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WikiPedia: Hoàng đạo http://adsabs.harvard.edu/abs/1983CeMec..31..329K http://articles.adsabs.harvard.edu/full/1986A&A...... http://stars.astro.illinois.edu/celsph.html http://astro.unl.edu/naap/motion1/animations/seaso... http://aa.usno.navy.mil/data/docs/EarthSeasons.php http://www.astrologyclub.org/articles/ecliptic/ecl... //dx.doi.org/10.1007%2FBF01230290 http://www.dur.ac.uk/john.lucey/users/solar_year.h... https://books.google.com/books https://books.google.com/books?id=8CwSAAAAYAAJ&dq=...