Every day, the sun rises in the east and sets in the west. This is the basic apparent motion we all know. But at what times exactly does the Sun actually rise in the East itself and set in the West?
Today, we know that the Earth rotates around the North-South axis, leading to the day-night phenomenon because each part of the planet’s surface receives light from the Sun in turn.
However, the time of the Sun’s illumination each day is not exactly 12 hours (half a day) but fluctuates with the seasons. The reason for this is that the Earth orbits around the Sun every roughly one year, and its axis is not perpendicular to the orbital plane (90 degrees) but tilted about 66.5 degrees ( i.e. a deviation of approximately 23.5 degrees from the vertical axis).
Not every day of the year you see the Sun rising in the very East and setting in the West.
Summer in the Northern Hemisphere is when the Northern Hemisphere is facing the Sun more, then the Southern Hemisphere is winter and vice versa.
That motion results in the apparent position (i.e. the position as viewed by an observer, here on Earth) of the Sun changing with each day of the year (say, the same 9 o’clock). bright but in the same position, you see the Sun on two different days with different positions).
It also means that in fact it is not every day of the year that you see the Sun rise in the very East and set in the very West. Most times of the year, the rising and setting positions of the Sun are more or less deviated from these two points.
In astronomy, the zenith is the term for the vertical point above the observer’s head. That means that you are standing anywhere on Earth, when you look straight up, you are looking up at the zenith.
In addition to the fact that the Sun rises in the East and sets in the West, there is another thing that everyone knows is that at noon, the Sun is at its highest and its light shines vertically down.
However, even this is not entirely correct. On most days of the year, if you go outside at midday when it’s sunny, you’ll still see your shadow fall in some direction even though it’s very short. That means that the Sun does not shine vertically from above, or in other words, it is not at the zenith.
Gravity (the Earth’s gravity) tends to pull everything towards its center. So the direction from your feet to the top of your head when standing upright is (basically) the direction that connects the center of the Earth to your zenith.
If the Earth’s axis were not tilted, the Sun would always be at zenith only for people living at the equator, because observers at other latitudes have a different zenith view and the Sun can never be at the zenith. their. However, because the Earth’s axis is tilted 23.5 degrees, it is precisely the Sun that can reach its zenith at different times in the entire region stretching from 23.5 degrees North latitude to 23.5 degrees latitude. Male. These two latitudes 23.5 North and South are called the Tropic of Cancer and Tropic of the Earth.
Thanks to the Earth’s tilt axis, the entire region between the Tropic of Cancer and Tropic of Cancer has at least one time of year when the Sun crosses its zenith.
At the Tropic of Capricorn, the Sun crosses the zenith on the summer solstice (June 20, 21, or 22), and at the South Tropic of Capricorn, the Sun passes the zenith on the winter solstice (December 20, 21, or 22). ). At the Equator, the Sun crosses the zenith on the spring equinox (March 20 or 21) and the autumn equinox (September 22 or 23).
Unlike the concept of zenith, the concept of two directions East and West does not depend on the position of the center of the Earth but depends on the axis of rotation of the Earth. The parallels lie on planes that are perpendicular to the Earth’s axis of rotation. Therefore, only at the equator does the observer’s zenith view lie on the plane of latitude.
For that reason, the day the Sun rises in the very East and sets in the West does not depend on whether it passes the zenith at noon that day, but only on whether its rays are in the same direction. with the Earth’s equatorial plane or not . Thus, you only need to answer the question: When is the Sun’s light in the same direction as the Earth’s orbital plane? It is the spring equinox and the autumn equinox.
At the spring and autumn equinoxes, the Sun is positioned above the celestial equator (the extended circle of the Earth’s equator). At these two times, the Sun’s rays shine perpendicular to the Earth’s axis. Notice here that we get this because the Sun is so far away and many times larger in size than the Earth, that the rays from it are considered perfectly parallel when reaching the Earth.
Finally, to be even more precise, you should note that the spring and autumn equinoxes are only a point (the time when the Sun crosses the intersection of the ecliptic and the celestial equator) and not a day.
For example, when people say that the spring equinox falls on March 20, it means that the time falls on March 20. If the spring point is located near the middle of the night (for example, 23 o’clock on March 20), it is obvious that the time the Sun rises closest to it is March 21, not exactly March 20. Therefore, saying that the Sun rises in the East and sets in the West at the spring and autumn equinoxes is only an approximation, you can add an error of 1 day before and after the spring and autumn equinoxes you see above. calendars.