Days of solstices and equinoxes
Days of solstices and equinoxes
« E pur, si muove ! » - literally "And yet, it rotates !" -, claimed Galileo Galilei after he was sentenced for heresy in 1633. Yes, our Earth does rotate. It rotates around its own axis, on the one hand, revolves around the Sun, on the other hand. Viewed from earth, it is however the Sun which seems to make this daily rotation and this yearly revolution as well. Doesn't it effectively rise at the north-eastern corner of the local horizon and set at its north-western corner on summer solstice ? Doesn't it appear at the south-eastern corner of the sky and disappear at its south-western corner six months later, on winter solstice ? Aren't the length of its diurnal trajectory and the height of its upper transit in the southern meridian responsible for the duration of the day ? A duration which equals 12 hours on spring and autumn equinoxes.
Trajectory of the Sun above the terrestrial horizon on summer solstice (left),
on spring and autumn equinoxes (center) and on winter solstice (right),
viewed from the northern hemisphere.
This software invites you to accurately determine the days of solstices and equinoxes at any year between -4712 and 2025. To do so, it combines various astrometry (relative to the positioning of the stars in the sky) algorithms published within the scientific articles whose list appears below.
Free tests of this software are available within the Client Area. The access to the user interface of this software is made on payment : 7,50 euros, which can be paid via the Paypal secured paiement system (right button). This amount includes an unlimited access to the user interface and a free access to any future updates.
Price : 7,50 euros
Borkowski, K.M., "ELP 2000-85 and the Dynamical Time - Universal Time Relation", Astronomy and Astrophysics, 205 (1988), L8-L10.
Bureau des Longitudes, "Introduction aux Ephémérides Astronomiques", EDP Sciences 1998.
Centre de Données Astronomiques de Strasbourg : http://cdsweb.u-strasbg.fr .
Chapront-Touzé, Michelle et Chapront, Jean, "Lunar Tables and Programs from 4000 BC to AD 8000", Willmann-Bell, Richmond, 1991, pp 6-7.
JPL Horizons : http://ssd.jpl.nasa.gov/horizons.html.
Simon, J.L., Bretagnon, P., Chapront, J., Chapront-Touzé, M., Francou, G., Laskar, J., "Numerical expressions for precession formulae and mean elements for the Moon and the planets", Astronomy Astrophysics 282, 663-683 (1994).
Stephenson, F.R., "Historical Eclipses and Earth Rotation", Cambridge University Press, Cambridge, 1997.
Stephenson, F.R. et Morrison, L.V., "Long-Term Fluctuations in the Earth's Rotation : 700 BC to AD 1990", Philosophical Transactions of the Royal Society of London, Ser. A, 351 (1995), 165-202.
Stephenson, F.R. et Morrison, L.V., "Long-Term Changes in the Rotation of the Earth : 700 BC to AD 1980", Philosophical Transactions of the Royal Society of London, Ser. A, 313 (1984), 47-70.
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