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 this September 23. In this case, the Earth is equally exposed to sunlight on one half while the other is at night. Etymologically, the word equinox is formed from two Latin terms: “aequus” which means “equal” and “ nox, noctis” which means “night”. This word therefore 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 happens 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 Calculation of Ephemerides (IMCCE). This shift results from the fact that the Earth revolves around the sun in 365 days, 5 hours and 46 minutes, and not in 365 days altogether. The difference is cumulative and the readjustment is carried out 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 the ‘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 performs its course around the sun while being tilted (the ecliptic). This explains the variation of the duration of day and night according to the seasons. If the Earth remained perpendicular to the sun, the days would remain of the same length and the night/day alternation would be equal 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 over the length of the seasons. At the time of the Solstices, the declination of the Earth in relation to the Sun is maximum (23.5°) and the star of the day 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 of the day explained?
It is the inclination of the axis of the earth (the axis passing through the poles) which explains the more or less long exposure to the sun according to the seasons. In summer, this inclination favors the exposure of a 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. Seen from the surface of the earth, the course of the sun follows a curve throughout the year which rises until the summer solstice then stagnates for a few days at its highest level in June, before descending again to be at most low 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 sunshine per day. This loss in day length then decreases 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 in alignment with the Earth, the attraction is maximum and the tides rise higher: these are high tides. Two scenarios exist: either the Moon is on the same side as the Sun (New Moon), or on the opposite side (Full Moon), the main thing being 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 in alignment with the Moon. It is necessary to wait for this alignment Earth – Moon – Sun so that the high tides occur.