Dolmens and the Equinox

La Vía Láctea y el Dolmen Datal en Valencia de Alcántara (Cáceres, Extremadura). Crédito: J. C. Casado / STARRYEARTH.
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Next Sunday, September 23rd, at 01:54 UT the Earth will be at a specific point in its orbit round the Sun: the September equinox. The September and March equinoxes are the only days in the year when the Sun rises exactly in the east and sets exactly in the west, across the whole planet. In practical terms the days at both the vernal and the autumnal equinoxes are divided equally into daytime and night-time, with each lasting approximately 12 hours. In fact the literal meaning of the word equinox, is “equal night” and comes from the latin work aequinoctium.

Megalithic structures

In the region of Valencia de Alcántara, in Caceres, and in neighbouring regions of Portugal there are collectively more than 50 dolmens, which is one of the largest sets of megalithic monuments in the Iberian Peninsula. They were built with impressive architectural coherence.

The dolmens are megalithic monuments built mainly between 4000 and 2500 years prior to the Christian era, and their purpose was for funeral rites. They were places where the members of a community were buried together. It is also possible that they were built on sacred sites, associated with the ancestral religión, or with some guiding divinity.

As a result of corraborative work between researchers at Churchill College, Cambridge and the Instituto de Astrofísica de Canarias (IAC), we know that these dolmens for a coherent set, in terms of their orientation. All of them point towards the Eastern sector of the horizon in the various geographical areas analyzed, with a strong concentration around precisely the eastern direction. There is also a notable concentration of orientations towards  the points at which the sun rose on dates close to the autumnal equinox, close to which date the rains started, following the end of summer.

Live from Valencia de Alcántara

The channel will offer a live transmission of the sunrise, working within the Light Pollution Initiatives (LPI) of the STARS4ALL project, and with the collaboration of the Office of the Director General of Tourism of the Junta of Extremadura.

Participating in the broadcast will be Dr. Andrea Rodríguez, who belongs to the Archeoastronomy group of the Instituto de Astrofísica de Canarias. “The equinoxes have been considered moments of regeneration and transition by the cultures around the Mediterranean” says Rodríguez, and adds that “The orientations of these dolmens can give us information about the beliefs, the celestial mythologies, and the calendars of the people who built them.

The date is next Sunday, 23rd September at 06:00 UT (07:00 local time in the Canaries, 08:00 Central European Standard Time).

The vernal and autumnal equinoxes are the geometrical points in the Earth’s orbit around the Sun when a line from the centre of the Sun lies in the equatorial plane of the Earth. The people who live at the equator will see the Sun exactly overhead at local mid-day” comments Miquel Serra, an astronomer at the IAC, who is in charge of the broadcast.

STARS4ALL is  project funded by the H2020 Programme of the European Union under agreement number 688135. STAR4ALL includes 8 institutions (UPM, CEFRIEL, SOTON, ECN, ESCP Europe, IAC, IGB, UCM) in 6 countries.

Extremadura Buenas Noches (Extremadura, Good Night) is a project within the Night Strategy of the Junta of Extremadura (Extremadura 2030).

Three Spanish Supercomputer centres: The Extremadura Centre for Advanced Technologies (CETA-CIEMAT), the Council of University Servers of Catalonia (CSUC) and the Instituto de Astrofísica de Canarias (IAC) are collaborating in distributing the broadcast of the web portal (

Contact at the IAC: Miquel Serra-Ricart, mserra [at] (mserra[at]iac[dot]es)

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