Summer has come to an end in the southern hemisphere. This Wednesday afternoon (18.58 hours in Argentina), autumn officially begins, while the other half of the planet welcomes the spring, which starts at 22.58 and lasts until June 21.
The equinox -from Latin aequinoctium, which means night, is the same name for this phenomenon in which the duration of day and night coincide in practically any point of the Earth. During the autumn, the day and the night last the same, but, with the passage of the days, the hours of light will diminish of gradual form. In fact, each day will wake up a minute later and the Sun will be on the horizon for a minute and a few seconds before, so we will enjoy almost three minutes less of the Sun each day.
The equinox is the time of year when the sun is in the plane of the terrestrial equator. This instant in which the axis of the earth is perpendicular to the sun's rays only occurs twice a year: in March and in September.
The dates of the equinoxes vary from year to year due to the way in which the duration of the Earth's orbit around the Sun –known as the tropic year– fits in the sequence of leap years of the calendar. In this way, the welcome to fall may occur on three different dates of the calendar: the days 19, 20 or 21 March. The earliest start will occur in the year 2096, and the later one occurred in 2003, according to data from the National Geographic Information Center of Spain.