Thursday 22nd December was the longest day of the year, or the summer solstice.
More correctly, it was the day on which the solstice fell, at 5:35am ADST.
The word solstice means ‘Sun stands still’. Although the Sun never stops moving from east to west across the sky, it does stop moving in a north-south direction. Our summer solstice is the precise moment at which the Sun reaches its southernmost point in the sky, stops, then starts moving north again.
Of course the Sun itself is not really moving north and south. Its apparent movement in the sky is a result of the tilt in the Earth’s axis and the motion of this axis around the Sun. On the summer solstice the tilt points directly towards the Sun, which sits highest in the sky, and sunlight hits the ground the most directly.
The day on which the solstice falls is the longest day of the year, but not the day with the earliest sunrise or the latest sunset. This is because sun time does not keep exactly in step with clock time.
Around the summer solstice, sun time is going slightly slower than clock time, so sunset is getting later relative to the clock. The shorter day will not be large enough to offset this for about two and a half weeks, so sunset will keep getting later until then. Similarly the earliest sunrise happened two and a half weeks ago.
Nor is the solstice the warmest day of the year. Although the ground will start receiving less sunlight, it will keep heating up for about another month.