Why Auckland is sold short on longest day

The 24 hours we consider to be the length of a day is only an average figure.

The Sun is rising at its earliest time of the year about now, even though the longest day is not until December 22.

To complete the puzzle, the latest sunsets are not until after the summer solstice.

According to the US Naval Observatory, the reason for this is that our clocks and the Sun do not keep the same kind of time.

The 24 hours we consider to be the length of a day is only an average figure.

Instead, consider solar noon – the moment precisely midway between sunrise and sunset.

The time between one solar noon and the next – called apparent solar time – can be different to the 24 hours shown on our clocks which is known as mean solar time, or civil time.

The size of the difference depends on the time of year. During some parts of the year there can be as much as 16 minutes difference between the two types of time. Meanwhile, how much difference there is between the dates of the earliest sunrise, the summer solstice, and the latest sunset, depends on latitude.

But if we kept time by the Sun, and not mechanical or electrical clocks, the date of the earliest sunrise and the date of the latest sunset would also be the date of the summer solstice.

Why isn’t the time from one solar noon to the next always the same? The answer is complicated but is related to the small part of the Sun’s apparent daily motion that depends on the position of the Earth in its orbit around the Sun.

This component of the Sun’s apparent motion varies by a small amount over a year due to the elliptical shape of the Earth’s orbit and the tilt of the Earth’s axis. A consequence of the variation between the time shown on our clocks, and apparent solar time, is that sundials usually seem to be wrong.

Timeanddate.com shows the Sun rising in Auckland at 5.54am for the past week and then again on Sunday. After that the sunrise starts getting later.

The day of the summer solstice – December 22 – is 14 hours 41 minutes and 35 seconds long in Auckland.

The latest sunset is 8.43pm, starting in the last few days of December and lasting through the first third of January.

For Invercargill, by comparison, the earliest sunrise is at 5.47am – from Sunday through to Wednesday. The longest day is 15 hours 48 minutes and 57 seconds long. The latest sunset is at 9.41pm in the last week of December and the first week of January.