New angle on longest night

ANOTHER year has gone by, and we are about to experience the winter solstice of 2022.

It will be on Tuesday, June 21, and the exact moment will be 7.14pm. Tuesday will be the shortest day of the year for us, and the night of June 21-22 will be the longest night.

However, the difference between the length of day and night in the few days either side is very small indeed.

The solstice is the moment when the sun is farthest north on the imaginary celestial sphere, the imaginary globe that turns around us as the Earth rotates.

The apparent north-south motion of the sun over the course of a year is because Earth’s axis has a tilt of about 23.4 degrees. This influences the length of the day and the maximum height that the sun reaches in the sky throughout the year, which results in the occurrences of the seasons.

In case you’ve ever wondered, this is the reason that globes of Earth are always mounted in a way in which they appear to be “tipped over’’ slightly: 23.4 degrees from being “upright’’.

Perhaps the most dramatic effect of the tilt of Earth’s axis affecting the daylight hours is the situation in Earth’s Arctic and Antarctic regions at the time of the solstices.

Here in Tasmania, at the solstice we still have about nine hours between sunrise

and sunset — from Hobart, it is nine hours, 53 seconds — and of course we have a twilight period before and after that. However, as we travel farther south, the sun spends less and less time above the horizon, and we reach a point at which it does not rise at all: the place where the “shortest

day’’ is, literally, a day of zero length, except for the twilight.

Allowing for the fact that the sun appears as a disc of a certain size, and for the bending of light in Earth’s atmosphere (called refraction), the latitude south of which the sun does not rise is 67.23 degrees south. Both Mawson

Station and Davis Station are a little south of this line. However, they still experience a twilight period for a while, centred on “local noon’’.

So, it’s not correct to say that all of Antarctica experiences six months of day and six months of night, because that ignores the twilight period.

However, there is part of that continent that does experience periods of total darkness over complete rotations of Earth, without any glimmer of twilight at all. It is the South Pole and the circular region surrounding it from latitude 74.34 degrees southward. This is because from north of that latitude, a glimmer of twilight briefly appears.

As the days and weeks go by after the solstice, this region shrinks and by about August 2, a faint glimmer of twilight will begin to appear even at the South Pole. The sequence takes place in the opposite order in the weeks before the solstice.

Of course, at this time of the year the opposite is true in the northern hemisphere, with the North Pole currently being in perpetual sunlight.

Most other planets have tilts, too. The two that are most Earth-like in that respect are Mars (25.2 degrees) and Saturn (26.7 degrees). This is why you will sometimes read information such as a spacecraft surviving a Martian winter. The craziest tilts are those of Uranus (97.9 degrees) and Venus (178 degrees).

You may wonder how a tilt can be greater than 90 degrees, but this reveals that these planets rotate in the opposite direction to all the others as seen by looking down from the northern side of the solar system.

Of course, the long Tasmanian nights at this time of the year offer a great chance for stargazing! There is a beautiful view of the Milky Way across the sky, and within it there are many attractive star clusters that look great in binoculars. A fine place to look is on the opposite side of the Southern Cross to the “pointers’’ side, where there are several of my favourites.