Requirements for UTC and Civil Timekeeping on Earth
Interested readers can find more about leap seconds than they might have believed possible from the proceedings of two workshops held in 2011 and 2013 on the future of Coordinated Universal Time. Search on "Requirements for UTC and Civil Timekeeping on Earth" (URLs are not permitted in comments). A feisty if small community has been debating these issues for many years.
The fundamental tension that creates the need for leap seconds comes from the fact that atomic time and solar time are simply two different things. Mean solar time arises from the "synodic" day. The sidereal rotation of the Earth (relative to the stars) has small variations and a general slowing trend due to lunar tides. As we lap the Sun once per year, one rotation is unwrapped, and as a result there is one fewer day per year than sidereal rotations. While the solar clock subdivides calendar days, atomic clocks are metronomes in multiples or fractions of seconds, or better yet in units of frequency, Hertz. Solar time counts fractions of days and atomic time, multiples of SI seconds; there is not a fixed relationship between solar days and SI seconds. Wishing it will not make it so.
One somewhat bizarre result is that if leap seconds cease, the Prime Meridian will begin to move from Greenwich, England to Paris, France at a rate of about one football field (American or European) per year. Presumably this won't actually happen but nobody has offered any explanation of what will have to change to avoid such an issue. In fact some are relying on schemes to hide the embargoed leap seconds within the standard timezone system whose origin is the Prime Meridian.
While timekeeping issues at the level of seconds may be negligible for some purposes, they are quite significant for fields like aerospace, astronomy and navigation. Navigation these days means in the air and on land as well as at sea; GPS and other GNSS satellites themselves depend on solar time to operate. Ceasing leap seconds is a very expensive proposition for many communities, and a risky one for others that have not previously considered a distinction to exist between solar and atomic clocks.
- Rob Seaman, National Optical Astronomy Observatory