All about... the Venus transit

Credit: Wikimedia/MswggpaiThe transit of Venus is much like a solar eclipse, with the planet crossing in front of the Sun as viewed from Earth.

However while the Moon can completely obscure the Sun’s disc, Venus, being much further away, appears as a small black dot slowly making its way across it over the course of six to seven hours.

Like eclipses, however, the event would be far more common if the orbits of the bodies involved weren’t tilted.

Because of the incline, Venus usually appears above or below the Sun rather than directly in between it and the Earth. As a result, transits are among the rarest predictable astronomical events, occurring in a repeating pattern of pairs eight years apart – the last was in 2004 – separated by a gap first of 121.5 years and then of 105.5 years.

Johannes Kepler predicted the transit of 1631, but no observations were made. Subsequent transits (in 1639, 1761, 1769, 1874 and 1882) were used to make increasingly accurate calculations of an important astronomical measurement – the distance between the Earth and the Sun, which was previously unknown. Many other astronomical distances depend on this, from the size of the rest of our solar system to the distances to other stars.

By watching a transit of Venus and measuring the time taken for it to cross the entire Sun’s disc as viewed from two points on Earth separated by large latitudes, the distance from Earth to Venus can be worked out using the parallax method. Then, using Kepler’s third law, the interplanetary distance and the orbital periods of both bodies can be used to determine the Earth’s orbital radius.

Jeremiah Horrocks initially miscalculated the distance, arriving at a value two thirds of the currently accepted 93 million miles after the 1639 transit. The next pair were observed by a number of astronomers all over the world, and increasingly accurate results taken.

After the 1882 transit, Simon Newcomb combined data from the prior four events and determined the distance to the Sun to an accuracy of 0.31 million kilometres. Using other techniques such as radar, it’s now known to within around 30 metres. These figures allow easy calculation of the distance to other planets, stars, galaxies and ultimately the size of the observable universe.

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