It’s the Summer Solstice!

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Today, December 22nd is the Summer Solstice, the longest day of the year. The summer solstice marks the astronomical start of Summer. From today onwards the days will begin getting shorter again.

Sunrise, Sunset and Day Length Graph

Sunrise, Sunset and Day Length Graph

In Brisbane the day will be 13 hours, 52 minutes and 50 seconds long. Night will be just 10 hours, 7 minutes and 10 seconds long. However the earliest sunrise and latest sunset do not occur today, as shown on the graph above. Today just has the longest gap between sunrise and sunset.

The days and nights vary in length because the Earth’s poles are inclined at a 23.5 degree angle to the orbit of the Earth. This means that when the Earth is on one side of it’s orbit, the North Pole tilts towards the sun. When the Earth is on the other side the South Pole tilts towards the sun. Whichever hemisphere is tilted more towards the sun is experiencing summer.

Solar Terminator - Equinoxes on left, solstices on right.

Solar Terminator – Equinoxes on left, solstices on right.

It’s important to note that the Earth’s axis of rotation does not move, it stays nearly constant as the Earth moves in it’s near circular orbit around the sun.

Earth spins on a tilt of 23.4 degrees off vertical

Earth spins on a tilt of 23.4 degrees off vertical

This is known as axial tiltĀ of the Earth and is what is responsible for the seasons.