We know that the Earth rotates around its axis. We have the idea that one day is the time span for the Earth to make a complete turn. But what we may not all know is that the Earth's rotation speed is decreasing. And that the length of the solar day is not the same throughout the year. These facts require corrections in the so-called Civil Hour.
Civil Time, in any country, is determined in relation to the average position of the sun in the local sky. But since the length of the solar day is not constant throughout the year, there are differences between true solar time and Civil Time, which has a uniform progression.
Historically, the UT1 time scale was defined based on the average duration of the Earth's rotation, which is currently measured in relation to the celestial positions of the most distant quasars.
The Earth's rotation has been studied rigorously and measured with atomic precision, since 1972, by the International Service for Earth Rotation and Reference Systems (IERS, from the English International Earth Rotation Service), of the Paris Observatory.
The introduction of a network of atomic clocks to more accurately measure the passage of time and define the scale of the International Atomic Time (TAI) found that there was a difference of tens of seconds to the time then used (UT1).
Since then, regular corrections have been made to try to match astronomical clocks with atomic ones. The last hit was in 2012.
In fact, the need for coordination between the scales of Atomic Time, defined by atomic clocks, and Astronomical Time (scale UT1 defined by the Earth's real rotation) occasionally leads to the introduction of “intercalating seconds” in Coordinated Universal Time (UTC). so that the difference between them (UT1-UTC) remains less than 1 second.
To maintain this temporal proximity, the IERS determined that it was necessary to introduce an “intermediate second” in UTC, which currently defines the Legal Civil Time, at the end of June 2015.
Thus, and in mainland Portugal, and in practical terms, it will be necessary to “stop” the clocks for one second when it is 00:59:59 on the 1st of July.
It should also be said that the need for this adjustment comes from the fact that the duration of the "second" in the International System of Units (linked to the TAI scale) is shorter than the current duration of the "second UT1" of the Earth's rotation. That is, the Earth's complete rotation lasts longer and longer on the TAI scale.
The Lisbon Astronomical Observatory (OAL) is the institution that has the legal responsibility to maintain and distribute the Legal Hour in Portugal.
To set your clock, and your computer's, to the correct time and follow the introduction of the interim second, follow the instructions indicated on the OAL website: http://oal.ul.pt/hora-legal/como-acertar/
Author Antonio Piedade
Science in the Regional Press – Ciência Viva
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