Equinox and solstice

Meaning

The Earth’s axis is tilted about 23½° to the plane of its orbit, so over the year the Sun climbs north of the celestial equator and sinks south of it. Four moments mark the turns. At the equinoxes the Sun crosses the equator and day and night are close to equal everywhere. At the solstices it reaches its farthest point north or south, and the days are longest in one hemisphere and shortest in the other.

The tropical zodiac is built on these four moments. The Sun’s entry into Aries is the March equinox, into Cancer the June solstice, into Libra the September equinox and into Capricorn the December solstice. These four signs are the cardinal signs, from the Latin cardo, a hinge: each one opens a season.

Astrology reads each of them as a threshold. The March equinox, the start of the zodiac, is read as a new beginning; the June solstice as fullness and care; the September equinox as balance and exchange; the December solstice as structure and endurance. South of the equator the seasons are reversed, but the signs and their readings stay the same.

In practice

• The dates fall around 20 March, 21 June, 22 or 23 September and 21 or 22 December, shifting a little from year to year because the year is not a whole number of days.
• The seasons are not equal: the Earth moves faster when it is closer to the Sun, so the northern spring and summer together last about a week longer than the autumn and winter.
• Ingress charts: mundane astrology, the astrology of nations and cities, casts a chart for the exact moment of each equinox and solstice and reads it for the quarter that follows, the Aries chart above all for the whole year.
• In a birth chart, a planet at or near 0° of a cardinal sign is often read as tuned to its times, its themes meeting public life easily.

Across traditions

• The Persian New Year, Nowruz, begins at the March equinox.
• The Chinese calendar counts the equinoxes and solstices among its twenty-four solar terms. The winter solstice, dongzhi, is a festival of its own, while the Chinese astrological year begins at lichun, the start of spring, when the Sun reaches 315°, halfway between the winter solstice and the spring equinox.
• Indian astrology, with its zodiac fixed to the stars, keeps the Sun’s entry into its sidereal Capricorn, Makar Sankranti, in mid-January, more than three weeks after the solstice.

Origin and history

Greek astronomers measured the solstices and equinoxes with care. Hipparchus, in the second century BC, found that the equinox slowly moves back against the stars, about 1° every 72 years: the precession of the equinoxes, which Ptolemy reports in the Almagest. Ptolemy’s Tetrabiblos then ties the twelve signs to the equinoxes and solstices rather than to the stars, which is why the signs have since drifted from the constellations of the same names, by about 24° today. He reads the New or Full Moon before each of the four ingresses for the weather and events of the coming season. Astrologers writing in Arabic, Abu Ma’shar among them, made the chart of the Sun’s entry into Aries the chart of the world’s year.

How Ulune calculates it

• The Calendar finds the four moments as ingresses of the Sun into 0° Aries, Cancer, Libra and Capricorn: the Sun is sampled every twelve hours and each crossing is refined on the Swiss Ephemeris to the millisecond.
• The Sun’s position is its apparent longitude seen from the centre of the Earth, measured from the true equinox of the date. That matches the astronomical definition Jean Meeus uses, so the times agree with the published equinoxes and solstices.
• A day that holds an equinox or a solstice names it before its Moon phases and other sign changes; only eclipses and stations come first. Its reading says what begins in each hemisphere.
• In the Table, a body within 1° of 0° Aries is noted “0° Aries”. The other three cardinal points get no note of their own.

Correspondences