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Thread: Can the sun stop in the sky and reverse its course.

  1. #1 Can the sun stop in the sky and reverse its course. 
    Forum Radioactive Isotope mitchellmckain's Avatar
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    Oct 2005
    Salt Lake City, UTAH, USA
    Sure. It does so all the time. But only in the sky of mercury

    In the April 2010 issue of Scientific American there is an article by Edward Bell entitled "8 Wonders of the Solar System", and it explains:

    "Sunrise and Sunset on Mercury are spectacles to behold. Two and one half times larger in the sky than seen on Earth, the sun appears to rise and set twice during a Mecurian day. It rises, then arcs accross the sky, stops, moves back toward the rising horizon, stops again, and finally restars its jourey towards the setting horizon. These aerial maneuvers occur because Mercury rotates three times for every two orbits around the sun because Mercury's orbit is very elliptical."
    To explain the part about rising and setting twice, where the sun stops and turns around depends on where you are on mercury's surface. If the place it stops and turns around is just below the horizon then that means it will rise again in the same place it just set then turn around again and set again in the same place.

    The orbit and rotation of mercury exhibits an interesting phenomenon of tidal coupling where there is a ratio of 3 rotations for every 2 orbits. Only 1.5 rotations for every orbit has some rather peculiar consequences.

    To explain it is perhaps best to compare this with the earth which has 366.25 rotations for every orbit. Yes 366.25 even though there are 365.25 days in a year. Why? These 366.25 time periods are called sidereal days and it is the time it takes for a star to go 360 degrees through the sky of the earth. In other words a particular star rises and sets 366.25 times every year while the sun rises and sets only 365.25 times every year. This is because with the sun, you lose one earth rotation every year because of the orbit around the sun.

    Now consider what this means in the case of mercury. 1.5 rotations means 1.5 sidereal days per mercurian year. One day lost means only .5 mercurian days per mercurian year. Thus a day on mercury is twice as long as its year, or in other words, it is day for the whole time of one orbit around the sun and then night for the whole time of one orbit around the sun.

    But what someone discovered/realized is that mecury's highly elliptical orbit means that mercury moves so much faster in the part of its orbit that is closer to the sun that, the motion of the sun in sky due its orbit actually exceeds the motion in the sky due to its rotation and this causes it to move bacwards in the sky during that part of its orbit.

    Thus what I can add to Bell's description above, is that during this time when the sun is moving backwards, this is also when the sun is largest in the sky of mercury. In fact this means that it motion in the sky is more like that of a loop-de-loop. We see the planets do the same sort of thing from our perspective on the earth and this is portrayed in the Ptolemaic view of the solar system.

    The following shows the actual loop-de-loop paths of the planets in the sky as they simultaneously "go around the earth" and around their epicycles.

    It is one of the misconceptions that this Ptolemaic view is completely wrong. The truth is simply that its just unnecessarily complicated when you look at things from the geocentric perspective. And the above picture doesn't show all the complexities because for greater accuracy the little circles the planets have to loop-de-loop on (called epicycles) actually have smaller loop-de-loops on them as well (epicycles on the epicycles).

    The Copernican system is more useful in science especially for the big picture because of its greater simplicity. But now we can look back at the historical conflict as a little over-dramatized and see that the real point is that we should not feel confined to any particular perspective - though naturally it makes sense in most situations to choose the simplest one. But, in any case, the fact remains that the Ptolemaic system DOES represent the actual motion of the planets in our sky from our point of view.

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