Our sense of time is based on the Earth’s daily spin about its axis. A day is the time the Earth takes to complete one spin. Some definitions relating to time are:
Sunrise — the time when the top edge of the Sun’s disc appears above the horizon.
Sunset — the time when the top edge of the Sun’s disc disappears below the horizon.
Civil twilight — starts in the morning when the Sun’s centre is 6° below the horizon and finishes at sunrise. In the evening it starts at sunset and finishes when the Sun’s centre is 6° below the horizon. Generally lights are needed for outdoor activities in the mornings before the start of civil twilight and in the evenings after its end.
The dial plate of a pendulum clock from Sydney Observatory shows separate dials for hours, minutes and seconds. The division of a day into 24 hours comes from ancient Egypt, while the subdivision of an hour into 60 minutes is based on the practices of the ancient Babylonians. Photo MAAS
Nautical twilight — starts in the morning when the Sun’s centre is 12° below the horizon and finishes at the start of civil twilight. In the evening it starts at the end of civil twilight and finishes when the Sun’s centre is 12° below the horizon. Generally the horizon is not visible in the mornings before the start of nautical twilight and in the evenings after its end.
Summer time — all times in this guide are in Australian Eastern Standard Time (AEST) or summer time (AEDT), as appropriate. In 2016, summer time in NSW, Victoria, ACT, South Australia, Tasmania and New Zealand is expected to end on Sunday 3 April. In New Zealand it will begin again on Sunday 25 September, while for these Australian states the start date is Sunday 2 October.