On Thursday morning, when shivering through one of the coldest nights of the year, we also had the shortest amount of daylight, and longest night of the year - the winter solstice.
The Earth is tilted about 23.4 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.
As the Earth tilts, when it is the winter in a hemisphere, that means we are at maximum tilt away the sun.
Therefore, we get less sunlight, which gives us shorter days and less heat, so we are a bit cooler.
At the same time, though, it means the other hemisphere is tilted towards the sun so, more sunlight, which means longer days and more heat.
The further you are away from the equator, the more you are tiled with respect to the sun, meaning your daylight changes even more.
The difference in daylight in Canberra between the winter solstice and summer solstice is four hours and 46 minutes.
Melbourne is five hours and 15 minutes while Hobart is six hours and 21 minutes.
At the South Pole, the sun never rose on Thursday and at the North Pole, the sun never set.
Now, as we progress towards December, the amount of light in the day in the Southern Hemisphere will increase, while in the Northern Hemisphere, it will decrease.
You may be thinking, "Great - sunrise will now happen earlier, and sunset later".
However, we have to wait about two more weeks, until early July, before sunrise starts to change.
This is because of a second effect - solar day. In old westerns, this might have been called "high noon" - the time when the sun reaches due-north (in the Southern Hemisphere), and its highest point in the sky.
The Earth spins on its axis (a day) every 23 hours and 56 minutes. However, in the time the Earth has spun for a day, it has also orbited the sun for the same amount of time.
Therefore, it has to play a bit of catch-up for the sun to appear at the same point, sometimes about an extra four minutes.
The moon does something similar - it takes about 27 days for the moon to spin around the Earth but it has to play catch-up and takes about another two days to get to the same spot, and phase - which is why we see the lunar cycle as about 29 days.
However, this lag is dependent on the tilt and where you are on Earth.
You can picture in the summer, the sun is high in the sky, while in the winter, it is much lower.
Depending on the season, it takes a different amount of time for the sun to reach solar noon, and its position in the sky. In some cases, it is longer than 24 hours, like around the winter solstice.
Even though the tilt will mean more daylight, we are still taking a few extra minutes to catch-up to the sun's position in the sky.
This means sunrise still is getting later and later until early July, when we are all caught up, and the longer days finally mean a change in sunrise.
- Brad E. Tucker is an Astrophysicist and Cosmologist at Mt Stromlo Observatory and the National Centre for the Public Awareness of Science at the ANU.
