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Motion of the Luminaries
 

All the luminaries are constantly moving through the skies, and all of them make one revolution per day from the Earth's perspective. However, the patterns of the luminaries' movements are quite different.

The stars of the northern hemisphere's skies are the only visible stars for an observer who is located at the Northern Pole. These stars rotate around the North Star, and they do not recede beyond the line of the horizon.

Similarly, an observer located at the Southern pole may watch the stars of the southern hemisphere only. Both the northern and southern hemisphere stars may be observed from the equator.

The phenomenon of the luminaries' passage across the meridian is known as the transit. A luminary's elevation h is maximal at the upper transit and it is minimal at the lower transit. The period between the transits of the luminaries is equal to 12 hours (half a day).

The elevation of the luminaries at the upper transit is

h = 90? - φ + δ.

The elevation of the luminaries at the lower transit is

h = φ + δ - 90?.

This model illustrates the starry sky as it appears at the North Pole. "Show Constellation Lines" switch will familiarize you with the shapes of the constellations. The zenith is located at the upper edge of the model window.

Press the "Run" button to see the stars rotating around the pole, which is situated near the North Star. Pressing the "Stop" button will suspend the animation. "Reset" button will return the model to its initial state. A separate indicator displays the elapsed time.

Use the input window to change the geographical latitude of the observer's location. Observe the North Star rising to zenith or descending beyond the horizon as its line passes below the figure's border.

 
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