Earth's Orbit and Seasons

Earth’s Orbital Motion

Zodiac Constellations and the Ecliptic

• The twelve constellations the Sun moves through during the year are called the zodiac constellations.
• The path that the Sun travels along throughout the year is called the ecliptic.
• The zodiac constellations are found along the ecliptic path.

Seasonal Changes

• Seasonal changes to the night sky are due to Earth's motion around the Sun.

Ecliptic Plane and Solstices/Equinoxes

• The ecliptic is the plane of Earth’s path around the Sun.
• The plane is tilted at 23.5^\circ to the celestial equator.
• Northernmost point: Summer solstice (Sun is highest in the sky).
• Southernmost point: Winter solstice (Sun is lowest in the sky).
• Points where the path crosses the celestial equator: Vernal and autumnal equinoxes.

Causes of Seasons

• A combination of sunlight angle and day length causes the seasons.
• Primary reason for seasons is the 23.5^\circ tilt of the Earth’s rotational axis.
• This tilt allows the angle at which we receive sunlight to change throughout the year.
• Secondary reason is the variation in daylight hours.
  • Summertime: More hours of sunlight provide more heat.
  • Winter: Fewer hours of sunlight contribute to lower temperatures.
• The seasons are NOT due to changes in the distance between Earth and the Sun.
  • Earth is actually closer to the Sun during winter (in the Northern Hemisphere).

Solstices and Equinoxes

• Summer Solstice
  • Longest day of the year.
  • Sun is highest in the sky.
  • Hot temperatures.
• Winter Solstice
  • Shortest day of the year.
  • Sun is lowest in the sky.
  • Cold temperatures.
• Vernal and Autumnal Equinoxes
  • Occur in between the summer and winter solstices on the calendar.
  • Day lengths are in between the lengths for summer and winter solstices.
  • 12 hours of daylight everywhere on Earth.

Hypothetical Planet

• Imagine a planet whose rotational axis is perpendicular to its orbital plane.
• Its seasons would be constant (option C).