The Williamstown Time Ball Tower
This structure was built in 1849 and was both a lighthouse and a time ball tower at Point Gellibrand. It has been claimed that only five time ball towers remain today, including a working one in Greenwich. At precisely 1:00PM local time, the ball was lowered down the mast so that ships out in the bay could recalibrate their chronometers.
The principle is simple. Wherever ships were in the world, their navigators could know local time at one moment in the day, noon. If they knew Greenwich mean time at that moment, they knew what fraction of a day away from Greenwich they were, which then meant they were also the same fraction of a full circle around the world from Greenwich (in longitude).
Since Captain Cook's day, ships carried accurate chronometers for this purpose. However, over the course of a journey, mechanical clocks would inevitably drift away from precise GMT. Hence, if a ship called in at a port where there was trusted local time keeping, their chronometers could be reset to correct GMT. Since 1895, Melbourne's standard time has been ten hours ahead of Greenwich, which meant that after that date the adjustment could be made without reference to Melbourne's precise longitude.
The structure is made of bluestone (basalt here), locally quarried by convict labour sent down from New South Wales. Williamstown bluestone will be scattered around the globe, because it was also used as ballast to stabilise any ship leaving port with a significantly empty hold.
The Williamstown Time Ball Tower
This structure was built in 1849 and was both a lighthouse and a time ball tower at Point Gellibrand. It has been claimed that only five time ball towers remain today, including a working one in Greenwich. At precisely 1:00PM local time, the ball was lowered down the mast so that ships out in the bay could recalibrate their chronometers.
The principle is simple. Wherever ships were in the world, their navigators could know local time at one moment in the day, noon. If they knew Greenwich mean time at that moment, they knew what fraction of a day away from Greenwich they were, which then meant they were also the same fraction of a full circle around the world from Greenwich (in longitude).
Since Captain Cook's day, ships carried accurate chronometers for this purpose. However, over the course of a journey, mechanical clocks would inevitably drift away from precise GMT. Hence, if a ship called in at a port where there was trusted local time keeping, their chronometers could be reset to correct GMT. Since 1895, Melbourne's standard time has been ten hours ahead of Greenwich, which meant that after that date the adjustment could be made without reference to Melbourne's precise longitude.
The structure is made of bluestone (basalt here), locally quarried by convict labour sent down from New South Wales. Williamstown bluestone will be scattered around the globe, because it was also used as ballast to stabilise any ship leaving port with a significantly empty hold.