4.2 The Seasons
3 min read•june 12, 2024
Earth's is the key to understanding . This 23.5° tilt causes different parts of our planet to receive varying amounts of sunlight throughout the year, leading to seasonal changes in temperature and daylight hours.
The tilt's effects are most noticeable at higher latitudes. Near the poles, dramatic shifts occur between endless summer days and dark winter nights. Meanwhile, the equator experiences minimal seasonal changes, with consistent daylight year-round.
Earth's Axial Tilt and the Seasons
Earth's axial tilt and seasons
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- Earth's rotational axis tilted ~23.5° relative to its orbital plane around the Sun
- Tilt remains nearly constant throughout Earth's orbit
- Direction of tilt does not change; North Pole always points towards same direction in space (, the North Star)
- As Earth orbits Sun, tilt causes different hemispheres to receive varying amounts of sunlight
- During Northern Hemisphere summer, North Pole tilted towards Sun, causing longer days and more direct sunlight
- During Northern Hemisphere winter, North Pole tilted away from Sun, resulting in shorter days and less direct sunlight
- Opposite occurs in Southern Hemisphere during these times (Australian summer in December)
Sunlight variations by latitude
- depends on angle at which Sun's rays strike Earth's surface
- When Sun directly overhead (90° angle), sunlight intensity greatest (equatorial regions)
- As angle decreases, same amount of sunlight spread over larger area, reducing intensity (higher latitudes)
- Sunlight duration varies with latitude and season
- At equator, day length nearly constant throughout year, ~12 hours of daylight
- At higher latitudes, day length varies significantly between summer and winter
- In summer, days longer due to hemisphere's tilt towards Sun ( in Arctic regions)
- In winter, days shorter due to hemisphere's tilt away from Sun ( in Arctic regions)
- Variation in day length most extreme at poles
- At North Pole, 24 hours of daylight during summer and 24 hours of darkness during winter
- , or incoming solar radiation, varies with latitude and season due to these factors
Solstices vs temperature extremes
- Solstices and equinoxes mark key points in Earth's orbit and changing of seasons
- Summer solstice (around June 21 in Northern Hemisphere and December 21 in Southern Hemisphere): Sun reaches highest point in sky, hemisphere experiences longest day and shortest night
- Winter solstice (around December 21 in Northern Hemisphere and June 21 in Southern Hemisphere): Sun reaches lowest point in sky, hemisphere experiences shortest day and longest night
- Spring and fall equinoxes (around March 20 and September 22): Sun directly above equator, both hemispheres receive equal amounts of daylight and darkness
- Warmest and coldest periods in each hemisphere lag behind solstices
- Warmest temperatures typically occur few weeks after summer solstice
- Lag due to time it takes for Earth's surface and atmosphere to absorb and retain heat from Sun ()
- Coldest temperatures usually occur few weeks after winter solstice
- Lag due to time it takes for Earth's surface and atmosphere to cool down after reduced exposure to sunlight (seasonal temperature lag)
- Warmest temperatures typically occur few weeks after summer solstice
Earth's Orbit and Long-term Changes
- Earth's orbit is elliptical, not perfectly circular
- : point in Earth's orbit closest to the Sun
- : point in Earth's orbit farthest from the Sun
- The plane of Earth's orbit around the Sun is called the
- Earth's axis undergoes a slow wobble called , completing a cycle every 26,000 years
- This gradual change affects which star serves as the North Star over long periods
Key Terms to Review (19)
Aphelion: Aphelion is the point in the orbit of a planet, asteroid, or comet where it is farthest from the Sun. It is one of two extreme points in an elliptical orbit, the other being perihelion.
Aphelion: Aphelion is the point in a planet's orbit around the Sun when it is farthest from the Sun. This occurs once per orbit and is the opposite of perihelion, the point of closest approach to the Sun.
Axial Tilt: Axial tilt refers to the angle at which a planet's rotational axis is inclined relative to the plane of its orbit around the sun. This tilt is a critical factor in determining the seasons experienced on a planet's surface.
Ecliptic: The ecliptic is the apparent path that the Sun traces out in the sky over the course of a year as observed from Earth. It also represents the plane of Earth's orbit around the Sun.
Ecliptic: The ecliptic is the apparent path that the Sun appears to take across the sky over the course of a year, as viewed from Earth. It is the projection of Earth's orbit around the Sun onto the celestial sphere, and it serves as a reference plane for many important astronomical phenomena.
Equinox: An equinox is an astronomical event that occurs twice a year, around March 20-21 and September 22-23, when the sun is directly above the equator, and the length of day and night are approximately equal. This phenomenon is a crucial concept in understanding ancient astronomy, the seasons, and the calendar.
Insolation: Insolation, short for 'incident solar radiation', refers to the amount of solar radiation or sunlight that reaches a given surface on Earth. It is a crucial factor in understanding the seasonal changes and temperature variations experienced across the planet.
Midnight Sun: The midnight sun is a natural phenomenon that occurs in locations north or south of the Arctic or Antarctic Circles, where the sun remains visible at the local midnight. This is a result of the Earth's tilt on its axis and the way it orbits the sun, causing some regions to experience continuous daylight for a period of time during the summer months.
Perihelion: Perihelion is the point in the orbit of a planet, asteroid, or comet where it is closest to the Sun. At this point, the celestial body travels at its maximum orbital velocity due to the gravitational pull of the Sun.
Perihelion: Perihelion is the point in a planet's or comet's orbit when it is closest to the Sun. This is a crucial concept in understanding the motions and behaviors of objects within our solar system.
Polar Night: Polar night is a natural phenomenon that occurs in polar regions, where the sun remains below the horizon for an extended period, resulting in complete darkness throughout the day. This phenomenon is closely tied to the Earth's tilt and its revolution around the sun, which determines the seasons in different parts of the world.
Polaris: Polaris, also known as the North Star, is a prominent star in the northern celestial hemisphere. It holds great significance in the context of ancient astronomy, Earth's sky, the seasons, and the brightness of stars.
Precession: Precession is the slow, conical motion of Earth's rotation axis, which causes the celestial poles to trace out circles in the sky over a period of approximately 26,000 years. This phenomenon affects the position of stars and constellations over long time periods.
Precession: Precession is the gradual shift in the orientation of an object's rotational axis in relation to a fixed reference frame. This phenomenon occurs in various celestial bodies, including Earth, and has significant implications for the study of astronomy, timekeeping, and the calendar.
Seasonal Temperature Lag: Seasonal temperature lag refers to the delayed response of surface temperatures to changes in the amount of solar radiation received throughout the year. This phenomenon occurs due to the thermal inertia of the Earth's surface and atmosphere, which causes temperatures to peak and trough after the solstices and equinoxes.
Seasons: The seasons are divisions of the year marked by specific weather conditions, daylight hours, and ecological changes. They result from Earth's tilt on its axis and its orbit around the Sun.
Solstice: The solstice is the point in Earth's orbit where the Sun reaches its highest or lowest position in the sky at noon. This results in the longest day (summer solstice) or the longest night (winter solstice) of the year.
Solstice: A solstice is an astronomical event that occurs twice a year, marking the longest and shortest days of the year. It is a point in time when the sun reaches its northernmost or southernmost position in the sky, relative to the Earth's equator.
Sunlight Intensity: Sunlight intensity refers to the amount of solar radiation or energy that reaches a specific location on Earth's surface. It is a crucial factor that influences various aspects of the planet's climate, weather patterns, and the seasons.