Editing Second (section)

===Fraction of solar day===
{{see also|Seconds_pendulum#Defining_the_second}}

The earliest mechanical clocks, which appeared starting in the 14th century, had displays that divided the hour into halves, thirds, quarters and sometimes even 12 parts, but never by 60. In fact, the hour was not commonly divided in 60 minutes as it was not uniform in duration. It was not practical for timekeepers to consider minutes until the first mechanical clocks that displayed minutes appeared near the end of the 16th century. Mechanical clocks kept the ''mean time'', as opposed to the ''apparent time'' displayed by [[sundial]]s. 
By that time, sexagesimal divisions of time were well established in Europe.{{#tag:ref|It may be noted that 60 is the smallest multiple of the first 6 counting numbers. So a clock with 60 divisions would have a mark for thirds, fourths, fifths, sixths and twelfths (the hours); whatever units the clock would likely keep time in, would have marks.|group=nb}}

The earliest clocks to display seconds appeared during the last half of the 16th century. The second became accurately measurable with the development of mechanical clocks. The earliest spring-driven timepiece with a second hand that marked seconds is an unsigned clock depicting [[Orpheus]] in the Fremersdorf collection, dated between 1560 and {{nowrap|1570.<ref name="Landes">{{cite book | first1=David S. |last1=Landes |author-link1=David S. Landes |title=[[Revolution in Time]] | location=Cambridge, Massachusetts| publisher= Harvard University Press |year= 1983 | isbn = 0-674-76802-7 }}</ref>{{rp|417–418}}<ref>{{cite book |first1=Johann |last1=Willsberger |title=Clocks & watches |location=New York |publisher=Dial Press |year=1975 |isbn=0-8037-4475-7 |url-access=registration |url=https://archive.org/details/clockswatchessix0000will }} full page color photo: 4th caption page, 3rd photo thereafter (neither pages nor photos are numbered).</ref>}} During the 3rd quarter of the 16th century, [[Taqi al-Din Muhammad ibn Ma'ruf|Taqi al-Din]] built a clock with marks every 1/5 minute.<ref name="Selin1997">{{cite book |first=Helaine |last=Selin |author-link=Helaine Selin |title=Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures |url=https://books.google.com/books?id=raKRY3KQspsC&pg=PA934 |date=July 31, 1997 |publisher=Springer Science & Business Media |isbn=0-7923-4066-3 |page=934 |access-date=February 23, 2016 |archive-url=https://web.archive.org/web/20161120013247/https://books.google.com/books?id=raKRY3KQspsC&pg=PA934 |archive-date=November 20, 2016 |url-status=live }}</ref>
In 1579, [[Jost Bürgi]] built a clock for [[William IV, Landgrave of Hesse-Kassel|William of Hesse]] that marked seconds.<ref name="Landes" />{{rp|105}} In 1581, [[Tycho Brahe]] redesigned clocks that had displayed only minutes at his observatory so they also displayed seconds, even though those seconds were not accurate. In 1587, Tycho complained that his four clocks disagreed by plus or minus four seconds.<ref name="Landes" />{{rp|104}}

In 1656, Dutch scientist [[Christiaan Huygens]] invented the first pendulum clock. It had a pendulum length of just under a meter, giving it a swing of one second, and an escapement that ticked every second. It was the first clock that could accurately keep time in seconds. By the 1730s, 80 years later, [[John Harrison]]'s maritime chronometers could keep time accurate to within one second in 100 days.

In 1832, [[Carl Friedrich Gauss|Gauss]] proposed using the second as the base unit of time in his millimeter–milligram–second [[system of units]]. The [[British Association for the Advancement of Science]] (BAAS) in 1862 stated that "All men of science are agreed to use the second of mean solar time as the unit of time."<ref>{{cite book |url=https://books.google.com/books?id=540DAAAAQAAJ&pg=PR1 |title=Reports of the committee on electrical standards |year=1873 |publisher=British Association for the Advancement of Science |editor-first=Henry Charles Fleeming |editor-last=Jenkin |editor-link=Henry Charles Fleeming Jenkin |page=90 |access-date=February 23, 2016 |archive-url=https://web.archive.org/web/20161120015337/https://books.google.com/books?id=540DAAAAQAAJ&pg=PR1#v=onepage&q&f=true |archive-date=November 20, 2016 |url-status=live }}</ref> BAAS formally proposed the [[CGS|CGS system]] in 1874, although this system was gradually replaced over the next 70 years by [[MKS system of units|MKS]] units. Both the CGS and MKS systems used the same second as their base unit of time. MKS was adopted internationally during the 1940s, defining the second as {{frac|86,400}} of a mean solar day.