Solstice

Solstices are those moments of the year when the sun reaches its southernmost position (in December), or northernmost position (in June), at the Celestial Tropic of Capricorn or Celestial Tropic of Cancer, respectively.  The solstices are caused by the fact that the Earth's axis is not perpendicular to its orbital plane; it deviates by an angle of approximately 23.5 degrees of arc; this also causes seasonal changes in the weather.

The exact date of the two solstices vary slightly from year to year, falling on a date between the 20th - 22nd for the June solstice, and a date between the 20th - 23rd for the December solstice. 

Solstice festivals and celebrations were common (and held primacy) in most cultures of the ancient world.

Orrery
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Note:   An orrery is a model of the solar system. 

Behaviour of the sun during the solstices

On the solstices the length of the day and the altitude of the sun at noon are the maximum or the minimum of all the other days of the year.

June solstice  
(June 20-22:  Northern summer solstice, Southern winter solstice)

• At the equator the sun rises about 24° left from the East. Then it passes from North, where it reaches its maximum altitude of that day, that is 66°33'. Then it sets about 24° right from the West.

• At the Tropic of Cancer the sun rises about 26° left from the East. Then it passes from the zenith. Then it sets about 26° right from the West.

• At the Tropic of Capricorn the sun rises about 26° left from the East. Then it passes from the North, where it reaches its maximum altitude of that day, that is 43°06'. Then it sets about 26° right from the West.

• At the Polar Arctic Circle the sun touches the horizon at North without setting.

• At the Polar Antarctic Circle the sun touches the horizon at North without rising.

• At the North pole the altitude of the sun is always +23°27'.

• At the South pole the altitude of the sun is always -23°27'.

December solstice 
(December 20-23: Northern winter solstice, Southern summer solstice)

• At the equator the sun rises about 24° right from the East. Then it passes from South, where it reaches its maximum altitude of that day, that is 66°33'. Then it sets about 24° left from the West.

• At the Tropic of Cancer the sun rises about 26° right from the East. Then it passes from the South, where it reaches its maximum altitude of that day, that is 43°06'. Then it sets about 26° left from the West.

• At the Tropic of Capricorn the sun rises about 26° right from the East. Then it passes from the zenith. Then it sets about 26° left from the West.

• At the Polar Arctic Circle the sun touches the horizon at South without rising.

• At the Polar Antarctic Circle the sun touches the horizon at South without setting.

• At the North pole the altitude of the sun is always -23°27'.

• At the South pole the altitude of the sun is always +23°27'.

Exact Dates of the Solstices, 2000 - 2010

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