Because the southern cross rotates around the south celestial pole like a clock, it is possible to determine the time of night from its' position.
Below are some instructions on how to do this. Also there is a kit you can make that does the mathematics for you (CRUX-CLOCK). The CRUX-CLOCK is my invention, it won first prize in a competition run by the Australian ABC radio program, 'practicalities'.
The Night Sky
The Southern Cross is so much a part of Australia it is a good idea to learn how to recognise it. Acrux (A-kruks) is the brightest star in the cross as well as being the star at the foot of the cross.
The star at the top of the Cross is called Gacrux (ga-kruks).
The Southern Cross is not always upright as you see it on the Australian Flag. It rotates around the South Celestial Pole, and, in 24 hours will lie on one side, be upside down and lie on the other side before returning to an upright position.
The South Celestial Pole is a point in the sky around which all our stars rotate. There is no star there, just black night sky. You can point to the centre of this great celestial clock by facing true south and pointing up at an angle equal to your latitude. To face true south set 168° on your compass. Sydney, Australia is on a latitude of about 33°. If you raise your arm to 33° above the horizon you will be pointing to the South Celestial Pole. See now how a line through the Southern Cross also passes through the South Celestial Pole.
So regular is this rotation around the Pole that the Southern Cross can be used to tell the time. The method is outlined below, but if the maths seem a little daunting then there is a device called a "CRUX_CROSS" which does it all for you.
Time from the Southern Cross.
The night of the 29th of March is the best night of the year to use the Southern Cross to tell the time because there are no corrections to make.
Imagine the Southern Cross to be the hour hand of a 24 hour clock. The South Celestial Pole is the centre of the clock. Thus 0/24 hours is directly above 6 hours and is normally where 3 hours would be on a normal clock, 12 hours is closest to the horizon where 6 hours normally is on a clock. The diagram shows this. On this night of the year (29th) wherever the Southern Cross points on this clock is the correct time.
You can use this method any time of the year but there is a correction to make if the date is not March 29th. Firstly read the time off the Southern Cross. Next deduct 2 hours from that time for each month since 29th March (April 1st is close enough and is an easier date to remember).
Example: (see diagram above)
Date 31st May
Time on Star Clock, 3 hours
Borrow 24 hours (because we have a big number to take off).
3 hours + 24 hours = 27 hours
March 29th to May 29th = 2 months x 2 hours = 4 hours
May 29th to May 31st = 2 days x 4 minutes = 8 minutes
From 27 hours subtract 4 hours 8 minutes = 22 hours 52 minutes which is 10.52 p.m.
To make one you will have to download both the kit file (big, 60k) and the instructions file, if you are just curious click here for a peek at a thumbnail.
Print the kit file on an A4 shet of paper and glue it to a sheet of stiff, but thin, cardboard.
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