Weather of Arabia - the vernal equinox is defined as the day on which the length of night is equal or equal to the day in both the northern and southern hemispheres, and the spring equinox occurs in the month of March of each year in the northern hemisphere, and in the month of September in the hemisphere Southern. But why does this happen on the equinox day specifically? What causes the vernal equinox to occur?
The earth revolves around the sun at a time that its axis is inclined from the horizon by about 23.5 degrees. This means that the solar radiation is not distributed equally over the parts of the earth during the year , but rather varies according to the location of the earth in its orbit.
For all countries of the world, the sun rises in the east and sets in the west. However, on the surface it appears that the sun moves on the horizon towards the north during half the year, and towards the south during the other half of the year.
The northern hemisphere sees longer periods of daylight around July , while the southern hemisphere sees shorter periods of daylight. Around December , the opposite happens. Daylight hours increase in the Southern Hemisphere and decrease in the Northern Hemisphere.
But it happens twice a year (in March and September) that the Earth's tilt aligns with its orbit around the sun, so that the sun lies directly above the equator , and then both hemispheres get the same hours of day and night, and the line that divides the day and night becomes the line that divides the day and night, which is called The "gray line" or "twilight zone", passes through the north and south poles.
Although the number of daylight hours is equal to the number of hours of the night on the day of the equinox, they are not completely equal, as the earth gets a few minutes of the day more than the night , this is because the 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 an extra few minutes of the day 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.
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.
The equinox does not necessarily occur on the exact same day each year, as the vernal equinox occurs in the northern hemisphere between 19/20/21 March, and the autumn between 22/23 September, in this year the vernal equinox will occur on March 20, 2021, and the autumn on September 22, 2021 .
These changing dates are because the Earth year is not exactly 365 days, there is an extra quarter of the day (6 hours) that builds up each year, causing the date of the equinox to shift. The direction of the planet towards the sun also changes constantly, which changes the timing of the equinox.
Although the equinox marks the astronomical beginning of spring or autumn, the beginnings of these seasons in meteorology are March 1 and September 1.
People have been tracking the movements of the sun for thousands of years, and many ancient civilizations have linked the days of the equinox with cultural and religious traditions, as these solar changes not only marked the beginning of the seasons, but also determined when crops were planted and harvested.
There are also many ancient monuments associated with the equinox, such as the Hindu temple Angkor Wat in Cambodia, where the sun rises on the equinox day directly over 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 god of the serpent, where the light appears on the equinox in an optical illusion as if a snake descends down the side of the temple, and travels To the underworld.