Observations by SPACE in INDIA


One cannot measure the distance to the sun just by ordinary means which we use in our day to day lives. Astronomers since time immemorial have been trying to do that. Around 300 B.C., Aristachus of Samos attempted to derive the distance to the sun by observing the 1st quarter moon. Using concepts of geometry he estimated it to be about 20 Earth-moon distances. Around 200 B.C., Eratosthenes, using data acquired during lunar eclipses, attempted to measure the Earth-Sun distance and came up with a value of 804,000,000 stadia (about 133 million kms). Though these were major achievements for their time, we know today, that the true distance is about 400 Earth moon distances (~150 million kms). Then came the idea that observation of the transit of Venus across the disk of the sun could be the key to deriving the distance to the sun, the "Astronomical Unit" or AU.

Jeremiah Horrocks

The first known observation of a transit of Venus was made by Jeremiah Horrocks from his home at Carr House in Much Hoole, near Preston in England, on 4 December 1639.

Sir Edmond Halley (of Comet Halley fame!!) gave us plans for calculating the AU from transits of the sun. By observing the apparent shift in position of Venus against the background of the solar disk as seen from two different places on Earth, one can, using a bit of trigonometry, derive the distance to Venus which when coupled with Kepler's 3rd law of planetary motion will yield the distance from the sun to all the planets. Though he did not live to see the transit (he died in 1742 whereas the transit happened in 1761)

Edmond Halley

The most recent transit of Venus observed from Earth  took place on June 8, 2004. The event received significant attention, since it was the first Venus transit to take place after the invention of broadcast media. No human alive at the time had witnessed a previous Venus transit, since the previous Venus transit took place on December 6, 1882.

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