The simple answer is that it gets darker earlier. But why?

The Western Australia time zone is 8 hours ahead of Greenwich and is referred to as GMT+8 or more precisely UTC +8¹.

The meridian (line of longitude) where the time is 8 hours ahead of Greenwich is 120° East. This 120° meridian crosses Western Australia east of Perth at:

• Great Northern Highway 19 km east of Pardoo Roadhouse.
• Ripon Hills Road 28 km east of Marble Bar.
• Goldfield Highway 22 km west of Wiluna.
• Mt Magnet-Leinster Road 71 km east of Sandstone.
• Great Eastern Highway 33 km east of Yellowdine.
• South Coast Highway 4 km west of Ravensthorpe.

It is only along this 120° line of longitude where the time is UTC+8. Perth is 390.5 kilometres west of the 120° meridian – at longitude 115°51’38”.

When it is noon at 120°East (GMT+8 or Australian Western Standard Time) the mean solar time in Perth is 11:43:24 a.m.

At 32°South (Perth’s latitude) each degree of longitude is about 94.2 kilometres apart². As each degree of longitude is four minutes of time³ it can be seen that the mean solar time of Kalgoorlie (529 km east from Perth direct line) is about 22 minutes ahead of Perth.

So, we need to camp about 20 minutes earlier than we normally would.

NOTES

1. On 1 January 1972, GMT was superseded as the international civil time standard by Coordinated Universal Time (UTC) and starts at midnight.
2. The earth’s equator describes a circle (roughly) which is, of course, 360 degrees. However, the system of longitude uses only 180 degrees – but in two directions east and west of the Prime Meridian at Greenwich – to complete the 360 degrees of the earth’s circumference.
3. There are 1440 minutes in a solar day (24 hours x 60 minutes = 1440 minutes).
   The 180 degrees of East longitude and the 180 degrees of West longitude = 360 degrees.
   Therefore, each degree of longitude equals four minutes of time (1440÷360).