Days of solstices and equinoxes
Days of solstices and equinoxes
In the eyes of the terrestrial observer, the Sun follows a circular trajectory
- more or less extended, more or less inclined, according to the time of the year.
The winter and summer solstices materialize its perigee and apogee,
whereas the spring and autumn equinoxes occupy the intermediate positions.
With the help of modern tools, let's determine the occurrence of these key events ...
In brief
The succession of the seasons of the year, the alternation of periods of drought and humidity, of heat, warmth and coolness, govern life on Earth - both on the surface of the continents and at the bottom of the oceans. They result from slow but inexorable variations in the duration and intensity of the sunshine. In other words, the trajectory that our Sun seems to describe above the Earth's horizon. A trajectory that differs each day: more or less extended and inclined, depending on the time of the year.
The winter solstice, the spring equinox, the summer solstice and the autumn equinox are, at intermediate latitudes, the high points of each season of the year. Their occurrence materializes key moments, at which the Sun occupies particular positions: extreme or intermediate, two to two opposite, on the celestial vault centered on our Earth.
Variation of sunshine on the Earth's surface over the year (at 13:00 GMT),
from the winter solstice (left) to the summer solstice (right) through the spring equinox (center) (**).
This software invites you to accurately determine the instants at which the spring equinox, the summer solstice, the autumn equinox and the winter solstice occur since the year 4713 before our era. To do so, it combines various astrometry algorithms (relative to the positioning of the stars in the sky) published within the scientific articles whose list appears below.
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 (payment by PayPal account or by credit card). This amount includes an unlimited access to the user interface and a free access to any future updates. Free tests of this software are available within the Culture Diff' Client Area.
In detail ...
« 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. However, in the eyes of the Earth observer, it is the Sun that seems to make this daily rotation and this annual revolution around our planet. Indeed: doesn't it appear every morning in the eastern sky before culminating in the local meridian and then disappearing below the western horizon in the evening? Like the other stars, our Sun seems to travel across the sky from east to west. However, unlike other stars, our Sun describes a different trajectory each day - more or less extended according to the season - at intermediate latitudes of course (*):
- around summer solstice, the Sun appears in the northeast corner of the sky, culminates not far from the zenith then disappears in the northwest corner of the sky. At this time of the year, our Sun describes the longest trajectory. The length of the day is affected, as is the air temperature.
- at winter solstice, on the other hand, our Sun appears in the southeast corner of the sky, culminates low above the horizon and then disappears in the southwest corner of the sky. At this time of the year, the Sun describes the shortest trajectory. This results in short days and cool to freezing temperatures outside.
- at spring and autumn equinoxes, the Sun rises precisely in the east of the sky and disappears due west. The duration of the day is roughly 12 hours. The outside air temperature is intermediate between winter and summer..
(*) At the Earth's poles, the Sun seems to describe a trajectory strictly parallel to the horizon, whose height above the horizon varies according to the seasons of the year. There follows the alternation of a 24-hour day and a 24-hour night, interspersed with a twilight period (see The succession of day and night on Earth).
Variation of the apparent path of the Sun over the seasons of the year at intermediate latitudes.
The interactive map below shows the terrestrial zones illuminated by the Sun at the current time (date of the day and Greenwich Mean Time). Change the hour of the day, the day and the month of the current year and observe the effects of these variations on the insolation of the different terrestrial zones: continents, oceans, equator, polar latitudes, etc. The transition from one season to the next will become evident.
Simulation of the variations of insolation of the various terrestrial zones throughout the hours of the day and the seasons of the year (**).
By default, the display is for the present time. Modify the date and GMT time by positioning your cursors on the areas of the following form:
This software invites you to accurately determine the instants at which the spring equinox, the summer solstice, the autumn equinox and the winter solstice occur since the year 4713 before our era. To do so, it combines various astrometry algorithms (relative to the positioning of the stars in the sky) published within the scientific articles whose list appears below.
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 (payment by PayPal account or by credit card). This amount includes an unlimited access to the user interface and a free access to any future updates. Free tests of this software are available within the Culture Diff' Client Area.
Bibliography
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.
(**) This simulation was performed using the OpenLayers geographic mapping tool. The script to determine the day and night zones was borrowed from Jean-Marc Viglino. It is protected by the CeCILL-B compatible BSD OpenSource license.
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