The graph on this link I beleive shows a long term (1800-2200)declining trend of the date of the vernal equinox -- consistent with the explanation for such I provided earlier. http://scienceworld.wolfram.com/astronomy/VernalEquinox.html
Some other links that touch on and are consistent with a long run shift in the vernal equinox to ealier andealier dates. Particularly weather changes and changes in the nature of the seasons. --------- Precession causes the cycle of seasons (tropical year) to be about 20.4 minutes less than the period for the earth to return to the same position with respect to the stars as one year previously (sidereal year). This results in a slow change (one day per 58 calendar years) in the position of the sun with respect to the stars at an equinox. It is significant for calendars and their leap year rules. The following figure illustrates the effects of axial precession on the seasons, relative to perihelion and aphelion. The precession of the equinoxes can cause periodic climate change (see Milankovitch cycles), because the hemisphere that experiences summer at perihelion and winter at aphelion (as the southern hemisphere does presently) is in principle prone to more severe seasons than the opposite hemisphere. http://en.wikipedia.org/wiki/Precession Milankovitch cycles is the name given to the collective effect of changes in the Earth's movements upon its climate. The eccentricity, axial tilt, and precession of the Earth's orbit vary in several patterns, resulting in 100,000 year ice age cycles of the Quaternary glaciation over the last few million years. The Earth's axis completes one full cycle of precession approximately every 26,000 years. At the same time the elliptical orbit rotates, more slowly, leading to a 22,000 years cycle in the equinoxes. In addition, the Earth's tilt relative to the Sun changes between 21.5 degrees to 24.5 degrees and back again on a 41,000 year cycle. The Earth's axis today is tilted 23.5 degrees relative to the normal to the plane of the ecliptic. Precession of the equinoxes is the change in the direction of the Earth's axis of rotation relative to the Sun at the time of perihelion and aphelion. The Earth goes through one complete precession cycle in a period of approximately 25,800 years. When the axis is aligned so it points toward the Sun during perihelion, one polar hemisphere will have a greater difference between the seasons while the other hemisphere will have milder seasons. The hemisphere which is in summer at perihelion will receive much of the corresponding increase in solar radiation, but that same hemisphere will be in winter at aphelion and have a colder winter. The other hemisphere will have a relatively warmer winter and cooler summer. When the Earth's axis instead points toward the Sun during spring and autumn, the Northern and Southern Hemispheres will have similar contrasts in the seasons. At present perihelion occurs during the Southern Hemisphere's summer, and aphelion is reached during the southern winter. Thus the Southern Hemisphere seasons should tend to be somewhat more extreme than the Northern Hemisphere seasons. As the Earth spins around its axis and orbits around the Sun, several quasi-periodic variations occur. Although the curves have a large number of sinusoidal components, a few components are dominant. Milankovitch studied changes in the eccentricity, obliquity, and precession of Earth's movements. Such changes in movement and orientation change the amount and location of solar radiation reaching the Earth. This is known as solar forcing (an example of radiative forcing). Changes near the north polar area are considered important due to the large amount of land, which reacts to such changes more quickly than the oceans do. http://en.wikipedia.org/wiki/Milankovitch_cycles Due to this wobble a climatically significant alteration must take place. When the axis is tilted towards Vega the positions of the Northern Hemisphere winter and summer solstices will coincide with the aphelion and perihelion, respectively. This means that the Northern Hemisphere will experience winter when the Earth is furthest from the Sun and summer when the Earth is closest to the Sun. This coincidence will result in greater seasonal contrasts. At present, the Earth is at perihelion very close to the winter solstice. (From NOAA's National Geophysical Data Center) At times when Northern Hemisphere summers are coolest (farthest from the Sun due to precession and greatest orbital eccentricity) and winters are warmest (minimum tilt), snow can accumulate on and cover broad areas of northern America and Europe. At present, only precession is in the glacial mode, with tilt and eccentricity not favorable to glaciation Even when all of the orbital parameters favor glaciation, the increase in winter snowfall and decrease in summer melt would barely enough to trigger glaciation, not to grow large ice sheets. Ice sheet growth requires the support of positive feedback loops, the most obvious of which is that snow and ice have a much lower albedo than ground and vegetation, thus ice masses tend to reflect more radiation back into space, thus cooling the climate and allowing glaciers to expand. http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm ------------------------ Yahoo! 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