Autumn starts this Friday, September 22, at 21:02 pm and 21:02 pm in mainland Portugal and in the Autonomous Region of Madeira, and at 20:02 pm in the Autonomous Region of the Azores.
This is the time of the Autumn Equinox. This moment marks the beginning of Autumn in the Northern Hemisphere. This season continues until the next Solstice, which takes place on December 21st, at 16:28 pm, in mainland Portugal, at which time winter will begin.
The Lisbon Observatory recalls the meaning of the word “equinox”: it is the instant when the Sun, in its apparent annual movement, passes on the celestial equator.
The word of Latin origin aequinoctium add the nominative aequus (equal) with the noun noctium, genitive plural of nox (night). Thus it means “equal night” (to the day), because, it was thought, on these dates, day and night have equal duration.
In reality, stresses the Lisbon Observatory, "it's not quite like that". «The equinoxes are defined as the instant when the central point of the sun passes the equator and, therefore, the solar center rises at the east cardinal point and sets exactly to the west, lying for 12 hours above the mathematical horizon in anywhere on Earth these days».
This fact, however, "does not result in a solar day length of 12 hours", as the direct light on the ground appears when the upper edge of the sun rises, as it disappears at sunset, and the sun has an apparent diameter of 32′ ( arc minutes).
Furthermore, “there is atmospheric refraction: when the upper edge is on the horizon, the center of the sun is ≈50′ below the horizon, more than its diameter”.
«With these physical conditions and due to the movement of the terrestrial translation, only on September 26, 2017 there will be ≈12,014 hours with direct sunlight on the ground. On that day, the solar disk is born at 7:28 am and sets at 19:27 pm in Lisbon, with only 51 seconds of deviation at 12:00 am”, concludes that Observatory.
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