Autumnal Equinox Friday September 23 – News The Weather Channel


The equinox characterizes the moment during which the center of the sun is exactly vertical at the equator. The dates of the spring and autumn equinoxes therefore correspond to the days of the year during which these zenith passages take place. The first of the year (vernal equinox) took place on Sunday, March 20, and the second (autumnal equinox) is 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”. Hence, this word means “equal night”: the length 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 21st, but sometimes the equinox happens on September 22nd or even September 23rd. This year, the equinox falls on September 23 at 03:03 40 seconds French time (05:03 Universal Time) according to the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE). 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 readjustment is made by either adding a February 29 during leap years (every 4 years) or an extra day or two, 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 23.5° and the Earth travels around the sun while tilted (the ecliptic). This explains the variation in the length of day and night throughout the seasons. If the Earth remained perpendicular to the sun, the days would be of the same length and the night/day alternation would be the same at all times. Also, the distance from the Earth to the sun is not always the same: it varies from 147 million to 152 million km, a fact that does not influence the heat received from the sun, but causes differences in speed and, therefore, in the length of seasons. At the time of the solstices, the declination of the Earth in relation 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 at the equator. Credit: La Chaîne Météo What explains the decrease in daylight hours? It is the inclination of the earth’s axis (the axis that passes through the poles) that explains the longer or shorter exposure to the sun according to the seasons. In summer, this inclination favors the exposure of one hemisphere facing the sun. In winter, on the other hand, 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 that rises until the summer solstice and then stagnates for a few days at its highest level in June, before returning to drop to a maximum 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 of day length diminishes until the winter solstice on December 21st, when we slowly begin to gain it back. Halfway between the longest day and the shortest day (at the equinoxes), the slope of the curve is steeper. Why does the equinox affect the tides? At the time of the equinoxes, the tidal coefficients are the strongest: we also speak of “high 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 greatest and the tides rise higher: these are the 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 one being the alignment with the Earth. High tides do not necessarily occur on the day of the equinox, because the sun, despite being perfectly perpendicular to the Earth’s axis, is not always aligned with the Moon. You have to wait for this Earth – Moon – Sun alignment for the high tides to occur.
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