The Sun's path above the major circles of latitude

The Sun's path above the major circles of latitude

The apparent movement of the Sun is caused by the Earth's rotation around its axis.

Geography

Keywords

Sun, Earth, rotation, axis of rotation, Tropic of Cancer, Tropic of Capricorn, Equator, Arctic Circle, Antarctic Circle, horizont, zenit, ecliptic, celestial sphere, winter solstice, summer solstice, equinox, solstice, culmination, spring, summer, autumn, winter, season, circulatory system, year, calendar, month, months, angle of incidence, sunlight, sunshine duration, change of seasons, physical geography, astronomy, geography

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Scenes

The celestial bodies above us, including the Sun, seem to be moving during the day. This is caused by the Earth’s rotation around its axis from west to east. As a result, to the observer, the Sun appears to be moving from east to west. The apparent path of the Sun is an orbit in the celestial sphere, that is, the sky, which seems like a dome spreading above the Earth.

Knowing what the concept of the 'horizon' means is essential for celestial navigation. In simple terms, the horizon is where the Earth and the sky meet. In other words, it is the intersection of the celestial sphere and the plane that is tangent to the Earth.

An imaginary straight line drawn vertically above the head of the observer is perpendicular to the plane of observation. The point where the line intersects the celestial sphere is called the zenith; in other words, it is the highest point above the head of the observer.

If the Earth’s axis is translated to the place of the observer and it is extended, it intersects the celestial sphere at the north and south celestial poles.

The ecliptic is the plane of the Earth’s orbit around the Sun. The Earth’s axis and the ecliptic form a 66.5° angle; consequently the Sun’s rays are perpendicular to the Earth's surface at different places during the year, and they are perpendicular to the equator only twice a year.

On these two days, the Sun spends exactly the same amount of time above and below the horizon , meaning that day and night are of equal length. These days are called the vernal equinox and the autumnal equinox.

There are two days, one at the end of June and the other at the end of December, when the Sun's rays are perpendicular to the Earth at 23.5° lines of latitude North and South. In June, the Sun spends the longest period of time above the horizon in the Northern Hemisphere, while in December it does the same in the Southern Hemisphere. These two lines of latitude are called the Tropic of Cancer and the Tropic of Capricorn.

After these days, the Sun’s rays are perpendicular to the Earth at latitudes less than 23.5°. Therefore, both the Tropic of Cancer and the Tropic of Capricorn represent a change in the apparent path of the Sun. The time of this change is called the summer and winter solstice.

As a consequence, the noontime elevation of the Sun is higher in the Northern Hemisphere than in the Southern Hemisphere from the time of the vernal equinox until the time of the autumnal equinox. The angle of inclination of the Sun’s rays in the Northern Hemisphere is higher, and they supply more energy. As a result, it is summer in the Northern Hemisphere and winter in the Southern Hemisphere. Naturally, the situation is the opposite from the time of the autumnal equinox until the time of the vernal equinox.

During the summer solstice, the dividing line between day and night and the Earth’s axis form a 23.5° angle, so this line does not reach latitudes over 66.5° North or South. These are called the Arctic Circle and the Antarctic Circle, respectively.

During the summer solstice, latitudes over 66.5° in the Northern Hemisphere are fully illuminated for 24 hours, while latitudes over 66.5° in the Southern Hemisphere remain in complete darkness. During the winter solstice the situation is the exact opposite.

As days go by, except for these two days, the angle of the Earth’s axis and the dividing line between day and night will gradually become smaller than 23.5°. Thus the region where day or night lasts for 24 hours will become smaller and smaller.

Finally, during the vernal and autumnal equinoxes the dividing lines between day and night are at the poles. At that point, day and night are of equal length everywhere on Earth.

In the zones within the Arctic and Antarctic Circles, day and night alternate regularly, but their lengths differ. The length of the day depends on the geographic latitude and the positions of the Sun and the Earth, that is, the current date.

The dividing line between day and night bisects the equator so the days and nights are always the same length there: 12 hours each.

  • - On this day, the Sun spends exactly the same length of time above and below the horizon, meaning that the length of the day and the night are equal. Date: March 20 or 21.
  • - On this day, the Sun spends the longest period of time above the horizon in the Northern Hemisphere. This is when a change occurs in the apparent path of the Sun: it “heads” towards south. Date: June 20, 21, or 22.
  • - On this day, the Sun spends exactly the same length of time above and below the horizon, meaning that the length of the day and the night are equal. Date: September 22 or 23.
  • - On this day, the Sun spends the longest period of time above the horizon in the Southern Hemisphere. This is when a change occurs in the apparent path of the Sun: it “heads” towards north. Date: December 21 or 22.

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