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 hourhand (top star) is showing: straight upanddown is 12 o’clock, horizontal (East) is 9 o’clock, and so on. Double this number to make the time on a 24hour 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.
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