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South Australian Time Zone

QVMAG astronomer Martin George was asked by the ABC this week to comment on a suggested change in South Australia to move to either Eastern Australian Standard Time or Western Australian Standard Time. He said he felt that either of these possibilities would be a poor choice.

Time zones are necessary to ensure that entire countries, or well-defined regions of countries, share the same clock time. A major reason why this originally came about was that towns along railway routes all had a slightly different time, resulting in confusion over timetables and several accidents. Overall, each whole hour zone is based on longitude, with the difference of an hour equivalent to 15 degrees of longitude. The Eastern Australian time zone, for example, is based on longitude 150 degrees east, so we are 10 hours ahead of Coordinated Universal Time (UTC, which for these purposes is effectively the same as ‘Greenwich Mean Time’). The actual time zone divisions follow country and state borders and geographical features.

South Australia’s time zone is 9.5 hours ahead of UTC, corresponding to a longitude of 142.5 degrees east. However, this longitude meridian is to the east of South Australia’s eastern border, so no part of that state has that longitude. This means that South Australians’ clocks are already running fast in relation to the Sun.

If they were to adopt the same time as the eastern states, they would have a time zone a whole hour out of synchronisation with the Sun.

In late June, this would mean a 7:53 am sunrise in Adelaide. Even more significant would be the effects of a change to be in the same time zone as the west: a late June Adelaide sunset would take place at 3.41pm!

The most sensible arrangement, if there is to be a change, would be to adopt a time equal to UTC+9, corresponding to a longitude of 135 degrees east, which runs almost centrally through the State. This would place them an hour behind the east and an hour ahead of the west. It would be a quite sensible move, in my opinion!

Martin George, Astronomer

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