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Solar Declination   The Earth's axis-of-rotation is not perpendicular to the Sun/Earth plane, but rather is 23.5 degrees off.   This causes the North Pole to tilt toward or away from the Sun, depending on where the Earth is in its orbit around the Sun.   Solar Declination is the angle between the Sun's rays and Earth's equatorial plane.   (Technically, it is the angle between the Earth-Sun vector and the equatorial plane.)

 

The Solar Declination angle is zero during an equinox, and 23.5° during a solstice. By convention, the angle is considered positive when the North Pole points toward the Sun, negative when the South Pole points toward the Sun. The Declination angle is 23.5° during the Northern Summer Solstice, and –23.5° during the Southern Summer Solstice. It is between –23.5° and 23.5° the rest of the year.     Northern Summer Solstice   The North Pole reaches its maximum tilt toward the Sun on the Northern Hemisphere's Summer Solstice. In Visual Sun Chart's 365-day-year, this happens on June 21. On this day, the entire Arctic receives sunlight all day, and the entire Antarctic stays dark.   Here is the portion of the Earth that receives sunlight at 17:00 GMT on that day:

 

21 Jun 17:00 GMT

 

The Arctic Circle, Tropic of Cancer, Equator, and Tropic of Capricorn are shown as cyan (blue/green) lines. A black cross marks where the Sun is directly overhead, in this case on the Tropic of Cancer in the Bahamas.   The Tropic of Cancer is 23.5° North Latitude. The Sun will be directly over this latitude this entire day.   The Arctic Circle is 66.5° North Latitude (23.5° down from the North Pole), and receives Sun all day on this day.   The Antarctic Circle is just out of view at 66.5° South Latitude.   This is also called the Southern Winter Solstice. It is the day the South Pole points furthest away from the Sun's rays.     Autumn and Spring Equinox   An Equinox happens twice per year, every six months, when the Sun is directly over the Equator. On this day, the North and South Poles are the same distance from the Sun:

 

Equinox 17:00 GMT

 

All points on Earth receive 12 hours of daylight on the Equinox, with the Sun directly over the Equator all day. In this example, at 17:00 GMT the Sun is on the Equator over southern Colombia near Peru and Ecuador.     Southern Summer Solstice   The South Pole tilts closest to the Sun's rays on December 21, the Southern Summer Solstice. This is summer in the Southern Hemisphere. In the Northern Hemisphere this is the Winter Solstice, when the North Pole points furthest from the Sun's direction. On this solstice, the Antarctic receives daylight all day, and the Arctic receives no Sun:

 

21 Dec 17:00 GMT

 

The Sun is directly over the Tropic of Capricorn at 23.5° South Latitude all day. At 17:00 GMT, the Sun is directly over the Pacific Ocean near Antofagasta, Chile.

 
 

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Tuesday, 06-Jan-2009 09:32:44 GMT