Know that the Earth orbits the Sun once in approximately 365 days and use this to explain the periodic nature of the seasons

6.1.1 The Earth

Objective

Know that the Earth orbits the Sun once in approximately 365 days and use this to explain the periodic nature of the seasons.

Key Facts about the Earth’s Orbit

• The Earth follows an elliptical (nearly circular) orbit around the Sun.
• One complete revolution takes about \$365.25\$ days – one year.
• The average distance from the Sun (the astronomical unit, AU) is \$1.496\times10^{11}\,\text{m}\$.
• The orbital speed is approximately \$29.8\ \text{km s}^{-1}\$.

Orbital Parameters

Why Seasons Occur

1. Earth’s axis is tilted \$23.5^{\circ}\$ to the line perpendicular to its orbital plane.
2. During a year, different hemispheres are tilted toward or away from the Sun.
3. When a hemisphere is tilted toward the Sun, solar rays strike the surface at a higher angle, spreading over a smaller area and delivering more energy per unit area → warmer temperatures.
4. Conversely, when tilted away, the Sun’s rays are more oblique, covering a larger area and delivering less energy → cooler temperatures.

The combination of the orbital period and axial tilt produces a regular, repeating pattern of temperature changes known as the seasons.

Mathematical Description of Seasonal \cdot ariation

The solar declination \$\delta\$ (the latitude at which the Sun is directly overhead at noon) varies with the day of the year \$n\$ (where \$n=0\$ at the March equinox) approximately as

\$\delta \approx 23.5^{\circ}\,\sin\!\left(\frac{2\pi}{365}\,n\right)\$

This sinusoidal variation explains why the Sun’s apparent height in the sky changes throughout the year, giving rise to the seasonal cycle.

Summary

• The Earth completes one orbit around the Sun in about \$365\$ days.
• The axial tilt of \$23.5^{\circ}\$ causes the Sun’s rays to strike different parts of the Earth at varying angles during the year.
• These variations in solar intensity produce the periodic pattern of seasons.