The Earth experiences seasons due to its axial tilt, not because of its distance from the Sun. The simple answer to why we have seasons is that it’s the angle of Earth’s axis in relation to its orbit around the Sun that causes seasons to change.
- When a hemisphere tilts toward the Sun, sunlight strikes directly and it’s warmer.
- When the hemisphere tilts away from the Sun, sunlight strikes indirectly. The energy passes through a lot more atmosphere before it hits the ground, so it’s colder.
- The Sun’s energy hits the equator pretty much the same year round. Temperature does not vary much, but there are wet and dry seasons due to heating/cooling of the oceans to the north and south.
The Misconception About Distance from the Sun
A common misconception is that the Earth is closer to the Sun in the summer and further away in the winter. In reality, the Earth is actually closest to the Sun in January, a point known as perihelion (about 91.4 million miles away), and farthest in July, known as aphelion (about 94.5 million miles away). Even though there is a difference of millions of miles, this difference in distance does not significantly affect the seasons.
The distance from the Sun partially explains why summer may be hotter in the Southern Hemisphere. But, the ratio of ocean to land also plays a significant role.
The True Causes of Seasons: Axial Tilt and Parallelism
Seasons are primarily the result of the Earth’s axial tilt—a fixed angle of about 23.5 degrees relative to the plane of its orbit around the Sun. This tilt remains constant as the Earth orbits the Sun, a phenomenon known as axial parallelism. The North Pole always points in the same direction relative to the stars, towards Polaris, the North Star.
When the North Pole tilts towards the Sun, the Northern Hemisphere experiences summer because sunlight strikes this hemisphere more directly. Conversely, when the South Pole tilts towards the Sun, the Southern Hemisphere enjoys summer, while the Northern Hemisphere experiences winter. Because of axial parallelism, the seasons in the Northern Hemisphere and Southern hemisphere are comparable, but opposite to one another.
Other Factors Contributing to Seasonal Changes
The Earth’s tilt is the most significant reason for the seasons. But, several other factors also contribute to seasonal temperature changes:
- Distribution of Land and Water: Continents and oceans absorb and release heat differently, influencing weather patterns and seasons.
- Ocean Currents: Ocean currents transport warm or cold water, affecting the climate of nearby landmasses.
- Altitude: Higher altitudes often experience cooler temperatures year-round.
- Atmospheric Circulation: The movement of air masses redistributes heat across the planet.
What Is a Season?
A season is a period of the year characterized by specific weather conditions and daylight hours, resulting from Earth’s orbit around the Sun and its axial tilt. The primary seasons—spring, summer, autumn (fall), and winter—each have distinct weather patterns and daylight hours.
The Significance of Seasons
Seasons have a significant impact on the environment and human activities. They affect plant growth cycles, animal behavior, and agriculture. Human cultures organize calendars and celebrations around the progression of seasons.
Reckoning the Seasons: Solstices and Equinoxes
Seasons are often reckoned based on solstices and equinoxes. A solstice is when the Sun is at its greatest distance from the equator, marking the start of winter or summer. An equinox occurs when day and night are of equal length, signaling the beginning of spring or autumn.
However, this method does not work everywhere. Near the equator, the length of day and night remains nearly constant year-round, and temperature variations are minimal, leading to less pronounced seasons. Conversely, regions near the poles experience extreme variations in daylight hours and temperatures, leading to a different understanding and experience of seasons.
Are There Seasons on Other Planets?
Other planets with a significant axial tilt also experience seasons. The nature and length of these seasons depends on differences in axial tilt, orbital eccentricity, and rotation period.
Here’s a brief overview of how seasons work on a few other planets:
Mars
Mars has seasons similar to Earth’s because its axial tilt is roughly the same, at about 25 degrees. However, Martian seasons are nearly twice as long because Mars takes about 687 Earth days to orbit the Sun. Additionally, Mars has a more elliptical orbit than Earth, which means the difference between perihelion and aphelion is greater. This causes more variation in seasonal temperatures than Earth experiences.
Venus
Venus has an axial tilt of just about 3 degrees, which is almost upright. This minimal tilt means that Venus does not experience significant seasons. Its thick atmosphere also leads to a strong greenhouse effect, making its surface temperature extremely hot and relatively constant throughout its year.
Jupiter
Jupiter has an axial tilt of just over 3 degrees, so it experiences only slight seasonal changes. However, because it is a gas giant, the concept of seasons doesn’t apply in the same way as it does to terrestrial planets. Jupiter’s rapid rotation (about 10 hours for a full spin) leads to extreme weather and temperature patterns that differ greatly from what we define as seasons on Earth.
Saturn
Saturn’s axial tilt is about 27 degrees, similar to Mars and Earth, so it does experience seasons. However, each season lasts for more than seven Earth years because Saturn takes about 29.5 Earth years to complete one orbit around the Sun. Like Jupiter, Saturn is a gas giant, and its seasonal changes are not as obvious in terms of surface conditions. Scientists observe changes in its atmospheric conditions and the tilt of its spectacular ring system.
Uranus
Uranus has an extreme axial tilt of about 98 degrees, essentially rolling on its side as it orbits the Sun. This leads to extreme seasonal variations, with each pole getting 42 Earth years of continuous sunlight, followed by 42 years of darkness.
Neptune
Neptune, much like Uranus, has a significant axial tilt at 28 degrees. It experiences seasons that last for over 40 Earth years each. Due to its great distance from the Sun, the seasonal changes are not very intense in terms of temperature. However, they cause shifts in wind speed and atmospheric conditions.
References
- Khavrus, V.; Shelevytsky, I. (2010). “Introduction to solar motion geometry on the basis of a simple model”. Physics Education. 45 (6): 641–653. doi:10.1088/0031-9120/45/6/010
- Lerner, K. Lee; Lerner, Brenda Wilmoth (2003). World of Earth Science. Farmington Hills, MI: Thomson-Gale. ISBN 0-7876-9332-4.
- Meeus, J.; Savoie, D. (1992). “The history of the tropical year“. Journal of the British Astronomical Association. 102 (1): 40–42.
- Petersen, J.; Sack, D.; Gabler, R.E. (2014). Fundamentals of Physical Geography. Cengage Learning. ISBN 978-1-285-96971-8.
- Rohli, R.V.; Vega, A.J. (2011). Climatology. Jones & Bartlett Learning, LLC. ISBN 978-1-4496-5591-4.