so it appears only as a tiny dot in the sky. Hence, though it cannot block the Sun, it is still an eclipse situation.

The method of measuring the Earth-Sun distance during Transit uses the difference in the apparent positions of an object when viewed from different positions. This is called parallax, and is what you observe when you see the position of a nearby object change against a distant background when you move your viewing position. Such parallax effect is commonly visible when we are in a car or a bus. When we look at nearby trees and objects, they appear to rapidly change position against the distant sky as the car or bus drives forward.

During the Venus eclipse situation, parallax causes Venus to appear in different positions on the background Sun when it is observed from different places on Earth. Comparing this to our example of bus and trees, Venus would be the trees that change position when seen from different places on Earth (bus).

Transit of Venus was first used by a brilliant young scientist Jeremiah Horrocks who observed the 1639 Transit and calculated the Sun-Earth distance as 96 million kilometres (he tragically died at the age of 22). This value was off by more than 50 million kilometres, but it provided a catalyst for more accurate attempts during the next Transits of Venus in 1761-69 supported by Edmund Halley (of Halley’s Comet fame) who first suggested the use of the method of parallax positions of Venus. This resulted in a more accurate value of 153 million kilometres.

However, an error arising from the “backdrop effect” that merged the black dot of Venus too early with the edge of the Sun’s disk, made the calculated value still not reliable.

This galvanized even more international efforts to cooperate to make still more accurate measurements during the next Transit events in 1874-82. As a result, a much more reliable value of 149.50 million kilometres for the Earth-Sun distance was established.

Hence, the Transit of Venus has provided a setting for an extremely slow series of experiments conducted over more than 300 years and over wide areas of our world to provide a reliable measurement of the distance of Earth from Sun. It has showed how the solid theoretical knowledge of Newton and Kepler was used by experimentalists to persevere over several centuries in their quest to get reliable measurements. It was indeed a triumph of the scientific process driving discovery, and eventually human progress.

Progress in other areas of science, such as radar, has allowed extremely accurate measurement of the distance to nearby planets. The distance to the Sun can now be calculated far more accurately without using the Transit of Venus. The current Transit event of June 6 provides a valuable educational activity to challenge the minds of young scientists and gives a historical perspective to the progress of science with international collaboration. In this age of the Internet, such collaboration is extremely easy even for budding scientists in schools.

Another significant use of this Transit event is to test the methods and instruments for finding exo-planets that orbit distant stars. This method involves measuring the change in the brightness of a star when a planet comes in front of it. This similar to what will happen when Venus comes in front of the Sun on June 6.

The decrease is brightness of the star (or Sun) is extremely small. Can you imagine noticing any change in brightness of a headlamp of a car if a fly were to come in front of it? During this June 6 transit, scientists will check if their instruments can show any decrease in the extreme brightness of the Sun due to the tiny dot of Venus.

Transit of Venus will also allow scientists to measure the diameter of Venus, and study the atmosphere of that planet by measuring the light that has passed through the planet’s atmosphere.

On early morning of June 6, we in Tanzanians will see the end of the transit event from sunrise at 6:34 am until 7:49 am. At sunrise, the tiny dot of Venus will be close to the top left edge of the Sun, preparing to leave its bright face. You will need eclipse glasses to see this event since the Sun is extremely bright and your eyes can be blinded if you force your eyes to look at it.

If you do not have eclipse glasses, you can project the image of the Sun through a hole in a large cardboard and project it on the wall or on the floor.

As you look at the tiny dot through the eclipse glasses from sunrise on June 6, you will see the tiny dot of Venus shift slowly towards the edge of the Sun and eventually move out of the Sun, which will be the end of the Transit. By synchronizing your watch with International Standard time at, for example, you can measure the time of the last two contacts.  These are the time that the dot of Venus first touches the Sun’s edge and time that it finally leaves the edge.  You can report these data to for example

The exact line up of Earth-Venus-Sun is an extremely rare event. This happens only about once every century. However, when it does occur, it does so in pairs. The last Transit of Venus was in 2004 and the second of the pair is what we await at dawn on Wednesday June 6, 2012. The next transit after this one will not occur until after 105 years in 2117. So this is a once in a lifetime opportunity to see a rare astronomical event and appreciate its historical and scientific significance.

See for more details.