The Ephemeral and the Transient. An Equation of Time
Artist: Merche Pereira
When the Sun reaches its highest point, it is solar noon. The center of the Sun crosses the local meridian, which connects the celestial north pole to the south pole, passing through the observer’s zenith. We can see the projection onto the slab as a straight line that precisely marks the North-South direction: the meridian line. Each day, the sunlight passing through the circle that crowns the totem forms a small illuminated ellipse on the slab. When its center crosses the meridian line, it is solar noon.
But the Sun does not move across the sky at a constant rate throughout the year, since its apparent movement is a consequence of the Earth’s movement in its elliptical orbit around the Sun. According to Kepler’s second law, the Earth moves faster near perihelion, the point in its orbit closest to the Sun (January 3), than at aphelion, when it is farthest from the Sun (July 5). Furthermore, Earth’s orbit is tilted relative to the equator: the Earth’s Northern Hemisphere tilts towards the Sun between the vernal equinox (March 21) and the autumnal equinox (September 22), and away from it for the other half of the year.
To measure time and ensure its uniform flow, astronomers defined the mean sun, a hypothetical sun that travels along the celestial equator at a constant speed and revolves around the Earth every 24 hours. The time we use in practice is based on this mean solar time, which depends on geographical location. If we are located on the Greenwich Meridian, it is the Universal Time Coordinated (UTC; formerly Greenwich Mean Time or GMT). Civil time in Valencia is UTC+1, although currently an hour is added during summer time.
The difference between true solar time and mean solar time is the equation of time. If we record the position of the Sun in the sky at the same mean solar time throughout the year, we obtain an eight-shaped figure, called analemma, which is simply a diagram of the equation of time that graphically shows in the sky the difference between true solar time and mean solar time for each day of the year (see the image on the right).
At 1:00 PM (2:00 PM in summer time if there is a seasonal time change), the sunlight passing through the center of the circle at the top of the totem marks a point on the slab that traces an elongated eight-shaped figure throughout the year: the projection of the celestial analemma. The days of the year are marked on the analemma drawn on the slab; therefore, this structure is, in fact, a solar calendar. Since this installation is not located exactly on the Greenwich Meridian, there is also a slight deviation of the entire analemma from the meridian line, which, in this position, corresponds to 102 seconds.
The constellations over which the Sun is located at different times of the year have been added to the slab. As can be seen, they do not coincide with the position of the zodiac signs, which are also included, due to the precession of the equinoxes. Furthermore, since each zodiacal constellation -marked with green lines- has a different size, the Sun is over each one for a variable period of days, ranging from 7 days in Scorpio to 45 in Virgo, while the Sun remains in each sign -marked with red lines- for one month.
Constellations corresponding to civil noon on December 11, with the Sun located above the constellation Ophiuchus. The analemma represents the position of the Sun at civil noon throughout the year. The sky is calculated for the coordinates of the totem pole, so the local meridian, represented in green, is not aligned with the analemma, due to the difference in longitude between the local coordinates and the Greenwich meridian for which civil time is defined. The ecliptic is represented in light orange..
Program CULTURA RESIDENT/RESIDIR EN LA INVESTIGACIÓN by Vicerectorat de Cultura i Societat de la Universitat de València
Line of research: EXTRAGALACTIC ASTROPHYSICS AND COSMOLOGY
team: Vicent J. Martínez (IP), Amelia Ortiz Gil, Fernando J. Ballesteros, Alberto Fernández Soto, Juan Fabregat, Pablo Arnalte Mur y Enric Marco.
