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Motion of Visible Planets
 

This model examines the reasons for a loop pattern in the motion of the planets that are visible from the Earth.

Since the motion of planets around the Sun is obscured by Earth's motion along its orbit during their observation from the Earth, the planets appear to be moving across the sky dome either from East to West (direct motion) or from West to East (reverse motion). The instances when the planetary movement seems to change direction are called standings. The charts of the planetary paths appear to include a loop-like phenomenon. The greater is the distance between the planet and the Earth, the smaller is that loop. Planets make these loops rather than merely travel along the straight lines because the planes of their orbits do not coincide with the ecliptic plane.

The planets are classed into two groups: lower (inner) planets - Mercury and Venus; and upper (outer) planets, comprising the other six planets. (Select the planet type via the model switch.) The pattern of the planetary motion depends on whether a particular planet belongs to the lower or the upper group.

The maximal angular distance of a planet from the Sun is called elongation. Mercury has the highest elongation at 28?. Elongation of Venus is equal to 48?. In the case of eastern elongation, the inner planet is visible in the West during twilight, right after the sunset. In respect to western elongation, the inner planet is visible in the East at daybreak, just before the sunrise. Outer planets may be situated at any angular distance from the Sun.

Press "Run" button to initiate the model, and watch the planets rotate. Press "Stop" to pause the model, and press "Reset" to return the model to its initial state.

 
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