Autumn equinox this Friday, September 23

The equinox characterizes the moment during which the center of the sun is exactly vertical to the equator. The dates of the spring and autumn equinoxes therefore correspond to the days of the year during which these passages at the zenith take place. The first of the year (spring equinox) took place on Sunday, March 20, and the second (autumn equinox) takes place on September 23. In this case, the Earth is equally exposed to sunlight on one half while the other half is at night. Etymologically, the word equinox is formed from two Latin terms: “aequus” which means “equal” and ” no x, noctis” which means “night”. Therefore, this word means “equal night”: the duration of day and night is everywhere equal to 12 hours.

Why do the dates of the equinoxes vary?

Most of the time, the date of the equinox falls on September 21, but sometimes the equinox occurs on September 22 or even September 23. This year, the equinox falls on September 23 at 03:03 and 40 seconds French time (05:03 in universal time) according to the Institute of Celestial Mechanics and Ephemeris Calculus (BMICE). This change results from the fact that the Earth revolves around the sun in 365 days, 5 hours and 46 minutes, and not in 365 days in total. The difference is cumulative and the adjustment is made both by adding a February 29 during leap years (every 4 years) and one or two additional days, in certain years, during the solstice and equinox.

How are the seasons explained?

Our planet is not perpendicular to the sun: its axis of rotation is tilted by 23.5° and the Earth travels around the sun while it is tilted (the ecliptic). This explains the variation of the length of day and night according to the seasons. If the Earth stayed perpendicular to the sun, the days would have the same length and the night/day alternation would be the same at all times. Likewise, the distance from the Earth to the sun is not always the same either: it varies from 147 million to 152 million km, which does not influence the heat received from the sun, but causes differences in speed and therefore in its length. . seasons. At the time of the Solstices, the declination of the Earth with respect to the Sun is maximum (23.5°) and the day star is at the zenith of the tropics (of Cancer or Capricorn), while at the equinox, this apparent declination is zero, the Sun being vertical to the equator.



Credit: The Weather Channel

How is the reduction in working hours explained?

It is the tilt of the earth’s axis (the axis that passes through the poles) that explains the more or less prolonged exposure to the sun depending on the seasons. In summer, this inclination favors the exposure of one hemisphere facing the sun. In winter, on the contrary, this inclination reduces this exposure. In spring and autumn, the respective exposures of the southern and northern hemispheres are balanced. Viewed from the earth’s surface, the sun’s course follows a curve through the year that rises until the summer solstice and then stalls for a few days at its highest level in June, before sinking again to be as maximum in December at the time of the winter solstice. The longest day corresponds to the longest course of the sun, which then rises in the northeast and sets in the northwest. The shortest day corresponds to the shortest course of the sun.

At the end of September, from the equinox, we lose up to 4 minutes of theoretical sun per day. This loss in day length then subsides until the winter solstice on December 21, when we slowly begin to regain it again. Halfway between the longest day and the shortest day (at the equinoxes), the slope of the curve is steepest.

Why does the equinox influence the tides?

At the time of the equinoxes, the tidal coefficients are the strongest: we also speak of “high equinox tides”. High tides are linked to the attraction of the Moon and the Sun. When these two stars are aligned with the Earth, the attraction is maximum and the tides rise more: they are high tides. There are two scenarios: either the Moon is on the same side as the Sun (New Moon), or on the opposite side (Full Moon), the main thing being the alignment with the Earth. High tides do not necessarily occur on the day of the equinox, because the sun, although perfectly perpendicular to the axis of the Earth, is not always aligned with the Moon. We have to wait for this Earth – Moon – Sun alignment for high tides to occur.

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