The December solstice, when the Sun reaches the most southerly point in the sky, marks the longest day of the year in the southern hemisphere.

Normally, it falls on December 22 in Australia, but this year it was a day earlier thanks to the leap year.

But while it is the longest day, surprisingly, it does not coincide with the earliest sunrise or latest sunset time of the year.

"If you live in Sydney, for example, the earliest sunrise and the latest sunset are about a month apart with the solstice in the middle," said Professor Fred Watson, astronomer-in-charge of the Australian Astronomical Observatory.

The earliest sunrise and the latest sunset do not happen on the summer solstice even though it is the longest day of the year. ( ABC: Julie Ramsden )

This mind-bending fact is caused by the combination of the Earth's tilt of 23.5 degrees and its elliptical orbit.

The tilt, of course, gives us our seasons and dictates where the Sun appears in the sky from day to day. Our orbit, on the other hand, affects how we measure time.

"If we used a sundial, these three days would completely coincide on December 21," Professor Watson explained.

But sundial time — based on the movement of the Sun — is not as regular as the 24-hour clock we set our watches by.

"The Sun is a very poor time keeper," Professor Watson said.

That's because the Earth's orbit is not a perfect circle; it's an ellipse, and our planet moves more quickly through its orbit when it is closest to the Sun in January than it does in June.

Those changes cause the time between two solar noons — when the Sun is at the highest point in the sky each day — to vary throughout the year.

Sometimes it is slightly greater than 24 hours of clock time, and at other times slightly less. This effect is known as the equation of time.

"From early November until late January the difference between clock time and sundial time is rapidly changing," Professor Watson said.

As we approach the solstice, the solar day is half a minute greater than the 24-hour clock day."

At the same time, the southerly movement of the Sun caused by the Earth's tilt gets slower and slower (solstice means to stand still), so the difference between when the Sun rises or sets from day to day is now just a matter of seconds instead of minutes.

"The time of sunrise and sunset changes so very slowly near the solstice, that the solar vs clock time effect completely overwhelms it, and this separates the dates out," Professor Watson said.

This effect continues until late January when the solar/clock time difference starts to shrink again.

"You get to a point where the daily change in the rise and set times, which is due to the Sun's changing position in the sky, becomes a bigger effect [than the solar/clock time difference]," Professor Watson said.

Latest sunset depends upon latitude

While this year's solstice falls on December 21 in Australia, the date of the earliest sunrise or latest sunset depends upon how far north or south you live.

Latitude determines the angle along the horizon at which the Sun travels. This changes faster at higher latitudes closer to the poles so sunrise and sunset times vary more.

That means the further south you live in Australia, the longer the day gets during summer, but the shorter the gap between the earliest sunrise, the solstice, and the latest sunset.