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Nature of Eclipses
 

This model simulates the causes of solar and lunar eclipses. Observe the Earth and the Moon rotating around the Sun in the model's upper left corner. Note that the Moon either becomes progressively lighter and rises over the ecliptic plane (as if emerging from water), or becomes darker and descends under the ecliptic plane over time.

Celestial sphere crosses the terrestrial orbital plane, circumventing it by an ecliptically shaped path. The plane of the lunar orbit is inclined to the ecliptic plane at an angle of 5?09'. Lunar orbit plane forms a large circle known as the lunar track. The ecliptic plane and the lunar track intersect with each other on a straight line that is called the line of nodes. The points alongside which the line of nodes and the ecliptic plane intersect are called the ascending and descending nodes of the lunar orbit. The lunar nodes continuously move towards the Moon, (i.e., to the West), and complete the revolution in 18.6 years. The latitude of ascending node decreases annually by approximately 20?.

Since the lunar orbit plane is inclined to the ecliptic plane at the angle of 5?09', the Moon may be situated far from the ecliptic plane during new Moon or full Moon periods. From the Earth's perspective, in such a case, the lunar disk will pass higher or lower than the solar disk. However, these conditions do not create the right circumstances for an eclipse.

The Moon must be in the vicinity of the ascending or descending nodes of its orbit, that is, not far from ecliptic during new Moon or full Moon periods, as a pre-condition for the occurrence of a solar or lunar eclipse. For simplicity's sake, the model considers such circumstances to occur twice per every revolution of the Earth. Observe umbra and penumbra appear at the model's bottom left side. They are the phenomena that are responsible for the eclipse.

The windows on the model's right side display the sky as it is seen from two independent observatories located on the Earth's surface. Point A (upper window) continuously observes the Sun. Solar eclipses may occasionally be observed at Point A (once per period, for simplicity's purposes). Point B (lower window) is situated at the opposite side from Point A. Point B tracks the Moon. Lunar eclipses may be observed from Point B once per revolution of the Earth.

Use "Run", "Stop", and "Reset" buttons to control this model.

 
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