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Winter solstice

Author
by Martin Bush
Publish date
21 June 2012
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Comments (3)

Martin is the programmer at the Planetarium at Scienceworks.

Today is the shortest day of the year, also known as the winter solstice. More correctly, it's the day on which the solstice fell, at 9:09am AEST.

Solstice means 'the Sun stands still'. Although we never see the Sun stop moving across our sky from east to west, it does stop moving in a south-north direction. Our winter solstice is the precise moment at which the Sun reaches its northernmost point in the sky, stops, then starts moving south again.

After the solstice, the Sun starts rising higher in the sky, and the points on the horizon where the Sun rises or sets start moving south.

Analemma Image of an analemma taken over the course of a year by Robert Price in Bethanga, Victoria, consisting of 48 images of the Sun superimposed on a single background image. The winter solstice occurs when the Sun is at the lowest point in this image.
Image: Robert T. Price
Source: Robert T. Price
 

Of course the Sun is not really moving south and north. Its apparent movement is a result of the Earth’s tilted axis moving around the Sun. On the winter solstice the axis is tilted away from the Sun, the Sun rises lowest in the sky and the sunlight's energy is the most diluted across the ground. You can learn more about the cause of the seasons in the Melbourne Planetarium show Tilt!

The day on which the solstice falls is the shortest day of the year, but not the day with the latest sunrise or the earliest sunset. This is because Sun time is not exactly the same as clock time.

At the winter solstice, Sun time is drifting later relative to clock time because the solar day is a little bit longer than 24 hours. For a few days after the solstice the small increase in the length of a day is not enough to overcome this drift, so the time of sunrise as measured by our clocks keeps getting later. Similarly the earliest sunset was a few days before the solstice.

Nor is the solstice the coldest day of the year. This is because of what is known as thermal inertia. It takes a lot of energy to heat up the ground and the oceans. At the moment the ground is still too warm to be heated by the amount of sunlight we are receiving, so it is continuing to cool. In around a month the balance will change. The ground will be a bit cooler and the sunlight a bit stronger, and the earth will start warming up again.

Links:

Infosheet: The Sun and the Seasons

Infosheet: The path of the Sun

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