2 September 2005

transit of Greenwich

Tick-tock goes the atomic clock
Australia's clocks officially go atomic from today.New national laws, which come into effect from 1 September 2005, have moved Australia to a new time standard based on the super-accurate atomic clock.

The system, known as co-ordinated universal time (UTC), replaces Greenwich Mean Time (GMT).

GMT is based on the average time it takes the Earth to rotate on its axis from noon to noon, a standard that is becoming increasingly unsatisfactory as more sophisticated technology demands more miniscule units of time.

UTC is based on the vibration of caesium atoms as a pendulum, which make it accurate to a nanosecond, or a billionth of a second. Because the Earth's rotation is relatively imprecise, GMT can be out by several thousandths of a second, and can be affected by tides, currents in the Earth's molten core, seasonal change or major tectonic movements.

Irregularities can have implications for global positioning, high-speed computing, astronomers, security networks and electronic transaction time records.

The move to UTC brings Australia in line with New Zealand, Singapore, some US states and most of the European Union.

Losing GMT cuts us off from one part of our history. Timekeeping brought the British empire into the Pacific.

Transactions of the Royal Society
1768, with the rank of lieutenant, he [James Cook] was appointed to the command of the Endeavour, accompanied by Mr. Green, astronomer, to observe the transit of Venus at Otaheite, in the South Seas; and an account of their observations on that occasion is given in the article above. Along with them also sailed Mr., now Sir Joseph Banks, and Dr. Solander. After the transit was discovered, Mr. Cook sailed on a voyage of discovery, in which he discovered and visited a number of new lands; as the Society Islands, New Zealand, Nevis Holland, Botany Bay, &c. In June, 17, 1771, he arrived in England, and was appointed a commander in the navy, an account of the voyage being published by Dr. Hawksworth.

James Cook and the transt of Venus
The size of the solar system was one of the chief puzzles of 18th century science, much as the nature of dark matter and dark energy are today. In Cook's time astronomers knew that six planets orbited the sun (Uranus, Neptune, and Pluto hadn't been discovered yet), and they knew the relative spacing of those planets. Jupiter, for instance, is 5 times farther from the Sun than Earth. But how far is that … in miles? The absolute distances were unknown.

Venus was the key. Edmund Halley realized this in 1716. As seen from Earth, Venus occasionally crosses the face of the Sun. It looks like a jet-black disk slowly gliding among the Sun's true sunspots. By noting the start- and stop-times of the transit from widely spaced locations on Earth, Halley reasoned, astronomers could calculate the distance to Venus using the principles of parallax. The scale of the rest of the solar system would follow.

But there was a problem. Transits of Venus are rare. They come in pairs, 8 years apart, separated by approximately 120 years. Halley himself would never live to see one. An international team did try to time a Venus transit in 1761, but weather and other factors spoiled most of their data. If Cook and others failed in 1769, every astronomer on Earth would be dead before the next opportunity in 1874.

On his way back to London from Tahiti, Cook discovered and claimed the east coast of New Holland and the rest is, as they say, history.

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