Today is the longest day in the northern hemisphere, which means 21 June is the day of the year 2010 with the longest hours of daylight. This astronomical event, called as Solstice, happens twice every year and referred as summer solstice and winter solstice.
What is Solstice? And, why is 21 June the longest day?
This event is related to the Earth’s motion on its axis.
A solstice is an astronomical event that happens twice each year, when the tilt of the Earth’s axis is most inclined toward or away from the Sun, causing the Sun’s apparent position in the sky to reach its northernmost or southernmost extreme.
The cause of the seasons is that the Earth’s axis of rotation is not perpendicular to its orbital plane (the flat plane made through the center of mass (barycenter) of the solar system (near or within the Sun) and the successive locations of Earth during the year), but currently makes an angle of about 23.44° (called the "obliquity of the ecliptic"), and that the axis keeps its orientation with respect to inertial space. As a consequence, for half the year (from around 20 March to 22 September) the northern hemisphere is inclined toward the Sun, with the maximum around 21 June, while for the other half year the southern hemisphere has this distinction, with the maximum around 21 December. The two moments when the inclination of Earth’s rotational axis has maximum effect are the solstices. [ source: Wikipedia ]
Generally, the summer solstice falls on 21st June in northern hemisphere and 21st December in southern hemisphere. The winter solstice occurs frequently on 21 December in northern hemisphere and 21 June in southern hemisphere. These days are celebrated by people around the world as a lucky day.
