Halley’s Comet returns after 75 years

Press, 3 December 1985, Page 29

Halley’s Comet returns after 75 years

By

JOHN HARFORD

Halley’s Comet is again within sight of Earth. With the advent of the Space Age, more interest than ever before is being shown in the most famous of all comets. This Is the first in a series of five articles which will background the once-in-a-lifetime event.

The 75-year wait is over. Halley’s Comet is again with us.

As yet, from the brightly lit southern hemisphere cities, only the trained ote server can pick the now hazy star-like light with the aid of a binocular or a telescope. Away from the cities, many people have now reported having seen the comet with the naked eye as it makes it first pass of the earth on its journey around the Sun.

Comet Halley will continue to grow larger and brighter in the night sky, though with only a short tail, until the glare from the near-full Moon will interfere with viewing in the last two weeks in December.

Apart from a brief glimpse in January, the comet will then swing out of sight around the Sun. It will reappear in March, comElete with tail, and be at its est for viewing from the

southern hemisphere in April. Halley’s Comet is an astronomical anomaly. It is the only bright comet with an orbit similar to man’s lifespan, which means that almost everyone since at least 240 B.C. has had the opportunity to see it once. Comets with larger orbits visit us so infrequently that it is difficult to predict the dates of their returns, while those with tighter orbits remain so close to the Sun that they are wasted by solar radiation and fade into insignificance.

Halley’s first recorded appearance was in 240 8.C.,

when it was seen by Chinese astronomers. Until recently, the comet’s visit in 164 B.C. was believed to be unreported. Babylonian tablets from that time, describing a comet similar to Halley in every particular, have been confirmed as sightings, making this the thirtieth recorded passage of Halley’s Comet. Although most periodic comets are named after their discoverers, Halley bears the name of Edmond Halley, Britain’s second Astronomer Royal. He was the first man to demonstrate that the comet journeys in a periodic orbit and is predictable in its appearances.

In 1705 he correctly predicted that his comet, last seen in 1682, would return in 1758. Halley, born in 1656, observed the Great Comet of 1680, not the one now known as Halley. This stimulated his interest in what was generally thought at the time to be an unexplained phenomenon, which heralded evil events, if not the end of the world. When he saw the comet, now known as his, in 1682, he set to work with his mathematical skills, Sir Issac Newton’s theory of gravity, and previous records of comets through the years.

Halley became convinced that the same comet had been seen three times since 1531. He calculated -its orbital period as 75 to 76 years and predicted its return in 1758.

He died in 1742 but the comet was “recovered” on Christmas night, 1758, by a German amateur, three months before perihelion in 1759, and the comet was given Halley’s name. Halley’s Comet moves backwards (opposite to the motion of the Earth) around the Sun, and along a plane tilted at 18 degrees to that of our planet’s orbit. This time around Halley comes closest to Earth on November 27 (at a distance of 93 million kilometres), and on April 11, 1986 (at a distance of 63 million kilometres). It' will reach perihelion,

or its closet approach to the Sun, on February 9, 1986. It will then be behind the Sun, preventing people on Earth from seeing it in its most spectacular manifestation. Searching began for Halley’s Comet at two of the largest telescopes in the United States in 1977, eight years before perihelion. It was recovered on October 16, 1982. j

During its sojourn in deep space, the comet has had to deal with gravitational forces from all over the solar system, travel 5,280,000,000 km from the Sun, and beyond the orbit of Neptune, almost five light hours away. Yet it will reach its appointed rendezous with the sun less than 12 hours earlier than expected.

TOMORROW: What are comets?