Understanding equinoxes and solstices
Equinoxes and solstices are key astronomical events that mark the changing of the seasons and are directly linked to Earth’s orbit around the Sun.
What is an equinox?
An equinox happens twice a year—around 20 March (spring) and 22 September (autumn) in the Northern Hemisphere. On these days, the Sun crosses the equator, resulting in nearly equal lengths of day and night. The term “equinox” comes from Latin, meaning “equal night.” The spring equinox signals the start of spring, when days become longer than nights, while the autumn equinox marks the beginning of autumn, as nights start to outlast days.
The equilux
While the equinox is when day and night are nearly equal, the equilux is the precise day when they are exactly equal. This occurs a few days before the spring equinox and a few days after the autumn equinox. The difference is due to the Sun’s apparent size and atmospheric refraction, which slightly extends daylight on the equinox itself.
What is a solstice?
Solstices also occur twice a year: the summer solstice (around 21 June) and the winter solstice (around 21 December) in the Northern Hemisphere. The summer solstice is the longest day of the year, when the Sun reaches its highest point in the sky. Conversely, the winter solstice is the shortest day, with the Sun at its lowest midday position.
During the summer solstice, regions north of the Arctic Circle experience 24 hours of daylight, while areas south of the Antarctic Circle are in complete darkness. This pattern reverses at the winter solstice. The tilt of Earth’s axis causes these variations, leading to warmer temperatures and extended daylight in summer, and shorter, colder days in winter.
