What is the vernal equinox

Arab weather - The day of the spring equinox is known as the day on which spring begins, according to astronomical calculations, and the sun is perpendicular to the Earth's equator, so the length of the night is equal to the day in the northern and southern hemispheres.

The vernal equinox occurs in March of each year in the northern hemisphere, and in September in the southern hemisphere. But why does this happen on this particular day? What causes the vernal equinox?

Why does the vernal equinox happen?

The Earth revolves around the Sun at a time when its axis is tilted about 23.5 degrees from the horizon. This means that solar radiation is not distributed evenly over the parts of the Earth during the year , but rather varies according to the position of the Earth in its orbit.

Where the earth tilts towards the sun during the summer, and it tends away from the sun during the winter, but it happens twice a year (in March and September) that the earth’s tilt aligns with its orbit around the sun, so that there is no tilt of the earth relative to the sun, and then the sun’s rays perpendicular Directly with the equator , and both the northern and southern hemispheres get the same hours of day and night.

(An illustrative drawing of the sun’s rays perpendicular to the equator, which means that the Earth on this day is not tilted from the horizon (note: the size of the sun and the earth is illustrative and does not take into account the true ratio between them))

The line separating the night and daytime regions, called the "gray line," applies to the Earth's axis and passes through the north and south poles.

Sunrise and sunset on the vernal equinox

The sun rises on the day of the vernal equinox in all countries of the world from the true east, making an angle of 90 degrees with the horizon, and sets in the true west , at an angle of 270 degrees with the horizon. After that, it begins to appear on the horizon towards the north in the northern hemisphere during the spring and summer months, and the daylight hours begin to gradually increase at the expense of the night hours, until the day of the summer solstice, when the longest day and the shortest night are in the year.

Are night and day exactly the same on the day of the vernal equinox?

Although the number of hours of day is equal to the number of hours of night on the equinox, they are not completely equal, as the Earth gets a few minutes more of the day than the night , this is because sunrise occurs when the tip of the sun is above the horizon, and sunset is defined as The moment when the other edge of the sun disappears below the horizon.

Because the sun is a disk and not a point source of light, the Earth sees a few extra minutes of daylight during the equinox. The atmosphere also refracts sunlight and continues to travel to Earth shortly after night begins, even after the sun has set below the horizon.

Thus, the length of the day on the equinox ranges about 12 hours and 6 and a half minutes at the equator, 12 hours and 8 minutes at 30 degrees latitude, and 12 hours and 16 minutes at 60 degrees latitude.

Why does the timing of the vernal equinox differ from year to year?

The equinox does not necessarily occur on the exact same day each year, with the vernal equinox in the northern hemisphere occurring on March 20 or 21, and the autumnal equinox on September 22 or 23.

These changing dates are because Earth's year isn't exactly 365 days , an extra quarter of the day (6 hours) accumulates each year, causing the equinox date to shift. The planet's orientation toward the sun also changes constantly, adjusting the timing of the equinox.

Although the day of the equinox marks the astronomical beginning of spring or autumn, the beginnings of these seasons in meteorology are March 1 and September 1.

The day of the vernal equinox in ancient civilizations

People have been tracking the sun's movements for thousands of years, and many ancient civilizations associated the days of the spring and autumnal equinoxes with cultural and religious traditions, where these solar changes not only marked the beginning of the seasons, but also when crops were planted and harvested.

There are also many ancient monuments associated with the equinox, such as the Hindu temple of Angkor Wat in Cambodia , where the sun rises on the equinox directly above its central temple. Built between 1113 and 1150, the complex is the largest religious monument in the world.

There is also a Mayan temple in Mexico , which was built between the eighth and twelfth centuries, known as the Temple of El Castillo , dedicated to the serpent deity. the underworld.