The Crux Perpetual — one hand, read from north and south
Look South to see the Crux Perpetual in the Southern Hemisphere sky, in the Northern Hemisphere Look North. All hands aligned at 12 o'clock, zero degrees at the End of March and beginning of April. So at precisely the end and beginning of each new day, the Crux Perpetual points at the date within its month. It can only do this by running ahead of time slightly. It is guided by the Cross! The Southern Cross! The Crux Perpetual and The Crux constellation are one and the same!
Same hand, same month
Both sky hands sit on one line through the sky. Follow either hand out to the month ring and you read the same month.
Mirror, not a new clock
Face south and the sky wheels clockwise. Face north and it wheels anticlockwise. The northern face is the southern one seen in a mirror.
Upright on 1 April
At midnight on 31 March/1 April both hands stand straight up at XII. Every month after, they step one section further round.
Questions people ask
What is the Crux Perpetual?
The Crux Perpetual is the name of the hand itself: a line drawn from the South Celestial Pole that bisects the angle between Acrux and Gacrux, the two stars of the Southern Cross that point toward the pole. It turns with the sky once every sidereal day and needs no winding.
Why do the Southern and Northern faces turn in opposite directions?
Because they're read from opposite ends of the Earth. Facing south, the sky appears to wheel clockwise around the South Celestial Pole; facing north, it wheels anticlockwise around the North Celestial Pole. It's the same rotating Earth, seen from two ends.
Do the two hands really point to the same month at the same time?
Yes, at each face's own local midnight. The Northern hand is built as a mirror of the Crux Perpetual, synchronized to the same reference constant, so both step around their month rings together through the year.
Is the alignment exactly 0 degrees at midnight on 1 April?
No - very close, not exact. Using the current bisector definition of the hand (the equal-angle line between Acrux and Gacrux, as seen from the pole), the hand reads about -0.4 degrees at Parkes Observatory local midnight on 1 April, not a perfect zero.
What are the two hands actually tracking?
The Southern face's hand is the Crux Perpetual, following the Southern Cross around the South Celestial Pole. The Northern face's hand mirrors it around the North Celestial Pole, near Polaris and the Big Dipper.