All you need to know about Mercury’s transit across the Sun

What is a transit?

  • When an inner planet (Venus or Mercury) passes between the Sun and Earth, it is called a transit.

How does it look?

  • Mercury appears like a small dot on the surface of the Sun. As the transit progresses, the dot-like silhouette of Mercury moves from the left edge of the Sun to the right, appearing to move down across the sun’s disc. Somewhat counterintuitively, it is called “ascending transit.” In India, we will be able to view the beginning of the transit, from 4:30 p.m. to about 6:30 p.m. As the Sun sets, we will not be able to view it till the end whereas the Americas will be able to view the last part of the transit.

Importance of the transit

  • An accurate measurement of transits can contribute to checking formulas involving planetary dynamics. So it has educational value.

Historical significance

  • Until the end of the 17th century, there was no mechanism for calculating the distance from the Sun to the Earth. The relative distances between planets were known, for example, that Jupiter is about five times as far from the Sun as the Earth is. To calculate this, at least one absolute distance was needed, namely, the Earth-Sun distance.
  • Sir Edmund Halley observed a mercury transit in 1677 and realised that if the time taken for the planet to transit were measured from two different spots on Earth, this could be used to measure the Earth-Sun distance, which could be used to calculate the distances of other planets, absolutely.

How often does transit of Mercury take place?

  • Unlike solar eclipses, Transits of Mercury are somewhat rare. In a century, there are about 13-14 transits of mercury. They happen in pairs separated by about three years.

When do they happen in a year?

  • Transits of Mercury can happen in May or November. The May transits (ascending mode) are about half as frequent as the November ones (descending mode).

Who first predicted it?

  • Johannes Kepler (1571-1630), the astronomer who discovered the laws obeyed by celestial bodies, was the first one to predict, using these laws, that there would be a Transit of Mercury on November 7, 1631.

When it was first observed?

  • Pierre Gassendi first observed it on November 7, 1631, in England.
  • The second observation was at India. Jeremiah Shakerly, an astronomer and mathematician, observed a Transit of Mercury in Surat, on November 3, 1651.

When is the next transit of Mercury?

  • The next one will be on November 11, 2019. But this will be visible on Earth only from some countries including South America and Africa. It will not be visible in India. The following one, to occur on November 13, 2032, can be viewed from India.


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