Earth’s rotation

• Why do we have seasons?

Earth’s rotation

• The Earth rotates on its axis (imaginary vertical line around which Earth spins) every 23 hours & 56 minutes.
• One day on Earth is one rotation of the Earth.
• One Day on Earth is when our side of the Earth faces the sun.
• Night on Earth is when the side of Earth we are on faces away from the sun.

Earth’s revolution

• It takes the Earth 365.25 days (or rotations) to travel or revolve around the Sun once.
• This is called a year.

• Orbit

Motion Terminology

• Rotation – to spin on an axis

Motion Terminology

• Revolution – the motion of a body around another body

Motion Terminology

• Orbit – the path that is followed around another body in space

Why do we have seasons?

• The Earth’s orbit around the sun is NOT a perfect circle. It is an ellipse.
• Seasons are not caused by how close the Earth is to the sun.
• In fact, the Earth is closest to the sun around January 3 and farthest away from the sun around July 4.

• Ellipse

Why do we have seasons?

• Seasons are the result of the tilt of the Earth's axis.
• Earth’s axis is tilted 23.5°.
• This tilting is why we have SEASONS like fall, winter, spring, summer.
• The number of daylight hours is greater for the hemisphere, or half of Earth, that is tilted toward the Sun.
• Seasons Interactive

Why do we have seasons?

• Summer is warmer than winter (in each hemisphere) because the Sun's rays hit the Earth at a more direct angle during summer than during winter

Why do we have seasons?

• Also the days are much longer than the nights during the summer.
• During the winter, the Sun's rays hit the Earth at an extreme angle, and the days are very short. These effects are due to the tilt of the Earth's axis.

Seasons…in a nut shell

Solstices

• Occur twice a year, when the tilt of the Earth's axis is oriented at its extremes. Tilted the farthest or closest
• Winter solstice is the shortest day of the year. In the Northern Hemisphere. It occurs on December 21 or 22 and marks the beginning of winter.
• The Summer Solstice is the longest day of the year. It occurs on June 20 or 21 and marks the beginning of summer.

Equinoxes

• A day lasts 12 hours and a night lasts 12 hours at all latitudes.
• Equinox literally means "equal night".
• Sunlight strikes the earth most directly at the equator.
• This occurs twice a year.

Equinox

• The vernal (spring) equinox occurs March 20.
• The autumnal (fall) equinox occurs September 22 or 23.

The Earth's seasons are not caused by the differences in the distance from the Sun throughout the year.

Review Look closely at where the Sun is hitting the Earth during each season: