Southern Cross Clock

[glenc (Glen)]

You can tell the time by looking at the southern cross (Cru) and the south celestial pole (SCP) as a clock.
Imagine a 24 hr clock, not the usual 12 hr clock.

The SCP is at the centre of the circle.
0 when Cru is at top
6 on the RHS
12 at bottom
18 on the LHS
24 at the top again.

On April 1 the Cru-SCP clock is correct.
After that take off 2 hours for every month.
Add 24 if the answer is negative.
That should give the time.
e.g.
On September 1 at midnight the Cru-SCP clock reads 10.
Take off 5 months times 2 = 10.
It is midnight.

[RB (Andrew)]

That's so cool, thanks for sharing that.

[Argonavis (William)]

Glen

I hope I don't confuse anyone; the method I learnt was that the 00 hour was at the bottom (ie lower culmination for gamma crucis) and deduct 2 hours for each month and 4 minutes for each day that has elapsed since 1 October.

I have always been too lazy to find out why 1 October (or 1 April for your method) are the coincidental dates - I am sure it has something to do with the RA of the long axis of the Cross and the sidereal time at these dates.

[Argonavis (William)]

Glen

BTW its good to see you active on this forum, you are an astronomical asset, so keep it up.

[glenc (Glen)]

On April 1 Cru is above the SCP at midnight and on Oct 1 Cru is below the pole at midnight so both methods work. I got the idea from Rumker who was at Parramatta in the 1820s. Earlier this year I visited his grave in Lisbon Portugal and cleaned it up so the writing on it is now readable. Brisbane employed Rumker as an astronomer but he walked out after a dispute and Dunlop took over.

See
http://www.austehc.unimelb.edu.au/fam/1524.html

[ballaratdragons (Ken)]

Back 16 years ago when I was a member of the (censored) Astro Society, I was chairman for the month and one of the members gave a similiar talk on this subject.

It was meant to be a talk about telling time using Crux!

She spent 45 mins explaining how in January the Crux is at such and such an angle at 10pm, then in February at 10pm it is at such and such an angle, and so on and so on. Everyone was SOOOO enthralled!

After her very lengthy version of the Crux clock, I stood up and thanked her for her very detailed talk.

Then I couldn't refuse; I finished her talk for her with "It's good to know that if you are lost in the bush at 10pm, you know exactly what month it is!"

The poor woman was flabbergasted as the whole room broke up in laughter

[glenc (Glen)]

Good one.

[sheeny (Al)]

Thanks Glen!

Al.