The Summer Solstice falls on Friday 21 June in 2013 in the Northern Hemisphere, the day on which the Sun rises highest in the sky in the Northern Hemisphere (and lowest in the Southern Hemisphere) and days are at their longest in the north (or shortest in the south).
The solstices are entirely a product of variation in the Earth's rotation on its axis, which is at an angle of 23.5° to the plain of the Earth's orbit about the Sun. This means that in December the Earth's Southern Pole is tilted towards the Sun, while the Northern Pole is tilted away from it. This means that around the Southern Solstice the Southern Hemisphere is receiving radiation from the Sun over a longer part of the than the Northern, and at a steeper angle (so that it to pass through less atmosphere to reach the planet), creating the southern summer and northern winter.
The tilt of the Earth at the Northern Solstice. Wikimedia Commons.
See also Total eclipse of the Sun visible from Australia, Papua New Guinea and the Solomon Islands, 9 May 2013, The March Equinox, The Earth approaches its perihelion, The Southern Solstice and The Earth reaches its aphelion.
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