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Solar longitude, commonly abbreviated as Ls, is the ecliptic longitude of the Sun, i.e. the position of the Sun on the celestial sphere along the ecliptic.It is also an effective measure of the position of the Earth (or any other Sun-orbiting body) in its orbit around the Sun, [1] usually taken as zero at the moment of the vernal equinox. [2]
It rotates with a sidereal period of exactly 25.38 days, which corresponds to a mean synodic period of 27.2753 days. [9]: 221 [1] [2] [5] Whenever the Carrington prime meridian (the line of 0° Carrington longitude) passes the Sun's central meridian as seen from Earth, a new Carrington rotation begins.
The mean longitude of the Sun, corrected for the aberration of light, is: L = 280.460 ∘ + 0.9856474 ∘ n {\displaystyle L=280.460^{\circ }+0.9856474^{\circ }n} The mean anomaly of the Sun (actually, of the Earth in its orbit around the Sun, but it is convenient to pretend the Sun orbits the Earth), is:
Then mean longitude is also [1] L = ϖ + M. Another form often seen is the mean longitude at epoch, ε. This is simply the mean longitude at a reference time t 0, known as the epoch. Mean longitude can then be expressed, [2] L = ε + n(t − t 0), or L = ε + nt, since t = 0 at the epoch t 0. where n is the mean angular motion and t is any ...
Newcomb gives the Right ascension of the fictitious mean Sun, affected by aberration (which is used in finding mean solar time) as [10] τ = 18 h 38 m 45.836 s + 8 640 184.542 s T + 0.0929 s T 2. Authors citing this expression include McCarthy & Seidelmann (p. 13) and the Nautical Almanac Offices of the United Kingdom and United States (p. 73).
One astronomical unit (about 150 million kilometres; 93 million miles) is defined as the mean distance between the centers of the Sun and the Earth. The instantaneous distance varies by about ± 2.5 million kilometres (1.6 million miles) as Earth moves from perihelion around 3 January to aphelion around 4 July. [36]
Evolution of the solar luminosity, radius and effective temperature compared to the present-day Sun. After Ribas (2009) [3] The uncrewed SOHO spacecraft was used to measure the radius of the Sun by timing transits of Mercury across the surface during 2003 and 2006. The result was a measured radius of 696,342 ± 65 kilometres (432,687 ± 40 miles).
When the mean equator and equinox of J2000 are used to define a celestial reference frame, that frame may also be denoted J2000 coordinates or simply J2000. This is different from the International Celestial Reference System (ICRS): the mean equator and equinox at J2000.0 are distinct from and of lower precision than ICRS, but agree with ICRS ...