Halley's comet

A comet is an irregular body a few kilometers across consisting of rock, ice and frozen gases. Most comets which may be seen by us travel around the Sun in long, oval orbits. When a comet is near the Sun, it develops a long, tenuous tail of gas and dust pointing alway approximately straight away from the Sun.

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Halley's comet is named after Edmond Halley. He thought that comets were bodies of the Solar System in orbits around the Sun. He suggested that a certain comet was a regular visitor, returning every 76 years, and was, in fact, the same one which had been observed since 240 BC, but in particular in the years 1531, 1607, and 1682. In 1682 he calculated its orbit and predicted it would return again in 1758, and sure enough, the comet arrived in March 1759. Halley's comet made a particularly bright appearance in 1910. The last appearance in 1986 was much more modest.

Halley's comet

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This simulation shows the orbit of Halley's comet starting on January 1, 2009. Note: The simulation shows a static picture of the comet. The brightness of the comet and the size and direction of the tail varies greatly throughout the orbit.

  1. Halley's comet is predicted to return to its closest point from the Earth in the summer of 2061. Turn on View, Distance and verify this by running the simulation. Slow down the speed of the simulation when the comet approaches the Sun, watch the distance 2 - 7 and stop it when this distance is a minimum. Note the time and the approximate value of the closest distance from the Earth.

  2. Let the simulation run for 2 or 3 revolutions about the Sun. You will notice that the orbit does not exactly retrace itself. This is because of perturbations from the planets. The simulation includes most of the planets. Which planet do you think ha greatest influence on the orbit? Go to the Parameters page, set the mass of this planet to 1 kg and run the simulation again to see if your guess was correct.

  3. Turn on View, Speed and rerun the simulation. Explain by using conservation of energy that the speed of the comet increases when its distance from the Sun decreases.

  4. The plane of the comet's orbit makes a large angle with the planes of the planetary orbits. The comet also revolves about the Sun in a clockwise direction when looking down from afar on the north pole of the Earth. This is the opposite direction of the planets. Watch the orbit in 3D and projected down into the xz- and yz-plane to get a good picture of the orbit.