# Why do we have days? We have day and night because

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Why do we have days? We have day and night because
Warm-Up March 15 Why do we have days? We have day and night because the Earth rotates on its axis.

By the way . . . When where you are is pointed toward the Sun, it is day. Then the Earth rotates you away from the Sun, and it is night. Sunlight Daytime Nighttime

Learning Standard: Today I’m creating a model to show the tilt of Earth on its axis because it explains what causes seasons. Opening: Luck of the Draw (questions about seasons) Work Session: Notes about seasons Cut and paste activity - seasons Closing: Homework – Seasons worksheet

Seat 20 What is a season? Luck of the Draw
One of the major divisions of the year, generally based on cyclic (recurring) changes of climate. Spring, Summer, Fall, and Winter Occurs in both the Northern and Southern Hemispheres (just at different times of the year). Luck of the Draw Seat 20

What causes the seasons?
The earth is tilted on it’s axis at 23.5˚. Luck of the Draw Seat 2

What’s the difference between rotation and revolution?
Revolution is when one object orbits another. Rotation is when an object turn around. Luck of the Draw Seat 12

Before we continue, let’s review

Important Latitudes to remember

Why is earth tilted at 23.5°? No one knows…
Astronomers have different theories: About 5 billion years ago, when the Earth was still very young, it was struck by a Mars-sized planet. This impact could have tipped our planet over. As the cloud of dust and gas collapsed when the universe was forming, the solar system did not form uniformly, the spinning of the gases and other planets is what made each different, hence the tilting of the planets Essentially, the numerical value of this axis tilt is an artifact of the way the Earth formed. It didn't have to have any specific value, and in fact the other planets all have different axis tilts owing to the differing details of their formation.

What would happen if earth wasn’t tilted?
Earth is tilted at 23.5 degrees, Uranus is tilted at about 98 degrees If you were standing on Uranus (if you could) the only difference you would notice was that the sun and the other planets would appear to rise and set in the north and south rather than east/west like on Earth. Whatever the reason, it's a good thing - if the Earth did not tilt, countries near the poles would be cold and dark all year round. If it tilted too much, the seasons would be very extreme – like on the planet Uranus. Here the winter lasts for 42 years in total darkness

What does the tilt do? It allows the sun’s rays to shine more directly and for longer periods of time on some locations than other places of Earth.

Who gets the most sun? Sun Over Equator (March 21)
Sun Over Tropic of Cancer (June 21) Sun Over Tropic of Capricorn (December 21) vernal equinox: the first day of spring in the Northern Hemisphere (March 21), when the Sun is perpendicular to the equator. summer solstice: first day of summer in the Northern Hemisphere (June 21), when the Sun is perpendicular to the tropic of Cancer. autumnal equinox: first day of autumn in the Northern Hemisphere (September 22), when the Sun is perpendicular to the equator. winter solstice: first day of winter in the Northern Hemisphere (December 21), when the Sun is perpendicular to the Tropic of Capricorn. Spring equinox - day and night are each 12 hours long and the Sun is at the midpoint of the sky. Summer solstice - the longest day of the year, when the Sun is at its most northern point in the sky. Autumn equinox - day and night are each 12 hours long and the Sun is at the midpoint of the sky. Winter solstice - the shortest day of the year, when the Sun is at its most southern point in the sky. Summer The day the north pole is nearest the Sun is called the 'summer solstice'. (You can see this from the picture on the right). Looking from Earth, the Sun reaches its highest point in the sky all year. This means it takes the most amount of time to cross the sky. So this is the longest day of the year. Its called the 'summer solstice' and happens around 21 June. Astronomers call this the start of summer and after this date, days start getting shorter. Autumn As we continue our journey around the Sun, the north pole moves away from the Sun. The Sun rises lower in the sky so the days continue getting shorter. When the Sun is at its mid-point in the sky, we reach the 'autumn equinox', around 22 September. Day and night are both 12 hours long and its the beginning of autumn. Winter The day when the north pole is furthest from the Sun is called the 'winter solstice'. The Sun crosses the sky at its lowest point all year. Therefore it crosses the sky in the quickest time so this is the shortest day of the year. Winter solstice happens around 22 December and marks the start of winter. From then on, the days start getting longer. Spring The Earth continues on its path, and our north pole starts moving towards the Sun again. The Sun moves upwards in our skies and the days continue getting longer. Again, we reach a midpoint when day and night are both 12 hours long. This is called the 'vernal (or spring) equinox' and happens around 21 March. Sun Over Equator (September 21)

Sunlight Reaching Earth at…
Winter Solstice Longest day of the year in the Northern Hemisphere. During the winter solstice (pictured above), the Northern Hemisphere day lasts fewer than 12 hours and the Southern Hemisphere day lasts more than 12 hours. During the winter solstice, the North Pole has a 24-hour night and the South Pole has a 24-hour day. During the winter solstice, sunlight strikes the earth most directly at 23.5 degrees South (the Tropic of Capricorn). During the summer solstice (not pictured), the Northern Hemisphere day lasts more than 12 hours and the Southern Hemisphere day lasts fewer than 12 hours. During the summer solstice, the North Pole has a 24-hour day and the South Pole has a 24-hour night. During the summer solstice, sunlight strikes the earth most directly at 23.5 degrees North (the Tropic of Cancer) Shortest day of the year in the Southern Hemisphere.

Tilt of Earth and Days The length of a day changes because the earth spins at a tilt and the Sun’s rays hit the earth more directly at different times of the year. Summer solstice (June 21) Northern Hemisphere - The day is longer than 12 hours Southern Hemisphere The day is shorter than 12 hours North Pole has a 24-hour day South Pole has a 24-hour night.

What happens during the Equinox?
Latin – equal) The day and night have an EQUAL amount of sunshine. The Sun’s rays hit the earth more directly at the equator.

Earth is ALWAYS tilted to the right.

Word Bank for homework Sunlight Summer Solstice Length
Revolution Closer Longer September 22 More time Spring Southern Away Indirect Autumn Directly Nights

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