The following passage was discovered on the web, and in the interest of our readers we decided to ask ourselves a few specific questions in order to provide a comprehensive understanding of the term "fundamental year".

http://pweb.jps.net/~tgangale/mars/other/allison2.htm

"A 'year' refers to a planet's period of orbital revolution. But the precise reckoning of this period depends upon the specifically adopted reference for the average interval between the beginning and end of an orbit. The anomalistic year refers to the repetition of the planet's "mean anomaly" and corresponds to the mean interval between successive passages of its perihelion or closest approach to the Sun. The sidereal year refers to the planet's mean orbital period as referenced with respect to the stars. The tropical year, often but imprecisely described as the mean interval between successive passages of the vernal equinox, is defined by astronomers as the slightly different interval during which the Sun's mean longitude, referred to the mean equinox of date, increases by 360°. (The tropical year is shortened with respect to the sidereal year by the longitudinal precession of the planet's pole vector, and can be estimated as the average of the four mean intervals for the repetition of each of the equinox and solstice seasons.) All these measures of the orbital year also change slowly with time.

Although the mean Gregorian calendar year of 365.2425 days is closely matched to the Earth's current "vernal equinox year" of 365.2424 d, astronomers still use the Julian Century of 36525 Earth solar days or 100 Julian Years as a fundamental unit for ephemeris work. This definition has the advantage of a short and exact decimal representation for conversion to or from the Julian Date, and serves the specification of standard astronomical epochs. The current J2000 epoch is defined as JD 2451545.0 (2000 Jan 1.5), for example, exactly 36525 days after J1900 = JD 2415020.0 (1900 Jan 0.5)."

Dr. Michael Allison, Goddard Institute for Space Studies, NASA

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S.R.G. Comment:

Why would a "vernal equinox year" have 365.2424 days?

Maybe it is a misprint, misinformation, a misconception or just Mist (German for rubbish).

What is the fundamental year?

According to the IAU, it is the tropical year for 1900.0 of exactly 31,556,925.97474 seconds. Such a year is not based on estimates of any kind, but on precise measurement.

How does one measure successive passages of the vernal equinox, which is essentially an imaginary point in the sky?

In order to determine such a year, an inertial or non-moving point of reference is required that remains fixed relative to the position of the sun. Only the absolute rotation period of the Earth on its axis can serve as the basis for the determination of this year; i.e. for the 360-degree orbit period of the absolute center of the Earth around that of the sun. As we know, the mean sidereal day of 86164.0905382 seconds is the actual 360-degree rotation of the Earth on its axis.

It was said that the tropical year is the time interval of 31,556,925.97474 seconds during which the Sun's mean longitude or the mean equinox of date increases by 360-degree. But what about the sidereal year, which is referenced with respect to the stars? Doesn't that refer to the actual 360-degree orbit period of the Earth?

According to the theory of lunisolar precession, the positions of both the sun and the stars remain fixed to each other. The longitudinal precession of the planet's pole vector is not affecting the absolute rotation of the Earth on its axis. That means any changes measured in the orientation of the earth's axis relative to inertial space (and the sun) must be removed in order to determine the actual 360-degree orbit period of the absolute center of the Earth around that of the sun. Simple trigonometry. No astronomy.

In view of the incontrovertible documented evidence that the sidereal year for 1900.0 has also 31,556,925.97474 seconds, why is it then that astronomers claim a sidereal year must have 31,558,149.5 seconds?

Unfortunately, astronomers are not able to answer this simple question. That is because they are under the illusion that the sidereal day, as measured relative to the stars, must be about 3.34 seconds longer than the absolute rotation of the Earth on its axis.

But in reality, the difference an observer on Earth would measure relative to a star is on average only 0.00912 seconds per absolute rotation, due to the longitudinal precession of the planet's pole vector. This comes to about 3.34 seconds of rotation time (50.26 arc seconds) per tropical year. Why would the observer on the Earth have to rotate another 1220 seconds or so in order to measure the star?

He (or she) doesn't - as the 360° orbit relative to the Sun's mean longitude is completed, the sun has already moved further in its orbit around its dual. Measured are only the average 9.12 ms per rotation (except relative to Sirius, as repeatable experiments, i.e. long-term observation of transit periods can easily prove).

One must not mistake the moving axis of the Earth with the observer on the rotating Earth. Otherwise, one is faced with the dilemma of the non-existing 20 minutes per 360-degree orbit!