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Edmund Halley (1656-1742) a contemporary of Isaac Newton (1642-1727) began addressing himself to the problem of cometary motion. With Newton's help, he began cataloguing and calculating the paths of comets using Newton's principle of gravitation, and discovered that one particular comet appeared in the same path about every 76 years. In 1705, he predicted that that comet would reappear in 1758, and it has been known as Halley's Comet since. It's currently outside the orbit of Neptune. The comet moves in an elliptical orbit, speeding past the Sun in about 2 years, but spending about 50 years slowly curving around its distant point, outside the orbit of Pluto. Its furthest distance will be in December 2023. One can calculate its distance from known formulas; however, in 2003, the Very Large Telescopes at Paranal, Chile observed the comet. Astronomers concur that the new telescope technology will be able to track Halley's at any point in its 76 year orbit from now on!
Now it may be beyond the Kuiper Belt
It will back on the year 2062. We know this by calculating 76 years, which the Halley comet is coming. It came at 1986.
Haley's comet visited us in 1986.
The periodicity of this comet is around 76 years.
The motion of the comet is retrograde or apparently from west to east to our eyes. Hence it looked approaching the sun during early morning before surise during the last visit in1986.
When the comet goes away from the sun from its nearest (perihelion) position, visiblity for our eyes is after sunset.
Haley was at the nearest postion (called perihelion) to its focus (that is the sun) in 1986.
By calculation the this comet has to be at its farthest from the sun (that is at aphelion) after 76/2 = 38 years
Now, by John Keplers's 2nd law of planetary motion, the radius vector , line 'comet-sun' covers equal areas in equal interval of time. You can interpret now that the speed of the plannet is more when it is nearer to the sun than when it is away from the sun.
Now it is 2009 year = 1986+23. The time that elapsed after prihelion postion of the comet is 23 years is nearlly 61% of the semi orbital period. From the above para, we can guess that comet has finished very much longer than 61% of the semi orbital distance. Again, the speed of the comet is more when it is nearer to the sun during the 61% of the time than that in the other farther half.
Therefore, the comet on its path away from the sun , is approaching its aphelion postion. The ascending postion of the comet maybe after midday but hours before sunset.
Hope this helps.
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