Monday, June 21 at 1.32pm, we will be at the winter solstice. That means it is the shortest day of the year and longest night of the year here in the Southern Hemisphere.
Likewise, in the Northern Hemisphere, they'll be having the longest day of the year as they celebrate the summer solstice. We have the Earth's tilt to thank for this fun part of life here on Earth.
As of today, the Earth is tilted 23.44 degrees. However, the Earth's tilt does wobble a bit, like a spinning top slowing down, so it can range from 22.1 to 24.5 degrees. But tilted to what?
Our solar system is a giant disc or frisbee, and all the planets lie on it and go around the sun on it. This imaginary line is called the ecliptic. The tilt is the angle between how a planet spins (its rotational axis) with respect to how we go around the sun (its orbital axis).
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If we spin essentially straight up and down with respect to how we go around the sun, we'd have a tilt of 0 degrees. If we spin in the same direction we go around the sun, we would tilt around 90 degrees, like we are on our side. In our solar system, the planets all have different tilts. Mercury has no tilt and Jupiter has a tiny one of only 3 degrees. Mars' tilt is about 25 degrees, making it very similar to the Earth. Uranus has a tilt of almost 98 degrees, meaning it is like a ball on its side rolling around. All of these tilts make the seasons different on each planet.
As the Earth tilts, when it is the winter in a hemisphere, like Monday at 1.32pm, for us here in the Southern Hemisphere, that means we are at maximum tilt away the sun. Therefore, we get less sunlight, which gives us shorter days, and less heat.
At the same time though, it means the Northern Hemisphere is tilted towards the sun, so more light, which means longer days and more heat. This tilt also means that where you are on the hemisphere matters.
If you think of how long daylight is in Darwin compared to Canberra or Hobart on the winter solstice, it is very different, and the extremes can be big.
In Hobart, Tasmania the daytime is six hours and 21 minutes shorter on the winter solstice (Mpnday) compared to the summer solstice (December 21). As you go further north, in Melbourne the day is five hours and 15 minutes shorter while in Canberra it is four hours and 46 minutes.
However for Darwin, which is only 12 degrees from the equator, the difference is only one hour and 28 minutes. As you get closer to the equator, the differences between summer and winter are a lot less, and as you go closer to the poles, the extremes are a lot more.
While Monday is the shortest day of the year, it is not the day with the latest sunrise or earliest sunset. The latest sunrise won't be until the end of the month, June 30 and 31, and the earliest sunset was last week.
Not only is the Earth tilted, its orbit around the sun is not a perfect circle, it is an ellipse. That means because of this, the time it takes for the sun to return to its same position every day is not quite 24 hours, so there is a bit of a lag between where the sun is (due to our orbit), and the length of our day (due to the tilt). The good thing is, the days will only get longer from here.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU