Tiny black Mercury flitting across Sun enthrals India

Post-noon on Monday, the grounds of Nehru Planetarium in New Delhi were filled with hordes of children accompanied by their teachers or parents, huddling around telescopes installed in the campus, to view the rare phenomenon of mercury transiting across the background of sun.

Nehru Planetarium had installed a massive Altaz-mounted computerised telescope – Celestron to track the movement of the sun. The telescope costs close to Rs 3 lakhs.

Mercury, the smallest and the fastest moving planet in the solar system made first contact with the edge of the sun, as seen from earth at 16:40:43 hrs. First contact indicates an appearance as if Mercury is touching sun from the outside. Barely ten seconds later, was a tiny black dot visible in the background of the fiery orange surface of the sun. It seemed as if Mercury had touched the sun from the inside. Mercury rapidly revolves in an orbit around the sun at the speed of 48 kilometers per second, while the speed of earth's orbital revolution is 30 kilometers per second. The sighting lasted uptil sunset that occurred at 19:01 hours.

Mercury being the smallest and the nearest planet to the sun, its orbit is smaller than the orbit of earth. Earth is third in row after Venus.

Mercury, periodically comes between earth and sun, just like a total solar eclipse phenomenon when moon comes in between the sun and the earth," said Arvind Paranjpye, director, Nehru Planetarium, Mumbai.

The phenomenon of Mercury transit is rare because it occurs not more than fourteen times in hundred years. "Not every transit of the mercury will be visible from India. This is because along with the revolution, earth rotates along its own axis. It is visible in India during May as Earth, Mercury and the Sun align linearly to make the sighting possible. The phenomenon will recur again in 2019, but it would not be visible from India. The next transit which will be visible from India will occur only in 2032 now," said Dr Nandivada Rathnasree, director of Nehru Planetarium, New Delhi.

Astronomy enthusiasts had carried along solar telescopes with inbuilt solar filters, that block harmful solar rays and point directly to the sun, for viewing. "Mercury is really tiny, 160th the diameter of the sun. It cannot be viewed with naked eye. Usage of telescope is a must," said Ananya Sharma, member, Amateur Astronomers' Association in New Delhi.

Apart from viewing the tiny planet which is rarely visible from earth due to its measly size, school children were conducting experiments to project the image of the sun and the black dot on white paper from unto the telescope.

Nehru planetarium authorities had installed an 8 x 8 feet cardboard black box with multiple viewing slits to observe the phenomenon. "Early viewing methods included projection of the image of sun onto the inside of the cardboard box from the telescope. We have attempted to recreate this. It is to get a better view of the planet as the image of Mercury is magnified upto 1.2 mm this way as also to cut out ambient light," said Dr Rathnasree.

Past discoveries

In the early 1600s, Johannes Kepler discovered that both Mercury and Venus would transit the sun in 1631. In the 17th century, astronomer Edmund Hailey had used mercury transit observations to pin down the distances between sun and earth. "The transits also serve as a proof of the theory propagated by Copernicus in 15th century, when he said that it is the sun and not the earth that is the centre of the solar system," said Paranjpye.

"When Mercury is in front of the sun, we can study the exosphere close to the planet," said NASA scientist Rosemary Killen in a press note. "Sodium in the exosphere absorbs and re-emits a yellow-orange color from sunlight, and by measuring that absorption, we can learn about the density of gas there."

Also, scientists use the phenomenon to track planets orbiting distant stars which are otherwise obscured by brightness of the sun. The transit causes a slight dip in the brightness as it blocks the tiny portion of sun's light.