The Southern Cross (determining due south, and telling the time)

To determine south from the Southern Cross (Crux) is very simple, provided that you can see it, ie you need to be suitable far south of the equator.  There are three easy methods for determining true south (within a reasonable degree of accuracy - in the diagrams the South Celestial pole is not necessarily at the indicated exact position, but can be assumed to be very close to this point):

1.  If you extend the long diagonal through its tail, by about 4.5 times the length of this diagonal, you end up very close to the South Celestial Pole, where there is no star, unlike for the North Celestial Pole.  Drop a line vertically from this pole to the horizon to determine true south.

2.  The intersection of the Southern Cross's extended long diagonal, and the perpendicular bisector of its pointers, is at the South Celestial Pole.

3.  Draw a line from Archernar and Beta Centauri (the pointer star closest to the Southern Cross).  The South Celestial Pole lies roughly half way between these two stars.

To determine the time, there are three things to remember:
1.  The earth makes one complete rotation relative to the stars in 24 hrs
2.  The star constellation makes one "rotation" per year (roughly 365.25 days)
3.  The Southern Cross points vertically up (12 o'clock position) at midnight, 29th of March.

Therefore, on the 29th of March, no corrections need to be made when telling the time from the position of the Crux.  Remembering that a normal clock makes two revolutions per day and that the Crux only makes one.  Thus its clock position needs to be multiplied by 2.  For example, if it is pointing in the 11 o'clock position, the time would be 22h00, and if it was pointing in the half past two position, the time would be 05h00.

But the relative star positions rotate throughout the year, and for each month after 29 March, 2 hours need to be subtracted from this time.  You can further subdivide the month for accuracy, and factor in about 4 minutes per day.  For example, if you are trying to determine the time on 15 August, which is 4.5 months further on from 29March, you need to subtract 4.5 x 2 = 9 hrs from the time position of the Crux, ie if the Crux was pointing at the 5 o'clock position, the time would be (5 x 2) - 9 = 01h00.  (If you end up with a negative number, add 24)

To cater for 29 March, you can use the following correction:
Add 6 (March = month 3, and 3 x 2 = 6) to the Crux time, and subtract 2 hrs for each month since 1 January.  ie in the above calculation the time would be (5 x 2) +6 -(7.5 x 2) = 10 + 6 - 15 = 01h00

Below is a Crux Clock, which can be used to determine the time.  Print and cut it out, and join the three items together with a pin or rivet, through the indicated centre.  To read the time, you will need to set the date and the position of the Crux.

Please note that it is difficult to determine a very accurate time with the above method (and with the "Crux Clock") as the accuracy will be depend on how accurately you can tell the position of the Crux in the sky, and also if you can make the necessary adjustments necessary of your position in your particular time zone.