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 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 Earth’s axis 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 axis tilting 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 result in different Sun angles during different periods. Sun is not in the ellipse’s center but is one of the focal points. The earth is 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 year 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 and changes the seasons.
The changing of seasons on Earth is primarily caused by the tilt of Earth’s axis and its orbit around the Sun. Here’s how it works:
Tilt of Earth’s Axis: Earth’s axis of rotation is tilted by approximately 23.5 degrees relative to its orbital plane around the Sun. This tilt remains constant throughout the year. This tilt is what gives rise to the seasons.
Revolution around the Sun: Earth orbits the Sun in an elliptical path, completing one revolution in approximately 365.25 days. This orbit is responsible for the variation in the amount of sunlight received by different parts of Earth during different times of the year.
Sunlight Intensity: Because of Earth’s tilt, the intensity of sunlight reaching different latitudes varies throughout the year. There are four key points in Earth’s orbit and axial tilt that mark the changing of seasons:
a. Summer Solstice: Around June 21st in the Northern Hemisphere (and December 21st in the Southern Hemisphere), the North Pole is tilted towards the Sun. This results in the longest day of the year in the Northern Hemisphere and the shortest day in the Southern Hemisphere. It marks the beginning of summer in the Northern Hemisphere and winter in the Southern Hemisphere.
b. Winter Solstice: Around December 21st in the Northern Hemisphere (and June 21st in the Southern Hemisphere), the North Pole is tilted away from the Sun. This leads to the shortest day of the year in the Northern Hemisphere and the longest in the Southern Hemisphere. It signifies the start of winter in the Northern Hemisphere and summer in the Southern Hemisphere.
c. Spring Equinox: Around March 21st, both hemispheres receive roughly equal amounts of daylight as the tilt of Earth’s axis is neither towards nor away from the Sun. This marks the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
d. Autumn Equinox: Around September 21st, similar to the spring equinox, both hemispheres receive roughly equal amounts of daylight. It signifies the start of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
Impact on Temperature and Weather: The changing angle and intensity of sunlight influence the amount of heat received by different regions on Earth. When a hemisphere is tilted towards the Sun, sunlight is more direct, leading to longer days and warmer temperatures. In contrast, sunlight is more spread out when a hemisphere is tilted away from the Sun, resulting in shorter days and cooler temperatures.
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: Earth tilts in the same direction in the summer months. The North Pole is tipping toward the Sun. Sunlight strikes the northern hemisphere more directly, and the Sun stays in the sky longer. Summer days are warmer than winter, 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. The Earth’s surface and the midday Sun form a 90-degree angle now. 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 day and night length is the same at all places on the Earth.
After this day, spring is in the northern hemisphere, where the day is longer than the night. It becomes autumn when the southern hemisphere’s 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 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 faces 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 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 the 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?
The axis pointed to a star in Draco’s constellation five thousand years ago. 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 a few temperature variations throughout the year as well. The axis points towards the Sun during the axis’s 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.
Aphelion and perihelion occur near the equinoxes when the Earth’s axis is aligned. The northern and southern hemispheres will have similar contrast in the seasons. At present, perihelion occurs during the southern hemisphere’s summer. Aphelion is reached during the southern winter. So, the southern hemisphere’s seasons are more extreme than the northern hemisphere’s when other factors are equal.
When sunshine strikes the North Pacific’s cold waters in spring, productivity skyrockets in phytoplankton, another microscopic photosynthesizer from the ocean food web base. 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.
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“Astronomy Answers AstronomyAnswerBook: Seasons,” Astronomical Institute, Utrecht University.