solstice and equinox

The Solstices and The Equinoxes

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     Solstice refers to either of the the two days during the year when the Earth is so located in its orbit that the inclination (about 23½°) of the polar axis is toward the Sun. This occurs on June 20 or 21 (summer solstice), when the North Pole is tilted toward the Sun; and on December 21 or 22 (winter solstice) when the South Pole is tilted toward the Sun. The adjectives summer and winter, used above, refer to the Northern Hemisphere; seasons are reversed in the Southern Hemisphere. The name is solstice is derived from the Latin sol (sun) and sistere (to stand still) because at the solstices, the Sun stands still in declination i.e. the apparent movement of the Sun's path north or south comes to a stop before reversing direction.

     The two equinoxes occur roughly midway between the solstices: the autumnal equinox (September 22 or 23) and spring/vernal equinox (March 20 or 21). The term equinox is derived from the Latin for equal nights, indicating that the day and night are of equal duration.
     The two solstices and the two equinoxes mark the beginning of four seasons.

* The dates drift with the difference between the actual solar years and 365 days, and are corrected by leap years
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Insolation  

Murali Krishna Ecology, Man and Biosphere

Insolation is the incident solar energy emitted by the Sun, which reaches a unit horizontal area of the Earth's surface. The term is a contraction of incoming solar radiation

Direct insolation is the solar irradiance measured at a given location on Earth with a surface element perpendicular to the Sun's rays, excluding diffuse insolation (the solar radiation that is scattered or reflected by atmospheric components in the sky). Direct insolation is equal to the solar constant minus the light reflected, scattered, or absorbed by clouds and atmospheric particles.

Albedo  

Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space. It is a measure of the reflectivity of the earth's surface.

Ice, especially with snow on top of it, has a high albedo: most sunlight hitting the surface bounces back towards space. (The word is derived from Latin albedo "whiteness", in turn from albus "white") Water is much more absorbent and less reflective. So, if there is a lot of water, more solar radiation is absorbed by the ocean than when ice dominates.

Albedo is not important at high latitudes in winter: there is hardly any incoming sunlight to worry about. It becomes important in spring and summer when the radiation entering through leads can greatly increase the melt rate of the sea ice

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Solar Constant 

     The solar constant is the amount of incoming solar electromagnetic radiation per unit area that would be incident upon the top of the Earth's atmosphere at a distance of one astronomical unit, or AU(1.496 × 108 km or 9.3 × 107 mi, roughly the mean distance from the Sun to the Earth). The value of solar constant is about 1.96 cal · cm⁻² · min⁻¹ (1367 W^-1 m⁻²).

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