13 interesting facts & trivia you should know about Spring Equinox.
1. An equinox is the moment in which the plane of Earth’s equator passes through the center of the Sun, which occurs twice each year, around 20 March and 23 September.
2. On an equinox, day and night are of approximately equal duration all over the planet
3. They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction. To avoid this ambiguity, the word equilux is sometimes used to mean a day in which the durations of light and darkness are equal.
4. Day is usually defined as the period when sunlight reaches the ground in the absence of local obstacles. On the day of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, so night and day are about the same length. In reality, the day is longer than the night at an equinox. There are two reasons for this:
First, from the Earth, the Sun appears as a disc rather than a point of light, so when the centre of the Sun is below the horizon, its upper edge is visible. Sunrise, which begins daytime, occurs when the top of the Sun’s disk rises above the eastern horizon. At that instant, the disk’s centre is still below the horizon.
Second, Earth’s atmosphere refracts sunlight. As a result, an observer sees daylight before the top of the Sun’s disk rises above the horizon. Even when the upper limb of the Sun is 0.4 degrees below the horizon, its rays curve over the horizon to the ground.
5. When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox. Because the Julian year (365.25 days) is slightly longer than the tropical year, the calendar “drifted” with respect to the two equinoxes — such that in 300 AD the spring equinox occurred on about 21 March. By 1500 AD, it had drifted backwards to 11 March.
6. Spring equinox and fall (or autumn) equinox: colloquial names based on the seasons. However, these can be ambiguous since the northern hemisphere’s spring is the southern hemisphere’s autumn, and vice versa. The Latinate names vernal equinox (spring) and autumnal equinox (fall) are often used to the same effect.
7. March equinox and September equinox: names referring to the months of the year they occur, with no ambiguity as to which hemisphere is the context. They are still not universal, however, as not all cultures use a solar-based calendar where the equinoxes occur every year in the same month (as they do not in the Islamic calendar and Hebrew calendar, for example).
8. The Vernal Equinox is another name given to the Spring Equinox and labels the March Equinox is the Northern Hemisphere.
9. Northward equinox and southward equinox: names referring to the apparent direction of motion of the Sun. The northward equinox occurs in March when the sun crosses the equator from south to north, and the southward equinox occurs in September when the sun crosses the equator from north to south. These terms can be used unambiguously for other planets.
10. The 2017 Vernal Equinox, is the same thing as the Spring Equinox and falls on March 20.
11. Equinox is a phenomenon that can occur on any planet with a significant tilt to its rotational axis. Most dramatic of these is Saturn, where the equinox places its ring system edge-on facing the Sun.
12. As a result, they are visible only as a thin line when seen from Earth. When seen from above – a view seen by humans during an equinox for the first time from the Cassini space probe in 2009 – they receive very little sunshine, indeed more planetshine than light from the Sun.
13. This lack of sunshine occurs once every 14.7 years on average. It can last a few weeks before and after the exact equinox. The most recent exact equinox for Saturn was on 11 August 2009. Its next equinox will take place on 6 May 2025.