1986年的哈雷彗星穿越銀河

Halley’s Comet, comet, or ball of ice and dust orbiting the Sun, that can be seen from Earth once about every 76 years. It travels in a very elongated path around the Sun, in a direction opposite to that of the planets in the solar system. The path of the comet extends from near the Sun to beyond the planet Neptune. The periods between its approach to Earth usually vary from 75 to 79 years because the gravitational forces of other planets change the comet’s orbit slightly. The comet’s scientific designation is 1P/Halley, the first comet listed in the official catalog of comets.

（注：本文非雙語(yǔ)對(duì)照）

美麗的哈雷彗星

Halley’s Comet has a rocky, irregularly shaped core, or nucleus, about 10 km (about 6 mi) across. The nucleus is composed of rock and dust held together by ice. Ice on the comet sublimes, or changes stage directly to gas, as the comet approaches the Sun. As this occurs, material escapes the nucleus, forming a coma, a distinctive cloud of gas and dust around the nucleus. A tail of gas and dust also forms as material begins to escape the nucleus.

As Halley’s Comet approaches the Sun, it loses rocky material. These chunks of rock continue orbiting the Sun in the same orbit as the comet. When Earth passes through the orbit, some of the particles enter Earth’s atmosphere, creating the Orionid meteor shower every year in October and the Eta Aquarids meteor shower every year in May (see Orionids).

哈雷彗星的軌道

Astronomers classify Halley’s Comet as a short-period comet, meaning that its orbital period is less than 200 years. Halley’s Comet belongs to the group of short-period comets with periods 20 years or longer, a group called Halley-type comets or intermediate-period comets. According to current theories, Halley’s Comet likely originated in part of the Oort Cloud, a vast sphere of icy bodies theorized to exist at a distance from the Sun of 20,000 to 200,000 astronomical units (AU). (An AU is the mean distance between Earth and the Sun, or about 150 million km [93 million mi].) Some calculations suggest that Halley’s Comet may have come from an inner, disklike region of the Oort Cloud.

When it first became a comet and entered the inner solar system thousands of years ago, Halley’s Comet may have been a long-period comet. Over time the gravitational pull of planets in the solar system may have reduced its orbital period, turning it into a short-period comet.

一起期待哈雷彗星的下一次降臨

?Humans have recorded the appearances of Halley’s Comet since the year 239 bc, when Chinese astronomers recorded the appearance of a “broom star.” English astronomer Edmond Halley published a treatise on the nature of comets in 1705. Halley was the first person to demonstrate that comets revolve in elliptical orbits around the Sun. He also predicted that the comet now known as Halley’s Comet would return in 1758. After his prediction proved accurate, the comet was given his name.

（注：本文非雙語(yǔ)對(duì)照）