Observations

What time should you hear the time gun fired at 1 pm each day from Fort Denison in Sydney Harbour?

Sydney time gun

The view from the time gun on Fort Denison in Sydney Harbour is towards the Opera House. A magnificent view, but it is no longer possible to see Sydney Observatory and its time ball as they are hidden by the building. Image and copyright Nick Lomb ©, all rights reserved

Over the last week there has been a fun discussion in the Sydney Morning Herald’s Column 8 about the one o’clock gun at Fort Denison and the time ball at Sydney Observatory. The discussion began with John Kimpton, of Cremorne Point asking, presumably sarcastically, ‘Am I the only person who enjoys hearing the 1pm Fort Denison gun fire at exactly 12.59pm each day?’. My Sydney Observatory colleague Allan Kreuiter then joined in reassuring readers that the Observatory’s time ball does drop at the correct time one 1 pm and not a minute early. Noting this Column 8 asked ‘Is the gunner at Fort Denison, er, taking his eyes off the ball, perhaps?’

There was a reply in Monday’s (11 April 2011) Column 8 from Bindi Bow, who is an employee at Fort Denison and as part of her duties fires the gun at 1 pm on weekdays. She explains that in the past the gun was fired when the time ball on the Observatory was seen to drop, but that is no longer possible due to the Opera House blocking the view of the ball.

Ms Bow says that she uses the speaking clock available by telephone on 1194 to obtain the time and fires the gun accordingly. Assuming that the provided time is correct (1194 is operated by a private company with reference to UTC (AUS)), what time should the time gun be heard by John Kimpton at Cremorne Point or by anyone else close enough to hear the sound?

Sydney time gun map1

A map indicating the times for sound to travel to different places near Fort Denison and Sydney Harbour. Each concentric circle denotes an extra second of travel time. Drawing Nick Lomb on a Google map of Sydney Harbour

A visual time signal such as from the time ball can be regarded as instantaneous due to the high velocity of light. A sound signal as from a time gun is not, as sound travels much more slowly. In dry air at 20°C temperature it travels at 343 metres per second. Using that figure I prepared the map above showing how long after the firing of the gun it would be heard. Such maps have been prepared for other time guns such as the 1861 map for the gun at Edinburgh. The map indicates that at Cremorne Point the gun should be heard at 1:00 pm and 3 seconds. Note that this does not take into account Ms Bow’s reaction time, any time delays in the gun itself and propagation differences due to temperature changes and wind.

Do you live in the vicinity of Sydney Harbour, can hear the gun fired as well as see the time ball drop at 1 pm? It would be good if you could time the delay from the ball drop to the time you hear the gun and let us know. The above map could then be verified experimentally.

8 responses to “What time should you hear the time gun fired at 1 pm each day from Fort Denison in Sydney Harbour?

  • The 1 o’clock gun today was at 2 minutes past one.

    I live above Kuttabul and our local school bells, St Vincents at Potts Point keeps my daily activities in order too.

    This 2 mins difference is indeed puzzling.

  • Here is a probably simple and useful idea to eliminate the Sydney Opera House problem.

    Put the time ball drop on webcam and/or cannon, and post it live on the internet. Here anyone wanting to set the time and see the event would have no problems with the cannon delay due to the speed of sound!

    Amusing and informative all at the same time!

    Cheers!

  • Probably a really dumb question, but I’ve actually forgotten why it is 1pm instead of at precisely noon?

    (it is funny, too. That I spoke of this last night at a lecture on navigation, and this very question popped into my head as I spoke of 1pm (paying no one would ask me that question!)

    • Hello again Andrew. It is a very sensible question as for the first few weeks of Sydney Observatory, back in 1858, the Government Astronomer dropped the ball at 12 noon. There are two theories of why he changed the time to 1 pm. 1. To know the right time at which to drop the ball, he needed to observe the Sun crossing the meridian, the imaginary overhead line running north to south. That happens at or close to 12 noon so he could not climb the tower to drop the ball at the same time. 2. Officers on the morning watch on board ships hand over to the afternoon watch at 12 noon, so they would have been too busy to watch the time ball drop.

    • Chris,
      This firing of the cannon is steeped in history. Just like the timing of the time ball being dropped that dates to the 1840s. It signifies to ships being able to set their chronometers for the long sea voyages. Knowing the time was essential for navigation, and right back to the time of Captain Phillip in 1788. This tradition is one of the oldest in Australia, and is something that joins us to our origins as a country.
      Frankly, I could not think of any better tribute!

      • Doh! The were talking planning for a time ball in the 1840s, and it appears on the early sketches around 1853. I think before that they used flags upon masts. Of course the time ball wasn’t in action to Sydney Observatory was completed!

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