An equinox is one of the two points of intersection of the line of equinoxes with the celestial sphere. One year has two equinoxes, the first between 19 and 21 March, the second between 22 and 23 September. Equinoxes are called the days of the year during which these zenith passages occur. The dates of the equinoxes are linked to early spring and autumn.
The axis of rotation of the Earth is inclined at about 23.437 ° relative to its orbit. For about half of the year, the northern hemisphere is oriented towards the Sun, while the orientation is in favor of the southern hemisphere during the other half. During an equinox, the two hemispheres are oriented also with respect to the Sun and this one is located directly at the zenith of the equator. The North and South poles are also located at this moment on the terminator and the day and the night divide exactly the two hemispheres. From the geocentric point of view, an equinox occurs when the Sun reaches one of the two intersections between the ecliptic and the celestial equator, its declination is then zero. Since the Sun is not a mere bright spot seen from Earth, its crossing of the equator takes about 33 hours.
It is often said that "at the equinox, the Sun rises in the East and sets in the West", but this is only approximately exact: this rule neglects the movements of the Sun during this day. The Sun can only rise exactly to the East if it rises at the precise moment of the equinox, which is the case over a whole meridian, but by the time the Sun sets twelve hours later its declination will have varied a little, and it will not go down exactly to the west. The difference is not very noticeable for the current observation.
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