Star gazers around the world watched with excitement as the Earth’s sister planet Venus trekked gracefully across the sun’s disk for over 6 hours earlier this month. It was a special event because no one alive today is likely to be around when the next Venus transit happens in 2117.
Slightly smaller than the Earth, Venus is the Earth’s nearest planet and comes as close as 40 million kilometres to us. It has no moon, no magnetic field and no water. The Earth’s 23.5° tilt causes our seasons, but Venus is tilted at a mere 3° which means there are no seasons on the planet. Its atmosphere has 96% carbon dioxide while its thick sulphuric acid cloud cover reflects 85% of the light falling on it.
Over 20 space missions visited Venus since 1964 — the first Venera spacecraft sent by the Soviet Union was crushed by the impossibly high atmospheric pressure of 92 times that on the Earth’s surface! NASA’s Mariner 10 made a fly by Venus on its way to Mercury as did Cassini-Huygens which flew by Venus twice, in 1998 and 1999, on its way to Saturn. NASA’s Pioneerand Orbiter missions delved deeper into the planet’s secrets and the sophisticated Venus Express sent by the European Space Agency in 2006 sent valuable information about cloud cover, volcanic activity and lightening on the planet.
Interestingly, seen from its North Pole, Venus has a clockwise spin whereas the rest of the planets in the solar system spin counter-clockwise. Further, its spin is so languorous that it takes 243 earth days to complete one rotation while it orbits round the sun in 225 earth days — a day on Venus is longer than a year on our planet!
Venus revolves around the sun in a near perfect circular orbit at a distance of 109 million kilometres while the Earth revolves in an elliptical orbit at an average distance of 149 million kilometres. In the course of their predictable movements Venus and the Earth are sometimes on the same orbital plane. Such alignment of planes happens in pairs of events separated by eight years and these events themselves happen once in 105 or 121 years. If Venus passes between the Sun and the Earth during such an alignment of orbital planes, it will appear to us like a small dot moving across the sun’s disk – this is what we call the Venus transit.
A transit is really like a solar eclipse, but solar eclipses are spectacular and occur at least two times a year, while Venus’ passage across the sun’s disk is rare and hardly noticeable. So what was all the excitement about?
Venus has a special place in human culture. The Maasai of Africa named it Kileken and have a charming tale about The Orphan Boy who comes down to Earth to help an old man. It is called Shukra in Hindu mythology. Son of Bhrigu, Shukra was the teacher of the asuras and he learnt the Sanjivani mantra which is the key to immortality. Shukra presides over Friday and has an enmity with Jupiter.
The ancient Egyptians mistook this celestial object for two different bodies: the Morning Star and the Evening Star. It was Pythagoras of Samos who pointed out in the 6th Century BCE that Phosphoros, the Bringer of Light and Eosphorus the Bringer of Dawn were the same — but he believed that this ‘wandering star’ orbited the Earth. The Greeks then named it Aphrodite, their goddess of love. Inheriting their mythology from the Greeks, the Romans named it Venus, after their own Goddess of Love and Beauty.
Venus has been the subject of exquisite sculptures like the 2,000 year old sculpture of Venus of Milo (now in the Louvre Museum in Paris) and the 1486 painting ‘Birth of Venus’ by Boticelli (in Florence).
The Mayans called it Quetzalcoatl and developed a religious calendar based on its motion, while the Babylonians, like so many other cultures, associated it the Goddess Ishtar and with notions of fertility and womanhood. The symbol of Venus in Alchemy is also the symbol for the female in biology — no wonder a popular book on gender differences claims that women are from Venus!
Venus is the brightest object in the earth sky after the Sun and the Moon. But the most illuminating effect of Venus was in 1610 when Galileo observed the phases of Venus with his telescope. These phases, Galileo concluded, could only be explained by the Copernican idea that Venus was orbiting the Sun rather than the Earth. That laid the foundation of the new astronomy.
Venus transits in 1761 and 1769 allowed astronomers to observe it from different parts of the world and calculate the distance between the Earth and the Sun by using a system of triangulation. This distance is now known as an Astronomical Unit and these calculations allowed the first understanding of the true scale of our solar system.
Afterthousands of years of ruling legend and lore, since its first observed transitin 1639 Venus has come to occupy a prime place in the history and future of science.