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Equinox
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A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun rises east and sets west. This occurs twice e
At Earth's poles the Sun stays near the horizon all day around equinox, marking the change between the half year polar night and polar day. The pictur
At Earth's poles the Sun stays near the horizon all day around equinox, marking the change between the half year polar night and polar day. The picture shows the South Pole right before March equinox, with the Sun appearing through refraction despite being still below the horizon.
The relation between the Earth, Sun, and stars at the March equinox. From Earth's perspective, the Sun appears to move along the ecliptic (red), which
The relation between the Earth, Sun, and stars at the March equinox. From Earth's perspective, the Sun appears to move along the ecliptic (red), which is tilted compared to the celestial equator (white).
Diagram of the Earth's seasons as seen from the north. Far right: December solstice.
Diagram of the Earth's seasons as seen from the north. Far right: December solstice.
Diagram of the Earth's seasons as seen from the south. Far left: June solstice.
Diagram of the Earth's seasons as seen from the south. Far left: June solstice.
Gregorian calendar
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The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it
Equinox
…occurred on about 21 March, and by the 1580s AD it had drifted backwards to 11 March. This drift induced Pope Gregory XIII to establish the modern Gregorian calendar. It was to conform with the edicts of the Council of Nicaea in 325 AD concerning the date of Easter, which means he wanted to move the vernal equinox…
Christopher Clavius (1538–1612), one of the main authors of the reform
Christopher Clavius (1538–1612), one of the main authors of the reform
Pope Gregory XIII, portrait by Lavinia Fontana, 16th century
Pope Gregory XIII, portrait by Lavinia Fontana, 16th century
First page of the papal bull Inter gravissimas
First page of the papal bull Inter gravissimas
Detail of the pope's tomb by Camillo Rusconi (completed 1723); Antonio Lilio is genuflecting before the pope, presenting his printed calendar.
Detail of the pope's tomb by Camillo Rusconi (completed 1723); Antonio Lilio is genuflecting before the pope, presenting his printed calendar.