Topic Tuesday #154 2015/06/30 "Leap Second"

Topic Tuesday #154 2015/06/30 "Leap Second"

Long ago, I wrote about the origins of the calendar. #24

But what I didn't discuss was corrections to timekeeping.
See the calendar that we eventually landed on is not perfect, by a long shot. This is why every 4 years we add a day (24 hours) to February. This keeps the calendar in sync with the planet's seasons. Today, June 30th, the last minute of the day has 61 seconds as we add a leap second in order to keep the time of day close to the mean solar time, or UT1. Without such a correction, time reckoned by Earth's rotation drifts away from atomic time because of irregularities in the Earth's rotation. Since this system of correction was implemented in 1972, 26 leap seconds have been inserted, including today's. Ideal implementation adds a positive leap second between "second 23:59:59" of a chosen UTC calendar date (the last day of a month, usually June 30 or December 31) and "second 00:00:00" of the following date.

Peter Whibberley, senior research scientist at the UK's National Physical Laboratory (NPL), said: "Because they depend on measurements of the Earth's rotation, which varies unpredictably, leap seconds occur at irregular intervals. Leap seconds are announced only six months in advance. This means computers and software cannot be supplied with leap seconds programmed in, and they must be inserted manually," he explained. "Getting leap seconds wrong can cause loss of synchronisation in communication networks, financial systems and many other applications which rely on precise timing. Whenever a leap second occurs, some computer systems encounter problems due to glitches in the code written to handle them. The consequences are particularly severe in the Asia-Pacific region, where leap seconds occur during normal working hours."

Due to these complications many would like to do away with the practice of adding leap seconds. The International Telecommunications Union (ITU) is set to discuss the topic at the World Radiocommunication Conference in Geneva this November.

There was some jargon in there, like "Mean Solar Day" and "UT1". Let me do the work for you and tell you what those are.

The duration of daylight varies during the year but the length of a mean solar day is nearly constant, unlike that of an apparent solar day. An apparent solar day can be 20 seconds shorter or 30 seconds longer than a mean solar day.

UT1 is the principal form of Universal Time. While conceptually it is mean solar time at 0° longitude, precise measurements of the Sun are difficult. Hence, it is computed from observations of distant quasars using long baseline interferometry, laser ranging of the Moon and artificial satellites, as well as the determination of GPS satellite orbits. UT1 is the same everywhere on Earth, and is proportional to the rotation angle of the Earth with respect to distant quasars, specifically, the International Celestial Reference Frame (ICRF), neglecting some small adjustments. The observations allow the determination of a measure of the Earth's angle with respect to the ICRF, called the Earth Rotation Angle (ERA, which serves as a modern replacement for Greenwich Mean Sidereal Time). UT1 is required to follow the relationship
ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians
where Tu = (Julian UT1 date - 2451545.0)

Around the world, we try to use UTC, (oddly acronymed for Coordinated Universal Time, because French.)
UTC is an atomic timescale that approximates UT1. It is the international standard on which civil time is based. It ticks SI seconds, in step with TAI. It usually has 86,400 SI seconds per day but is kept within 0.9 seconds of UT1 by the introduction of occasional intercalary leap seconds. As of 2015, these leaps have always been positive (the days which contained a leap second were 86,401 seconds long). Whenever a level of accuracy better than one second is not required, UTC can be used as an approximation of UT1. The difference between UT1 and UTC is known as UT1. 
For most purposes, UTC is considered interchangeable with GMT, but GMT is no longer precisely defined by the scientific community.