Weather of Arabia - The world will witness an important astronomical event on Wednesday, September 22, 2021 at 11:20 pm Mecca time, as the earth's axis does not tilt relative to the sun for the second day this year (the first time was in the month of March).
The sun is perpendicular to the Earth's equator. On this day, the northern and southern hemispheres have the same number of hours of day as the number of hours of night.
This day marks the official start and first day of autumn in the Northern Hemisphere - the Autumn Equinox.
And the first day of spring in the Southern Hemisphere - the "Spring Equinox".
Because of the Earth’s rotation around the Sun during the year, and the tilt of the Earth’s axis during its rotation around the Sun at an angle of approximately 23.5 degrees, the apparent path of the Sun changes towards the north and south throughout the year, i.e. between the Tropic of Cancer and the Tropic of Capricorn, passing through the Equator. On the day of the autumnal equinox, the sun reaches its southward journey just above the equator and its rays perpendicular to it.
Also, the apparent sunrise and sunset point in the sky changes north and south throughout the year, but on the equinox, the sun rises from the true sunrise point (making an angle of 90 degrees with the horizon), and sets from the true sunset point (making an angle of 270 degrees with the horizon), Therefore, the equinox is an opportunity to determine the exact directions with the help of the sun.
The timing of the autumnal equinox in the northern hemisphere each year depends on the time when the sun arrives above the equator and collides with it. Therefore, the date of the autumnal equinox in the northern hemisphere ranges from September 22 to 23. This change occurs due to several influences, namely:
The equinox is the day on which the northern and southern hemispheres receive the same amount of sunlight, so the length of the day and night is equal in all regions on Earth.
Since the sun is in the middle of the sky today, you will notice that your shadow at noon is the shortest of the year.