Four markers, two different jobs
Solstices mark the year’s extremes in the Sun’s north–south position; equinoxes mark its crossings of the equator. There are solstices in June and December, and equinoxes in March and September.
NOAA and NASA’s explanation connects these events with Earth’s tilt. The exact dates can vary, so “the June solstice” is a safer label than assuming the same numbered date every year.
Opposite hemispheres, opposite seasons
Earth’s axis is tilted. As the planet travels around the Sun, the northern and southern halves receive sunlight differently. At the June solstice, the Northern Hemisphere is tilted towards the Sun; at the December solstice, the Southern Hemisphere is.
For a place away from the equator and polar regions, that makes the summer solstice the day with the most daylight and the winter solstice the day with the least. June is summer’s solstice in London but winter’s in Sydney. December reverses those roles.
Near the poles, continuous daylight or darkness complicates the everyday idea of one longest or shortest day. The underlying tilt is still the same. The US Naval Observatory explains why the hemispheres have opposite seasons.
Nearly equal does not mean exactly twelve hours
At an equinox, the Sun crosses the celestial equator: the extension of Earth’s equator into the sky. Neither hemisphere is tilted towards the Sun in the way it is at its summer solstice.
Day and night are close in length, but the US Naval Observatory warns that they are not exactly equal. Sunrise and sunset use the edges of the Sun’s visible disc, and the atmosphere bends sunlight. Both affect the daylight we observe.
These four markers describe a year’s changing sunlight. They are not caused by Earth simply getting nearer to the Sun. Revisit what causes the seasons to connect the geometry, or day and night to separate the daily spin from the yearly orbit.