The Sun
Equinoxes and solstices
These are moments, not days. Each one is the instant the Sun reaches an exact ecliptic longitude — 0°, 90°, 180° or 270° — and that instant is what is listed here.
September equinox
in 18 days
December solstice
in 108 days
March equinox
in 197 days
June solstice
in 290 days
What an equinox actually is
The Earth's axis is tilted about 23.4°. As the planet goes round the Sun that tilt stays pointed the same way in space, so through the year the Sun appears to climb higher or sink lower at noon. An equinox is the moment it crosses the celestial equator — the projection of the Earth's own equator onto the sky.
The word means "equal night", and it is very nearly true: day and night are close to twelve hours everywhere. Not exactly, though, for two reasons. Sunrise is counted from the first sliver of the disc rather than its centre, and the atmosphere bends light around the horizon so the Sun is visible while still geometrically below it. Both push the balance towards daylight by several minutes.
And a solstice
A solstice is the opposite extreme: the moment the Sun reaches its furthest point north or south, stops, and turns back. The name says so — solplus sistere, the sun standing still. Around those dates the noon Sun sits at nearly the same height for several days, which is why ancient monuments could be aligned to them accurately without clocks.
The names on this page are given by month rather than season, because the seasons are reversed in the southern hemisphere. The June solstice begins summer in Oslo and winter in Melbourne; the moment is identical, the meaning is not.
Why the dates shift
The year is not a whole number of days — it is about 365.2422. Each equinox therefore lands roughly six hours later than the year before, drifting through the calendar until a leap day pulls it back. That is the entire reason leap years exist, and why the March equinox can fall on the 19th, 20th or 21st.
Where these times come from
Computed with the Swiss Ephemeris by solving for the moment the Sun's apparent ecliptic longitude reaches each exact value. Checked against published official times, they agree to within a minute.