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All through the year, the sun shines directly above a point on the surface of the earth. Solstices are when the Sun reaches its northernmost and southernmost points on the surface of the earth whereby it starts to go back to other extreme. So, there are two solstices in a year. When the Sun is directly above its northernmost point on earth, it is called summer solstice and when it is on earth's southernmost point, it is called winter solstice.

The word 'solstice' is derived from Latin word 'solstitium' which is a combination of two words, namely, sol (sun) and sistere (to come to a stop, make stand still). Thus, literal meaning of solstice is 'sun standing still'. The day at which this happens is also called solstice. Summer solstice is the longest day in the northern hemisphere.

The reason for solstice is that the earth not only rotates around an axis passing through its center, but it also revolves around the sun in an orbit. The axis of the rotation of earth makes an angle of about 23.44 with the plane in which it revolves around the sun. Due to this inclination, a hemisphere is directed either towards or away from the sun depending upon the position of earth on the orbital plane during the year.

In northern summer, northern hemisphere is directed towards the sun, exposing it to more sun light as the sun shines well above the horizon and stays for a longer time as it has to cover longer relative distances. On or about June 21, the sun shines directly above a point on the surface of earth, 23.44 north of equator, marking the northernmost point on earth where sun will shine directly overhead. A line joining all the points on surface of earth 23.44 north of equator is called tropic of cancer'. At this point, maximum of the northern hemisphere is exposed to the sun. The sun is at its zenith on tropic of cancer and thus has to travel a longer (relative) distance from east to west. Therefore, sunlight is available for maximum amount of time in a given day, making it the longest day of the year.

It must be kept in mind, though, that June 21 is not necessarily the summer solstice because of the wobbling of the earth whereby the angle of inclination of its axis of rotation, from the plane in which it revolves around the sun changes over time. This angle of inclination is between 22.1 and 24.5. Moreover, because of earth's tilt and orbital eccentricity, the point on orbital plane where northern summer and winter occurs will change in thousands of years from now.

Reference:

Solstice. (n.d.). Online Etymology Dictionary. Retrieved June 03, 2008, from Dictionary.com website: http://dictionary.reference.co m/browse/Solstice

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