[Diagrams on page 15]

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The elliptical orbit of Halley’s comet

Neptune’s orbit

Uranus’ orbit

Saturn’s orbit

[Diagram]

White box indicates area of the comet’s orbit visible from the earth

Jupiter’s orbit

Mars’ orbit

Earth’s orbit

Sun

Perihelion

However, we are cautioned not to expect the display of Halley’s comet this time to equal the one in 1910. Some astronomers even say that this appearance may be the most disappointing in 2,000 years. Why should one appearance differ from another? It depends mostly on where the earth is in its orbit when the comet goes by. Of course, the closer we are, the better our view of the comet. Also, the larger the angle between the comet and the sun, the longer it will be in the night sky. Now it happens that on February 9, 1986, when the comet makes its nearest approach to the sun, called perihelion, and is at the peak of its performance, the earth will be almost exactly on the opposite side of the sun. That means we will be separated by the maximum distance, nearly 150,000,000 miles,a while the comet is directly behind the sun! That is the worst possible lineup.

But the comet will be inside the earth’s orbit for several months, passing closer to us both before and after perihelion. On its way in, the comet will be in a good position for those living in the northern hemisphere, but then it will not be very bright. It will be brighter and will pass closer to the earth on the way out. Then it will be high in the southern skies but not so well placed for northern observers.

A Wanderer in the Solar System

Halley’s comet rides in a high orbit. It is not at all circular but is a long narrow ellipse. It stretches out across the orbits of all the planets from Venus to Neptune. At perihelion it is only 54,000,000 miles from the sun, but at its most distant point, it is more than 3 billion miles away.

At about the distance of Jupiter, the comet comes within range of telescopes, and after it crosses the orbit of Mars, it reaches naked-eye visibility. About this time the tail begins to form. It grows larger as the comet approaches the sun. It always points away from the sun, blown by a solar wind and by solar radiation.