by Dr Martin George
December is the month when we experience the longest day of the year —and the shortest night! In the northern hemisphere, it is exactly the opposite, with their shortest day and longest night. Here at the Ulverstone Planetarium, we are sometimes asked about the reasons for this.
The changing length of the day through the year is caused by the tilt of Earth’s axis. Earth orbits the Sun each year, but its axis of rotation — the line joining the north and south poles — is not aligned at a right angle to the plane of the orbit. In other words, the axis does not point exactly ‘up and down’. Instead, it is tipped over by an angle of about 23.44°. Globes of the Earth show this by being mounted at that ‘tipped-over’ angle. As our planet orbits the Sun, the axis remains pointed in almost exactly the same direction in space, so in December, our hemisphere is directed more toward the Sun, and the opposite is true in June. The December and June dates on which the extremes occur are called the solstices. The exact dates can be sightly different each year, and can also be affected by one’s time zone, but they are typically 21 June and 21 December, which are the dates for 2024 for our time zone.
Here in Tasmania, we have the longest summer days of all the Australian states, because we are the farthest south. In Ulverstone, the period between sunrise and sunset on 21 December is 15 hours and 9 minutes. By comparison, for Melbourne this period is 14 hours and 47 minutes, and for Sydney 14 hours and 25 minutes. The effect is hardly noticeable, though, in the equatorial regions, where the day length varies only slightly over the year.
Of course, we have much more light than just the period between sunrise and sunset, which are the times when the upper edge of the Sun is in the same direction as the horizon (that is, a sea horizon). The twilight period before sunrise and after sunset adds to this. The brightest part of twilight is called civil twilight, which on the solstice adds a total of 67 minutes to our useful hours of daylight.
As an astronomer, I prefer the winter for stargazing: the nights are long, and the evening view of the Milky Way is at its best in winter and early spring. However, for those summer evening barbecues and other outdoor activities, Earth’s tilt certainly helps!
Illustration:
Simulation of the view of Earth as seen looking back from the Sun on 21 December. The southern hemisphere is favoured with more sunlight, because of the tilt of Earth’s axis. DIAGRAM: Ulverstone Planetarium.