Our planet revolves around the sun with an inclination of about 23.5 degrees with respect to its orbital plane. This means that some parts of the Earth receive more or less sunlight depending on the time of year and the position of the planet in its orbit. Furthermore, in all the countries of the world the sun rises in the east and sets in the west.
However, the sun can also move north for half the year and south for the other half, depending on where it is. And, around the month of July, the northern hemisphere has longer days, while in the southern hemisphere the days are shorter. The opposite occurs in December, with more hours of daylight in the southern hemisphere and fewer hours in the northern hemisphere.
However, twice a year (in March and September) the Earth’s tilt aligns with its orbit around the sun. During the equinox, the sun is directly at the equator and both hemispheres receive the same hours of sunlight. In addition, in those moments the imaginary line that divides night and day (known as the gray line or twilight zone) divides the Earth in two and passes through the north and south poles.
However, day and night are not completely identical during the equinox, as the Earth receives a few more minutes of light than darkness. This is because sunrise occurs when the tip of the sun borders the horizon, while sunset occurs when the sun disappears below the horizon. Thus, the Earth presents a few more minutes of additional light instead of darkness during the equinox. Similarly, the atmosphere refracts sunlight and helps it travel around Earth (that is, the dark hemisphere) for a short period of time, even after sunset.
Where does the word come from?
According to the UK Met Office, the word equinox comes from the Latin ‘aequinoctium’, and translates to ‘equal night’ or ‘equal light’. On the other hand, despite the fact that the word equinox indicates that the hours of light and darkness are the same, in reality this is not fulfilled at that moment, but on the day of the equilux. In fact, equilux is the term used to indicate that day and night are exactly the same.
When do the equinoxes occur?
The equinoxes don’t necessarily occur on the same day every year. They usually happen around March 20 and September 23. For example, in 2021 the spring equinox took place on March 20, while the autumn equinox will take place on September 22.
The days vary because Earth years are not exactly 365 days long, but 365 days and 6 hours. These 6 hours are accumulated every year, which causes the date of the equinox to be different depending on the year.
