Other Clocks
Ways of telling the time that never quite caught on — or were never meant for Earth. See also the French Republican Calendar.
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Decimal time
The French Revolutionary decimal time divided the day into 10 hours, each hour into 100 minutes, and each minute into 100 seconds. One decimal second is 0.864 standard seconds.
History
Decreed by the National Convention in 1793 alongside the Republican calendar, decimal time was officially in use from November 1793 until it was suspended in April 1795 — about seventeen months. Clockmakers produced dials showing both systems, and a few survive in museums today.
Swatch Internet Time
One day = 1,000 .beats, with no time zones and no daylight saving. Everyone on Earth shares the same beat, counted from midnight in Biel (Biel Mean Time, UTC+1).
History
Launched by Swatch in October 1998, with Nicholas Negroponte of the MIT Media Lab, as a time standard for the internet age. It appeared on Swatch watches and in games such as Phantasy Star Online on the Sega Dreamcast. The beat is a decimal fraction of the day — the French idea again, just without the hours.
Coordinated Mars Time
A Martian solar day — a sol — lasts 24 h 39 min 35 s. Mission controllers keep Mars time by stretching an ordinary 24-hour clock to fit it. This is the mean solar time at Mars's prime meridian (Airy-0 crater).
How this is calculated
Using NASA's Mars24 algorithm (Allison & McEwen, 2000). Earth time is converted to Terrestrial Time (UTC + 69.184 s), then to sols since the Mars Sol Date epoch. Mars Years are counted from the northern spring equinox of 11 April 1955 (Mars Year 1). Rover sols count from each landing (sol 0) in local mean solar time.
Pulsar time
Millisecond pulsars are collapsed stars spinning hundreds of times a second, sweeping a radio beam past Earth like a lighthouse. A few are as steady over decades as the best atomic clocks, and networks of them — pulsar timing arrays — are used to build time scales independent of any clock on Earth.
PSR B1937+21
PSR J0437−4715
About these counts
Each count is modelled from the pulsar's published spin rate and its slow spin-down, as it would be ticked off at the centre of mass of the Solar System. A real observatory also corrects for Earth's orbit (up to ±8 light-minutes), interstellar dispersion and, for J0437−4715, its 5.7-day orbit around a white dwarf. The beams above are slowed down enormously for your eyes.
Spin data: B1937+21 period 1.557806567 ms, slowing by 1.05 × 10⁻¹⁹ s per second; J0437−4715 from its Fermi-LAT timing solution (173.687948 Hz).
Astronomer's clocks
What these are
Julian Dates count days since noon on 1 January 4713 BC and are how astronomers timestamp observations. Sidereal time tracks Earth's rotation against the stars rather than the Sun — a sidereal day is about 3 min 56 s shorter. TAI is the atomic time scale without leap seconds (37 s ahead of UTC); GPS satellites keep their own count, fixed at 19 s behind TAI since 1980. Unix time counts seconds since 1 January 1970.