Mercury makes a transit Science

Mercury transit: the first ever observed

This composite image of observations by NASA and the ESA’s Solar and Heliospheric Observatory shows the path of Mercury during its November 2006 transit.  

From using zebra crossings to improve pedestrian safety, we are going to make an astronomical leap and find out about celestial crossings. Only the transits of Mercury and Venus can be viewed from the Earth as these are the planets that lie between the Earth and the sun.

The event that astronomers call the transit of Mercury corresponds to the innermost planet of our solar system appearing to cross in front of the disk of the sun, visible from Earth as a tiny black dot against the sun’s bright source of light. Mercury passes between Earth and the sun about 13 times every century (14 times this century), but the first time this phenomenon was observed was only in 1631.

It was Johannes Kepler who made the discovery that both Mercury and Venus would transit the sun, a month within each other in 1631. November 7 marked the day when Mercury would transit the sun and it would be followed by a Venus transit on December 6. As the sight of a planet passing in front of a solar disk had never been observed till then, Kepler and Jacob Bartsch called out to all astronomers to be on the watch out for these events.

Kepler’s fear

But Kepler was uncertain about his calculations and hence suggested that prospective observers take to the task a day earlier and not to give up till a day later. Unfortunately for Kepler, he did not live to witness the transit as he died on November 15, 1630. His fears regarding his calculations, however, turned out be uncalled for as his predictions were within 5 hours of the actual event.

History suggests that early November 1631 experienced stormy weather in much of Europe. As a result, only three individuals are said to have witnessed the transit with only one among them - Pierre Gassendi - leaving detailed records of his observations.

Detailed observations

According to Gassendi’s account, he projected an 8 inch image of the sun from his telescope’s eyepiece on a white screen. Though there was scattered cloud cover, Gassendi was able to observe the transit of Mercury as a tiny black dot, much smaller than what he had expected, slowly moved across the sun at around 9 a.m. local time in Paris.

Till the 1960s, transit technique (exact measurement of the moment when Mercury or Venus began moving onto or off the disk of the sun) was employed to calculate the astronomical unit and with that, distances to planets. It was only in the last half century or so that radar measurements have become more accurate in determining distances to other planets in our solar system.

The most recent such transit occurred on May 9 this year and had been visible from India. In case you missed it, the next one visible from India is another 16 years away. Though Mercury will make a transit on November 11, 2019, it is only the November 2032 transit that will be visible from our country.


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Printable version | Jul 29, 2021 3:20:18 PM | https://www.thehindu.com/in-school/sh-science/Mercury-transit-the-first-ever-observed/article16441805.ece

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