The Equinoxes: An Astronomical Event

A rare and periodic celestial balance

The equinoxes are the two points of the year when the Earth's equatorial plane intersects the plane of the ecliptic, the apparent path of the Sun in the sky. This occurs when the center of the solar disk is exactly located vertically above the Earth's equator. At this precise moment, the Sun culminates at 90° above the equator, resulting in an equal duration of day and night across the entire planet (within a few minutes due to atmospheric refraction).

The two annual equinoxes

The equinoxes (from the Latin aequinoctium, "equality between day and night") are two key moments of the year. These dates vary slightly each year due to the precession of the equinoxes, a phenomenon caused by the gyroscopic rotation of the Earth.

• The March equinox (around March 20) marks the beginning of spring in the northern hemisphere and autumn in the southern hemisphere.
• The September equinox (around September 23) marks the opposite: autumn in the northern hemisphere and spring in the south.

N.B.: The precession of the equinoxes is a slow change in the orientation of the Earth's rotational axis, similar to the movement of a spinning top, which completes a full cycle in 25,800 years. The precession of the equinoxes results mainly from the combined gravitational attractions of the Sun and the Moon on the Earth's equatorial bulge.

Alternation of the seasons: a consequence of the Earth's tilt

Axial tilt and energy distribution

Contrary to popular belief, the seasons are not caused by the variation in the distance between the Earth and the Sun, but by the tilt of the Earth's rotational axis, fixed at approximately 23.44° relative to the perpendicular to the plane of the ecliptic. This tilt means that, throughout the year, the northern and southern hemispheres receive different amounts of solar energy.

Seasonal opposition between hemispheres

When the North Pole is tilted towards the Sun, the northern hemisphere experiences summer, with longer days and more direct sunlight, while the southern hemisphere experiences winter. Six months later, the situation reverses. Between these extremes, the equinoxes mark the moments of equal day/night duration and signal the beginnings of spring and autumn.

Geophysical and biological consequences

The alternation of the seasons is therefore a direct geometric consequence of the Earth's revolutionary movement combined with the obliquity of its axis. It has profound effects on biological, climatic, and societal systems, conditioning the cycles of vegetation, animal migrations, and the rhythm of agricultural civilizations.

Table of recent and future equinoxes

Source: Time and Date - Equinoxes, IMCCE - Observatoire de Paris.

The Solstices: Extreme Points of the Solar Cycle

The solstices represent the two annual extremes of the Sun's apparent path in the sky. On these dates:

• The Sun reaches its northernmost point (June solstice) or southernmost point (December solstice) relative to the celestial equator.
• They mark respectively: The longest day of the year in the northern hemisphere (June) and the longest day in the southern hemisphere (December).

Unlike the equinoxes where day and night are balanced, the solstices represent the peaks of light/darkness inequality of the year.

Sources: IMCCE - Observatoire de Paris, Time and Date - Solstices.