A year’s length on earth is affected by gradual and cyclical changes in its orbit and tilt. First, there is the precession of the Earth’s axis. Over about 26,000 years, the Earth’s axis traces out a circle in the sky. One result of this is that the North Star changes over time. The Earth’s axis points towards Polaris. The axis of the earth is an imaginary line on which the Earth rotates. It links up the two poles. Both the axis and the earth are tilted at an angle of 23.5 degrees during a revolution.
The tilting of the axis results in direct sunlight falling on different places during different seasons. This causes variations in days, nights, and seasons. Similarly, the earth’s revolution and the axis’s tilting results in different Sun angles during different periods. Sun is not in the center of the ellipse but one of the focal points. The earth is a little closer to the Sun every January and a little further away every July. Does that cause us to have seasons?
What causes the seasons to change on earth?
Earth orbits the Sun, and it takes a full year for our planet to complete its orbit. Earth also rotates like a slightly tilted spinning top. It remains tilted in the same direction all year round as we orbit the Sun. But it means the sun’s light shines differently on earth at different times of the year. The Earth moves changes the seasons.
Winter: Sunlight strikes the northern hemisphere at a shallow angle for a short period. This is why winter weather is generally cool with short days and long nights. While it’s winter in the northern hemisphere, it’s summer in the southern hemisphere. As Earth orbits the Sun, we move towards spring in the northern hemisphere. Earth is tilted neither toward nor away from the Sun as day and night are equal in length.
Summer: In the summer months, Earth still tilts in the same direction. The North Pole is tipping toward the Sun. Sunlight strikes the northern hemisphere more directly, and the Sun stays in the sky for a longer time. Compared to winter, summer days are warmer, and the Sun stays in the sky much longer. While it’s summer in the northern hemisphere, it’s winter in the southern hemisphere. Because of the earth’s tilt, the seasons are reversed.
When the Sun is directly overhead, we call this the overhead Sun. At this time, the Earth’s surface and the midday Sun forms a 90-degree angle. Different locations of the overhead Sun result in solar radiation variations in other areas and at different periods.
Geographical explanation: On the 21st or 22nd of March, The overhead Sun is over the equator. The equator receives the largest amount of solar radiation. The northern hemisphere is in the spring equinox, while the southern hemisphere is in the autumn equinox. The angle of the Sun decreases towards the poles. On this day, the two hemispheres receive a similar amount of solar radiation and the length of day and night is the same at all places on the earth.
After this day, it is spring in the northern hemisphere, where the day is longer than the night. In the southern hemisphere, it becomes autumn when the day is shorter than the night.
On the June 22nd solstice, the tilt causes the North Pole to face the Sun while the South Pole faces away. When the Earth’s orbit takes it around to the December 22nd solstice, the North Pole is tilted away from the Sun and the South Pole. This tilt explains why the North Pole experiences winter in January. While the South Pole is experiencing the opposite season of summer. The North Pole is facing away from the Sun and gets less overall sunlight. The South Pole is facing towards the Sun and gets more.
On the September Equinox, the Earth’s axis isn’t tilted toward or away from the Sun. So the poles have equal hours of night and day. Only this time, it’s fall in the Northern Hemisphere and spring in the Southern Hemisphere. Why is this time called an equinox? As the sun’s rays hit the surface, they are strongest at the equator directly in the middle, where they hit at a 90-degree angle.
On the 22nd or 23rd of September, the overhead sun is over the equator again. The equator receives the largest amount of solar radiation. The northern hemisphere is in the autumn equinox, while the southern hemisphere is in the spring equinox. The angle of the Sun decreases towards the poles.
- The length of day and night is the same at all places on the earth.
On December 21st or the December Solstice, the South Pole is tilted towards the Sun. Everything south of 66.5 degrees south latitude in the Antarctic Circle receives 24 hours of daylight. At noon, the Sun’s rays will be directly overhead the Tropic of Capricorn, or the latitude line marking 23.5 degrees South. This is the strongest insolation. The Southern Hemisphere receives, and it’s the peak of summer.
The Earth’s tilt means that the North Pole is tilted away from the Sun on the December Solstice. Everything north of 66.5 degrees North latitude in the Arctic Circle experiences a total of 24 hours of darkness. With weak insolation, the Northern Hemisphere is in winter. As the Earth continues along its orbit, the Sun’s angle with the Earth and the day length gradually increase in the Northern Hemisphere.
The North Pole is tilted towards the Sun, while the South Pole is tilted away. At noon, the Sun shines directly over the Tropic of Cancer, the latitude line marking 23.5 degrees North. The Tropic of Cancer and the Tropic of Capricorn mark the Earth’s boundaries where the Sun can shine directly overhead. At either pole, the Sun will never be directly overhead.
- Each solstice and equinox represents opposite seasons in opposite hemispheres.
The seasons are only possible because of processes set in motion before the Earth even existed. If the Earth didn’t rotate, half the planet would be an ice dungeon, and half would be on fire. If the Earth didn’t revolve, the day’s length would remain fixed yet drastically unequal worldwide. The motion of Earth significantly influences our life decisions. It guides where we decide to live, what food we eat, or even what weather we experience.
What is the revolution of the seasons?
Five thousand years ago, the axis pointed to a star in the constellation Draco. And twelve thousand years ago, the brilliant star Vega was the pole star. The equinoxes’ precession is caused primarily by the Sun’s gravitational forces in the moon acting on the earth. At the same time, the axial tilt is a primary cause of seasons on the earth. The Sun’s distance changes throughout the year because of the elliptical shape of the Earth’s orbit.
Contribute to few temperature variations throughout the year as well. When the axis is aligned, it points towards the Sun during the perihelion. One hemisphere will have a greater difference between the seasons, while the other will have milder seasons. The hemisphere in summer at perihelion will receive much of the corresponding entries in solar radiation.
When the Earth’s axis is aligned such that aphelion and perihelion occurred near the equinoxes. The northern and southern hemispheres will have similar contrast in the seasons. At present, perihelion occurs during the southern hemisphere’s summer. And aphelion is reached during the southern winter. So, the seasons of the southern hemisphere are somewhat more extreme than the northern hemisphere when other factors are equal.
When sunshine strikes the North Pacific’s cold waters in spring, productivity skyrockets phytoplankton, another microscopic photosynthesizer from the ocean food web base. And all ocean life responds to changes in the seasons. On land, forests grow green during the spring brightening the continents. During winter, continents in this field turn dark from a lack of photosynthesis.
Khavrus, V.; Shelevytsky. “Introduction to solar motion geometry on the basis of a simple model.” Physics Education.
“Fundamentals of physical geography,” Energy and Matter:(h) Earth-Sun Geometry.
Phillips, Tony. “The Distant Sun.” NASA. “Earth at Perihelion.” Science News.
“Astronomy Answers AstronomyAnswerBook: Seasons,” Astronomical Institute, Utrecht University.