Indeed, the barycentre of the system Sun-Earth is very nearly to the Sun's own barycentre. But, depending the scale of precision you need, it may complicate things too.
I'm attending a course of celestial mechanics now and we're just learning that matter. 2011/3/25 Patrick Powers <patrick_pow...@compuserve.com> > Well said. Not only that but the sun wobbles too with its barycentre > currently outside the sun’s disc. Whilst the Earth-Moon system isn’t the > major contributor to the overall wobble of the sun it’s one of the most > complicated. I like the link at > > http://www.orbitsimulator.com/gravity/articles/ssbarycenter.html > > which well shows the effects of the different planetary influences on the > sun’s barycentre. > > Patrick > > *From:* Kevin Karney <ke...@karney.com> > *Sent:* Friday, March 25, 2011 8:16 PM > *To:* Marcelo <mmanil...@gmail.com> > *Cc:* Sundial List <sund...@rrz.uni-koeln.de> > *Subject:* Re: varying speed? > > 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 <mmanil...@gmail.com> > >> 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 <bren...@verizon.net> >> >>> 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 > > > > ------------------------------ > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > >
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