The Summer Solstice - the longest day of the year

21 December 2012

The Summer Solstice – an early sunrise.

The Summer Solstice
– an early sunrise.
© Geoscience Australia

The summer solstice occurs once a year in December when the Sun’s track across the Australian sky reaches its highest point. It is a day that features the most daylight hours of any in the year.

The Summer solstice usually occurs on 21 December, but can vary to 20 or 22 December. This year it is on 21 December at 10:12pm eastern summer time.

An interesting idiosyncrasy relating to the solstice is that it does not feature the day with the earliest sunrise and latest sunset as is commonly expected.

The effects of the off-centre tilt of the Earth on its axis and its elliptical orbit around the Sun combine near the dates of the Solstice to make the Sun appear to move slightly slower than expected when measured by a watch or clock.

As a result, the earliest sunrise occurs before the date of the summer solstice, and the latest sunset happens after the solstice.

A similar situation occurs during the winter solstice around the 21 June. On this, the shortest day of the year, the sunrise is not the latest and the sunset is not the earliest. However, this day does have the least amount of daylight hours.

The other astronomical event, the Equinox, occurs when the Earth is directly aligned to the Sun and Earth’s tilt on its axis has no bearing. This occurs twice a year in March and September and, at the equator, the Sun can be seen directly overhead at zenith.

Topic contact: Last updated: October 4, 2013