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8. Explaining the seasons of the Earth

The tilt in the spin axis changes the intensity of sunlight through the year giving rise to periods of spring, summer, autumn and winter with their different average temperature, weather systems and the biological cycles of plants and animal life.

As the Earth orbits the Sun it also spinning on an axis of rotation roughly every 24 hours.

The spin axis is tilted at 23.5^o away from an imaginary vertical line from the plane of the Earth's orbit around the Sun - see the diagram below.

This tilt in the spin axis causes the different seasons on Earth by changing the intensity of the sunlight that falls on the Earth's surface.

The shallower the sunlight angle, the less intense the sunlight in terms of incoming radiation energy.

The steeper the sunlight angle, the more intense the sunlight in terms of incoming radiation energy.

Position 1 corresponds to the autumn equinox in the northern hemisphere.

Equal hours of night and daylight.

Equals the spring equinox in the southern hemisphere.

Moderate temperatures.

Position 2. corresponds to mid-winter in the northern hemisphere.

Spin axis tilted away from the sun, so reducing the intensity of sunlight and shorter length of daylight.

Equals the mid-summer in the southern hemisphere, spin axis tilted towards the sun.

The lowest temperatures on the Earth's surface in the northern hemisphere.

The highest temperatures on the Earth's surface in the southern hemisphere.

Position 3. corresponds to the spring equinox in the northern hemisphere.

Equal hours of night and daylight.

Equals the autumn equinox in the southern hemisphere.

Moderate temperatures.

Position 4. corresponds to mid-summer in the northern hemisphere.

Spin axis tilted towards the sun, so increasing the intensity of sunlight and longer length of daylight.

Equals the mid-winter in the southern hemisphere, spin axis tilted away from the sun.

The lowest temperatures on the Earth's surface in the southern hemisphere.

The highest temperatures on the Earth's surface in the northern hemisphere.

 

Apart from the Earth's rotation giving the rising and setting sun and the arced path in between, the position of the sun in the sky is also influenced by the tilt of the Earth's spin axis.

The sun is highest in the sky at midday in the summer and at its lowest 'high point' point in the sky at midday, mid-winter - see diagrams of the Sun's paths in the northern hemisphere.

Path 1 of the sun represents mid-winter in the northern hemisphere, shortest daylight time.

Path 2 of the sun represents its path at the equinoxes.

Path 3 of the sun represents mid-summer in the northern hemisphere, longest daylight time.

INDEX of my physics notes on ASTRONOMY

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Key points for Physics - Explaining the seasons of the Earth - the tilted spin axis of the Earth as it orbits the Sun

Why Earth Has Seasons: The Tilted Axis Explained
The changing seasons aren’t caused by how close Earth is to the Sun - they’re all about Earth’s axial tilt and its orbit around the Sun.

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The tilted spin axis of the Earth

• Earth’s axis is tilted at approximately 23.5° from vertical.
• This tilt stays constant as Earth orbits the Sun.
• Because of this, different parts of Earth receive different amounts of sunlight throughout the year.

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How the Orbit Affects Seasons

As Earth moves around the Sun:

• When the Northern Hemisphere is tilted toward the Sun:
  • Sunlight hits it more directly.
  • Days are longer, temperatures are warmer → Summer in the North, Winter in the South.
• When the Southern Hemisphere is tilted toward the Sun:
  • It receives more direct sunlight.
  • Summer in the South, Winter in the North.
• Spring and Autumn (Equinoxes):
  • Neither hemisphere is tilted toward the Sun.
  • Day and night are roughly equal in length.

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Key Seasonal Dates

Event	Approx. Date	Description
Spring Equinox	~March 21	Equal day/night; start of spring in Northern Hemisphere
Summer Solstice	~June 21	Longest day in the Northern Hemisphere
Autumn Equinox	~September 23	Equal day/night; start of autumn
Winter Solstice	~December 21	Shortest day in the Northern Hemisphere

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Keywords, phrases and learning objectives for astronomy

Be able to explain the Earth's seasons with reference to the tilt of the Earth's spin axis.

Describe the difference in sunlight intensity through the year causing the seasons of spring, summer, autumn and winter cycles of weather affecting plant and animal life.

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INDEX of my physics notes on ASTRONOMY