Using the Southern Cross  to find South and to Tell the Time

It is normal on flags to represent the Southern Cross 'standing up' with the brightest star at the bottom and three stars at the top. However the ordinary New Zealander will rarely see the constellation straight up and down: it makes various angles with the horizon depending on the time of year and time of night. The angle shown on the Star Fern Flag may be seen on Autumn evenings.

The Southern Cross is a very important constellation because it is the easiest way to find South, and hence all other directions of the compass. Here is how it can be done. The line joining the top and bottom stars of the Cross points directly to a piece of blank sky called the South Celestial Pole (SCP). All the stars in the southern hemisphere rotate around the SCP. However this point is not on the Southern Horizon but about (in New Zealand) about halfway up the sky.  If you can find and point to the SCP, then you simply drop your hand down to the horizon and that's South. Hence behind you is North, left is East and right is West.

Now the South Celestial Pole is also on a line that passes at right angles between the Pointers. So point your thumb and index finger towards the Southern Cross and Pointers. Then move your hand along the top-bottom line of the Cross, gradually bringing thumb and finger together. When thumb and finger touch, drop your hand to the horizon and you will be pointing approximately at due South.

If you can't see the Pointers, then extend the line from top star of the Cross through the bottom star and out the other side a bit more than an adults' handspan as seen at arms length.  (Handspan means the distance from tip of little finger to tip of thumb when stretched out as much as possible).   A more correct measurement is to say the SCP is 4 1/2 (four and a half) cross-lengths beyond the bottom star of the Southern Cross, but this is really hard to measure against the sky.   Of course if the Southern Cross is standing straight up  (for example soon after sunset at the beginning of July) or upside-down, then it points straight north-south. 

To Tell the Approximate Time  (can be done accurately to within about 30 minutes without any special instruments)

All Southern Hemisphere stars rotate around this South Celestial Pole (clockwise, of course!) So you can also find the time by the angle of the Southern Cross, using a simple calculation in your head.  Look at the 'hour' that the celestial hour-hand (top star) is showing: straight up-and-down is 12 o’clock, horizontal (East) is 9 o’clock, and so on. Double this number to make the time on a 24-hour clock, for example 9 o’clock stars become 18:00 hours.

Then find the number of months (including fractions) after 29 March: double that and subtract from the 'hours'. For example on 1 May we subtract 2 hours, while for 6 February one would instead add 3 1/2  hours, taking it to 21:30 hours.    This gives the correct local Sydney time,  but ...

In terms of the earth's rotation New Zealand is  90 minutes ahead of Sydney.  So you need to add an extra 1 1/2 hours. (Yes, the official time difference is 2 hours, as if New Zealand were perched right on the international dateline. But the stars don't know that). Finally If it’s daylight saving, we have to add one more hour.     All this implies "9 o’clock stars" on Waitangi Day will occur at 24:00 hours i.e. midnight.  At 10 o'clock the stars will be pointing to 8 pm.

Note careful observation, or homemade instruments can lead to very accurate measurement of the time. The addition of 0.5 hours during as a geographical adjustment from local time to  dateline time is approximate: it may depend on whereabouts in New Zealand you are observing from. Gisbourne is very close to the dateline, so adding 30 minutes to what you observe from the sky may not be necessary. For Southland the adjustment from star time to dateline time is closer to one hour.

Next Page:  What if the flag was changed like this...?



This page is copyright 2008 Barry McDonald,  Albany 0632,  New Zealand

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