What causes the Seasons? Ms. Bridgeland – 6th Grade

The Seasons happened by accident The Seasons happened by accident! The Earth was supposed to have stable conditions that did not change, but something happened that changed our planet forever!

Long ago, a protoplanet called “Theia” hit the Earth, and knocked its axis to the side.

Theia hit the Earth’s surface, causing a big crater in the Earth and dust and rubble to fly off the Earth’s surface. What are two important things that happened to Earth because of Theia?

Theia crashing into Earth caused… 1) The Earth to be forced to tilt to its side (–its axis is now tilted 23 degrees) 2) Caused the formation of the Moon!

“Theia” is a protoplanet that is the same size as Mars “Theia” is a protoplanet that is the same size as Mars. Theia is named after the Greek goddess who gave birth to the Moon goddess, Selene.

Theia crashing into the Earth also caused axial tilt Axial tilt: the tilt of the Earth’s axis 23.5 degrees

Why would axial tilt cause the seasons?

Because the Northern Hemisphere and Southern Hemisphere are getting different amounts of light

In this picture, which hemisphere is having summer?

Answer: The Northern Hemisphere The Northern Hemisphere is getting direct sunlight from the Sun, whereas the Southern Hemisphere is getting indirect sunlight, so it is WINTER there

Now look at this picture Now look at this picture. Why is it winter in the Northern Hemisphere now?

Answer: It is winter in the northern hemisphere because the southern hemisphere is exposed to direct sunlight

Important: Remember that the Earth’s axis is always tilted in the same direction

As it revolves around the Sun, the EARTH’S AXIAL TILT WILL NEVER CHANGE! It always points toward the North Star

What is the North Star? (Polaris)

North Star: Important because the Earth’s axis is pointed almost directly at it The North Star remains in the same spot over the northern horizon

North Star: Right above the North Pole

Remember that as the Earth revolves around the Sun, its axis points in the same direction. This is why during some seasons, the northern hemisphere gets more direct sunlight, and in others, the southern hemisphere gets more sunlight.

Why are days in summer considered “longer” than days in winter? 8 o’clock in the summer, it is still light outside 8 o’clock in the winter, it is dark outside

The answer is that during the summer, the Northern Hemisphere gets exposure to more hours of sunlight than in the winter

Solstice Occurs twice per year What IS it?

Solstice: The longest day of the year (for Northern hemisphere = June 21) The shortest day of the year (for Northern hemisphere = December 21)

Solstice: Summer Solstice: “The longest day of the year” means when the Earth is getting the most direct sunlight for the longest period of time - Means that the Northern hemisphere is facing as close to the sun’s direct rays as possible

Summer Solstice 2012: In Washington, D.C. Sunrise: at 5:43 am Sunset: 8:37 pm Length of daylight hours: 14 hours, 54 minutes

Solstice: Winter Solstice: December 21st “the shortest day of the year” –this means that the Northern hemisphere is getting the least amount of direct sunlight during daylight hours Meaning the Northern Hemisphere is tilted as far away from the sun’s direct rays as possible

Winter Solstice 2012: In Washington, D.C. Sunrise: 7:24 am Sunset: 4:50 pm Length of daylight hours: 9 hours, 26 minutes

What is an Equinox?

What is an Equinox? An equinox is when the sun is directly hitting the equator, which means that the Northern and Southern hemispheres are getting about the same amount of sunlight Both the Northern and Southern hemispheres have 12 hour days (equal amount of day and night)

Equinox Fall Equinox: September 21 (in Northern Hemisphere) – first day of Fall Spring Equinox: March 21 (in Northern Hemisphere) –first day of Spring

Note: A season cannot officially change without a solstice or an equinox!

Challenge Trivia: If during the winter, the Northern Hemisphere is facing away from the sun and does not get direct sunlight, why do we still have daylight during those months?

Answer: Rotation! Remember that while the Earth is revolving around the sun every 365 days, it is rotating on its own axis every 24 hours! So for about 12 hours, the planet still is exposed to daylight, just NOT at a direct angle.

Rotation

Rotation Which direction does the Earth rotate?

Rotation Counterclockwise! THE OPPOSITE OF HOW A CLOCK’S HANDS MOVE

Did you know the seasons influenced how we count days, months, and years? A year is 365 ¼ days because people noticed that there were 4 seasons With each season change, the moon went through its full cycle 3 times 4 season x 3 moon cycle per season = 12 months

Bill Nye http://www.youtube.com/watch?v=C2C2Jy2TAew