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Comets do not usually develop their tails until they are closer to the Sun, so the brightness of Hale-Bopp at that great distance raised hopes for a spectacular sight as it came closer to the Sun.The comet was in fact the brightest comet since comet West in 1976.

Records of comet sightings exist for as long as humankind has had the ability to create lasting archives. A bright comet makes an astonishing image, and some comets have been bright enough to see during daylight. For centuries in Europe, comets were considered evil portents, perhaps because of the coincidental passage of Halley's comet over the Battle of Hastings in 1066, when the Normans invaded England. Now about 1,000 comets have been observed more than once and have at least roughly calculated orbits. About 200 of these are short-period comets that have been observed multiple times and have well-documented orbits. Astronomers await the return of an additional 200 or so that have been viewed only once. Some 20 comets have

Comet Hale-Bopp was photographed in the constellation Andromeda at 8:14 PM- on March 31, 1997. During this 24-hour period, comet Hale-Bopp made its closest approach to the Sun. (George Shelton/NASA Kennedy Space Center)

been observed once and their return predicted but have not been seen again and are considered lost. Because of gravitational perturbations of large planets and accidents and perturbations in the distant outer solar system, comets with long periods are particularly difficult to predict and track.

Some of the brightest comets seen recently are listed in the table on page 127.The visual magnitude given is a measure of the brightness of a celestial body as seen from Earth. This scale has no dimensions but allows comparison among objects.The lower the magnitude number, the brighter the object; a decrease of one unit represents an increase in brightness by a factor of 2.512.The brightest star is Sirius, with a magnitude of —1.4; the full Moon is —12.7; and the Sun is —26.7. The faintest stars visible under dark skies are around +6. During its recent close opposition, Mars rose to an apparent magnitude of —2.9, when normally it is as dim as +1.8. Comets have appeared with magnitude —10 or even brighter, meaning that they shine as brightly as the full Moon and could cast their own shadows. Comet Skjellerup-Maristany was visible in the day next to the Sun with a magnitude of only —6.

Because orbiting through the inner solar system takes time, comets can stay visible from Earth for weeks or even months, apparently stationary to the unaided eye during a single night, their tails flaming behind them as if flying while standing still. Comet Hale-Bopp was visible to the unaided eye for a year and three months, a record for visual longevity in modern astronomy. Very large and close comets have had tails that stretched across a half or more of the sky, sometimes with multiple tails visible to the unaided eye, adding considerably to their magnificence.The Great September Comet of 1882 was the brightest comet seen in at least 1,000 years, far outshining the brightest full Moon.This comet visibly broke up over the period of its visit, appearing to have several attendant small comets at times.

Halley's comet is perhaps the most famous short-period comet. It is named for Edmund Halley, Savilian Chair of Astronomy at Oxford and contemporary of Sir Isaac Newton, who predicted its return near Earth in the year 1758 based on observations people had made in 1531, 1607, and 1682. Halley noted the periodicity and predicted

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