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where n is the Earth's mean angular orbital velocity in degrees per day, a.k.a. "the mean daily motion". = (+) where D is the date, counted in days starting at 1 on 1 January (i.e. the days part of the ordinal date in the year). 9 is the approximate number of days from the December solstice to 31 December.
Length of a decimal year. Since there are about 365 days in a year, there are about 365 / 10 = 36.5 days in a tenth of a year. Hence the year 2020.5 represents the day 2 July 2020. [22] More exactly, a "Julian year" is exactly 365.25 days long, so a tenth of the year is 36.525 days (36 days, 12 hours, 36 minutes).
Because of the Earth's axial precession, this year is about 20 minutes shorter than the sidereal year. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds, using the modern definition [13] (= 365.242 19 d × 86 400 s). The length of the tropical year varies a bit over thousands of years because the rate of axial ...
Thus, the sidereal day is shorter than the stellar day by about 8.4 ms. [37] Both the stellar day and the sidereal day are shorter than the mean solar day by about 3 minutes 56 seconds. This is a result of the Earth turning 1 additional rotation, relative to the celestial reference frame, as it orbits the Sun (so 366.24 rotations/y).
Since each lunation is approximately 29 + 1 ⁄ 2 days, [1] it is common for the months of a lunar calendar to alternate between 29 and 30 days. Since the period of 12 such lunations, a lunar year , is 354 days, 8 hours, 48 minutes, 34 seconds (354.36707 days), [ 1 ] purely lunar calendars are 11 to 12 days shorter than the solar year .
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The ERA may be converted to other units; for example, the Astronomical Almanac for the Year 2017 tabulated it in degrees, minutes, and seconds. [12] As an example, the Astronomical Almanac for the Year 2017 gave the ERA at 0 h 1 January 2017 UT1 as 100° 37′ 12.4365″. [13]
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