Mean Solar Time and Its Connection to Universal Time

Seago, John H.; Seidelmann, P. Kenneth

The passage of time has been historically reckoned by astronomical phenomena, particularly by the counting of solar days according to civil and religious calendars. Seasonal variations in apparent solar time have been known since antiquity, relative to an average solar rate. Improved determinations of the tropical year over the centuries increased the accuracy of mean solar time. Mean time became synonymous with clock time, and was employed by almanacs for navigational purposes.

Today, global timekeeping is based on Coordinated Universal Time (UTC), an atomic timescale with intercalary adjustments that maintain it within 0.9 s of the astronomical timescale Universal Time (UT1). UT1 is the best indicator of mean solar time presently maintained; its constant of proportionality is traceable to Newcomb's 1895 determination of the mean motion of the apparent Sun, also known as the fictitious mean sun, thereby allowing UT1 to remain a very close approximation to the mean diurnal motion of the Sun.

Modern numerical integrations of solar-system motion suggest possible refinements to the mean longitude of the Sun since Newcomb's time, and enable comparison to Universal Time. Solar ephemerides and mean orbital elements affirm that variability in the rotation rate of the Earth causes Universal Time to diverge from the original geometric concept of mean solar time by approximately ΔT/365.2422, where ΔT ("Delta-T") is the accumulated measure of non-uniform Earth rotation since 1900. After accounting for changes in the origins of the terrestrial and celestial reference systems since the end of the 19th century, the Sun on average crosses the celestial meridian about 0.2 s after the mean time implied by UT1.

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