Why do we Why do we have seasons?have seasons?

Earth’s rotationEarth’s rotation• The Earth rotates on its axis The Earth rotates on its axis

(imaginary vertical line around which (imaginary vertical line around which Earth spins) Earth spins) once every 24 hours. once every 24 hours.

• One day/night on Earth is one rotation One day/night on Earth is one rotation of the Earth.of the Earth.

• Day on Earth is when our side of the Day on Earth is when our side of the Earth faces the sun.Earth faces the sun.

• Night on Earth is when the side of Night on Earth is when the side of Earth we are on faces away from the Earth we are on faces away from the sun.sun.

Earth’s RevolutionEarth’s Revolution

• It takes the Earth 365 It takes the Earth 365 days (or rotations) to days (or rotations) to travel or revolve travel or revolve around the Sun once.around the Sun once.

• This is called a year.This is called a year.

Why do we have seasons?Why do we have seasons?• The Earth’s orbit around the The Earth’s orbit around the

sun is NOT a perfect circle. It sun is NOT a perfect circle. It is an ellipse.is an ellipse.

• Seasons Seasons are not are not caused by caused by how close the Earth is to the how close the Earth is to the sun. sun.

• In fact, the Earth is closest to In fact, the Earth is closest to the sun around January 3 and the sun around January 3 and farthest away from the sun farthest away from the sun around July 4.around July 4.

Ellipse Ellipse

Why do we have seasons?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 (experiencing summer).

Why do we have seasons?Why do we have seasons?• Summer is warmer than winter (in each hemisphere) because Summer is warmer than winter (in each hemisphere) because

the Sun's rays hit the Earth at the Sun's rays hit the Earth at a more direct angle a more direct angle during during summer than during winter summer than during winter

• Hemispheres experience opposite seasons (Summer in Hemispheres experience opposite seasons (Summer in Northern Hemisphere when its Winter in Southern Northern Hemisphere when its Winter in Southern Hemisphere)Hemisphere)

Why do we have seasons?Why do we have seasons?

• During summer, the days are much longer During summer, the days are much longer than the nights. than the nights.

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

Seasons in the Northern HemisphereSeasons in the Northern Hemisphere

SolsticesSolstices• SolsticesSolstices occur twice a year, when the tilt of the Earth's occur twice a year, when the tilt of the Earth's

axis is oriented directly towards or away from the Sun, axis is oriented directly towards or away from the Sun, causing the Sun to appear to reach its northernmost and causing the Sun to appear to reach its northernmost and southernmost extremes. southernmost extremes.

• Winter solstice is the shortest day of the year. In the Winter solstice is the shortest day of the year. In the Northern Hemisphere. It occurs on December 21 and Northern Hemisphere. It occurs on December 21 and marks the beginning of winter.marks the beginning of winter.

• The Summer Solstice is the longest day of the year. It The Summer Solstice is the longest day of the year. It occurs on June 21 and marks the beginning of summer. occurs on June 21 and marks the beginning of summer.

Tyrrhenian Sea and Solstice Sky Credit & Copyright: Danilo Pivato

SOLSTICESOLSTICE• During the winter the Northern Hemisphere During the winter the Northern Hemisphere

day lasts fewer than 12 hours and the day lasts fewer than 12 hours and the Southern Hemisphere day lasts more than Southern Hemisphere day lasts more than 12 hours.12 hours.

• During the winter solstice, the North Pole During the winter solstice, the North Pole has a 24-hour night and the South Pole has has a 24-hour night and the South Pole has a 24-hour day.a 24-hour day.

• The further north you go during Winter, the The further north you go during Winter, the fewer hours of daylight will be experienced. fewer hours of daylight will be experienced.

• Oppositely, the further north you go during Oppositely, the further north you go during the Summer, the more hours of daylight will the Summer, the more hours of daylight will be experienced. be experienced.

http://k12.ocs.ou.edu/teachers/reference/solstice.gif

EquinoxesEquinoxes• A day lasts 12 hours and a A day lasts 12 hours and a

night lasts 12 hours at all night lasts 12 hours at all latitudes.latitudes.

• Equinox literally means Equinox literally means "equal night"."equal night".

• Sunlight strikes the earth Sunlight strikes the earth most directly at the equator. most directly at the equator.

• This occurs twice a year This occurs twice a year during Spring and Fall.during Spring and Fall.

http://k12.ocs.ou.edu/teachers/reference/equinox.gif

EquinoxEquinox• The vernal (spring) The vernal (spring)

equinox occurs March equinox occurs March 21.21.

• The autumnal (fall) The autumnal (fall) equinox occurs equinox occurs September 21.September 21.

The Earth's seasons are not caused by The Earth's seasons are not caused by the differences in the distance from the the differences in the distance from the

Sun throughout the year. They are Sun throughout the year. They are caused by the earth’s tilt and revolution caused by the earth’s tilt and revolution

around the sun.around the sun.

ReviewReviewLook closely at where the Sun is hitting the Look closely at where the Sun is hitting the

Earth during each season:Earth during each season:

http://Search.Lycos.com/setup.asp?r=5&src=clear2&query=weather+savvy

http://www.nmm.ac.uk/uploads/gif/seasons-full.gif