Marcelo Nothing is constant in the heavens ! The 'tropical' year (from equinox to equinox) is 365.242190 days The 'sidereal' year (fixed star to fixed star) is 365.256363 days The 'anomalistic' year (perihelion to perihelion) is 365.259636 days - cycling over a period of some 21000 years (values for 2009 from Astronomical Almanac) But these are mean values having averaged out the effects of nutation (the wobbling of the Earth's axis) and various other effects.
Perihelion is even more complicated.... in 2010 - 3rd January 0 hrs in 2011 - 3rd January 19 hrs in 2012 - 5th January 1 hrs in 2013 - 2nd January 5 hrs in 2014 - 4th January 12 hrs (values from US Naval Observatory web site) This is strange behaviour - not just a leap year effect! I have heard that this is because - from the Keplerean point-of-view - the Earth and Moon rotate as a unit in an ellipse around the sun - like an out of balance dumbell - whose centre of gravity is somewhere in the Earth's core but not at its centre. So the actual moment when the Earth is closest to the Sun depends on the position of the Moon. This was explained to me some 50 years ago by my uncle who was a dedicated but amateur astronomer. I have never it confirmed by a professional astronomer. Best regards Kevin Karney Freedom Cottage, Llandogo, Monmouth NP25 4TP, Wales, UK 51° 44' N 2° 41' W Zone 0 + 44 1594 530 595 On 25 Mar 2011, at 16:14, Marcelo wrote: > Your question brought to my mind an old doubt. > > As the points of perihelion and aphelion are continually changing (in a very > slowly way, but they are), so the EoT is also changing from an year to > another, right? I mean, if a century ago perihelion and aphelion occurred not > in january and july, but in december and june (it's only an example, I don't > know how much time does it need to change), then the EoT was different. > > 2011/3/25 Marcelo <[email protected]> > Hello Brent, > > as long as I know, the Earth's speed really has a variation throughout the > year, for its orbit being ellliptical, with the Sun in one of the ellipse > focuses, it is faster when nearer to the sun (perihelion) and slower when its > at maximum distance from it (aphelion). > > Both the perihelion and aphelion are upon the ellipse's major axis. > > As a result, the sun's apparent ecliptical longitude changes a little slower > in july than it does in january. > > Further, as Earth's axis has a declination of ~ 23.5 degrees, that means that > the Sun's apparent longitude measured upon the Equator is slightly different > of its ecliptical longitude (measured upon the Earth's orbit plan). > > So, neither is the Sun moving from West to East regulary throughout the year, > neither is its movement on the ecliptic equal to that on the Equator - if Sun > moves 1 degree with relation to the ecliptic, it may move 58 minutes of arc > with relation to the celestial equator. > > > 2011/3/24 Brent <[email protected]> > Hello again; > > I read this at: > http://www.sundialsoc.org.uk/HDSW.htm > > Part 17 > >When we look at the Sun we are observing it from a moving > >platform. It is the varying speed around its elliptical > >orbit and the tilted axis which are responsible for the > >daily variations accounted for by the Equation of Time. > > I'm confused about the varying speed part. > Does the earth actually change speed as it travels around > the sun or is it just the way we perceive it? > > thanks again; > brent > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial >
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