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H1 Longitude Sea Clock

By Tech Center Labs

This app is only available on the App Store for iOS devices.


(note: for the high resolution iPad version search for H1 time keeper)
Enjoy and marvel at this marine timekeeper on your iPhone or iPod Touch and at the same time help support the National Maritime Museum in Greenwich, London. Read more ->

John Harrison's first marine timekeeper (known today as H1) is the first experimental sea clock made by Harrison, to enable navigators to find longitude at sea and is one of the great milestones in clock-making history.

The dials on H1 are different from ordinary clocks. The seconds hand (top) is double-ended and can be read from either end. It goes round the dial once in 2 minutes (30 seconds have passed when the hand moves from vertical to horizontal). The minute hand (left) is also double-ended and goes round its dial once in 2 hours. The hour hand (right) is also double-handed and goes round the dial once in 24 hours. The bottom dial shows the day of the month and goes round once every 31 days, straight up is the 16th.

The images are of the actual H1, built nearly 300 years ago, so the chapter rings are difficult to read. To see what the dials look like close up, have a look at the museum's website at: .

If you want to display GMT set your iPhone clock to UTC.

Tap the screen to turn on/off the background star field.

You can also dismiss the opening image by tapping the screen a couple seconds after starting the app.

"Built of brightly shining brass, with rods and balances sticking out at odd angles, its broad bottom and tall projections recall some ancient vessel that never existed. It looks like a cross between a galley and a galleon, with a high, ornate stern facing forward, two towering masts that carry no sails, and knobbed brass oars to be manned by tiers of unseen rowers. It is a model ship, escaped from its bottle, afloat on the sea of time." "No fanciful movie about time travel, despite the best efforts of Hollywood set design, ever presented a time machine as convincing as this one"
From Dava Sobel's New York Times Best Seller 'Longitude' .

Based on Dava Sobel's book, the truly great movie, Longitude, is a dramatic account of John Harrison's life and quest for the Longitude prize and of Rupert Gould's life-long project to restore the Harrison timekeepers to their former glory.

Further information is available in the following publications: Time Restored: The Harrison timekeepers and R.T.Gould, the man who knew (almost) everything and Harrison both by Jonathan Betts and available from the National Maritime Museum Shop,

This app contains images of H1 © National Maritime Museum, Greenwich, London by kind permission of the Ministry of Defence Art Collection, London, used under license.

A special Thanks to the people at the National Maritime Museum, Greenwich! It's been a great privilege to construct this animation of H1.

Gary / Tech Center Labs

What's New in Version 1.6

Thanks to the Maritime Museum we now have the actual sounds made by made by H1's Grasshopper Escapement. All sounds, bells & ticks, are toggled on/off with the bell icon.
Also now zooms higher resolution dials to full screen. Tap dials to zoom in/out.
Version 1.5 added sound to the app, ticks and ship's bell including the dog watch and New Year's Eve bells. Toggle bells on/off by tapping the bell icon.
Note: Ship's bell is not part of Harrison's H1, it's just an additional feature of the app.
As early as the 15th Century a bell was used to sound the time on board a ship. The bell was rung every half hour of the 4 hour watch. A 24 hour day was divided into 6 4 hour watches, except the dog watch (16:00 - 20:00 hours) which could be divided into two, 2 hour watches to allow for the taking of the evening meal.
The strikes of the bell do not accord to the number of the hour. Instead, on the 1st half hour of a watch 1 bell is struck, an additional strike is added each half-hour of a four-hour watch with eight bells signifying the end of one watch and beginning of the next. Bells would be struck in a pattern of pairs for easier counting, with any odd bells at the end of the sequence.

After a mutiny on the Nore in 1797, British ships modified the bell system on the dog watch (16:00 - 18:00) so that the mutiny signal of 5 bells was never again struck.

At midnight on New Year's Eve sixteen bells are struck - eight bells for the old year and eight bells for the new.
Thanks for the feedback!

iPhone Screenshots

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Customer Reviews

Where are we?

The app is great because of what it represents. The app would be even greater if the picture could be the screen wallpaper


Very nice rendering of H1. Can't wait for the wallpaper/screensaver.


would like to have sound( this clock has not just ticking sounds),
also some detail views please.

H1 Longitude Sea Clock
View in iTunes
  • $0.99
  • Category: Navigation
  • Updated:
  • Version: 1.6
  • Size: 2.9 MB
  • Language: English
  • Seller:

Compatibility: Requires iOS 2.0 or later. Compatible with iPhone, iPad, and iPod touch.

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