Editing Second (section)

===Fraction of an ephemeris year===
{{see also|Ephemeris time}}

Sometime in the late 1940s, quartz crystal oscillator clocks with an operating frequency of ~100&nbsp;kHz advanced to keep time with accuracy better than 1 part in 10<sup>8</sup> over an operating period of a day. It became apparent that a consensus of such clocks kept better time than the rotation of the Earth. [[Time metrology|Metrologists]] also knew that Earth's orbit around the Sun (a year) was much more stable than Earth's rotation. This led to proposals as early as 1950 to define the second as a fraction of a year.

The Earth's motion was described in [[Newcomb's Tables of the Sun|Newcomb's ''Tables of the Sun'']] (1895), which provided a formula for estimating the motion of the Sun relative to the epoch 1900 based on astronomical observations made between 1750 and 1892.<ref name="USNO">{{cite web
 | title=Leap Seconds
 | publisher=Precise Time Department, [[United States Naval Observatory]]
 | url=https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/Global-Positioning-System/USNO-GPS-Time-Transfer/Leap-Seconds/
 | access-date=May 13, 2025
 }}</ref> This resulted in adoption of an [[ephemeris time]] scale expressed in units of the [[sidereal year]] at that epoch by the [[International Astronomical Union|IAU]] in 1952.<ref>
{{citation
 |author=Nautical Almanac Offices of the United Kingdom and the United States of America 
 |url=https://archive.org/details/explanatorysupplement/page/n19
 |title=Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac
 |year=1961
 |page=9
 |quote=... defined ephemeris time ... [was] adopted by the [[International Astronomical Union]] in Sept. 1952.}}
</ref> This extrapolated timescale brings the observed positions of the celestial bodies into accord with Newtonian dynamical theories of their motion.<ref name="USNO" /> In 1955, the [[tropical year]], considered more fundamental than the sidereal year, was chosen by the IAU as the unit of time. The tropical year in the definition was not measured but calculated from a formula describing a mean tropical year that decreased linearly over time.

In 1956, the second was redefined in terms of a year relative to that [[epoch (astronomy)|epoch]]. The second was thus defined as "the fraction {{frac|31,556,925.9747}} of the tropical year for 1900 [[January 0]] at 12 hours ephemeris time".<ref name="USNO" /> This definition was adopted as part of the [[International System of Units]] in 1960.<ref>{{cite web
 |title=SI Brochure (2006)
 |work=SI Brochure 8th Edition
 |url=https://www.bipm.org/utils/common/pdf/si_brochure_8.pdf
 |page=112
 |publisher=[[BIPM]]
 |access-date=May 23, 2019
 |archive-url=https://web.archive.org/web/20190503133741/https://www1.bipm.org/utils/common/pdf/si_brochure_8.pdf
 |archive-date=May 3, 2019
 |url-status=live
 }}</ref>