LEARNING GOALS Investigation 3- Seasons and Sun Enduring Understanding: Seasons are related to the amount of solar energy transferred to Earth, and that amount of energy transferred is affected by latitude, tilt of Earth’s axis, revolution, and rotation. Learning Goals: I will graph monthly day-length data for a single location to look for a pattern. I will use a globe and light bulb to model daily and seasonal variations in day length. I will explain how the tilt of Earth’s axis and Earth’s revolution around the sun produces seasons. I will discuss seasonal variation in day length as a consequence of axis tilt, rotation, and revolution. I will use light sources and surfaces to model beam spreading. I will explain how beam spreading reduces the intensity of solar radiation.

Seasons and Sun

GLOBAL DAYLIGHT When one location on Earth has 14 hours of daylight, is all the rest of the world having 14 hours of daylight as well? Is the longest day of the year the same length all over the world? Is the longest day of the year the same day all over the world? Are the longest days always in the summer? Are the shortest days always in the winter?

What ideas do you have to explain why daylight hours change over a year?

SUN-EARTH MODEL How should I set up the model? Where should the Sun be and where should Earth be?

REVOLVE The movement of one object around another. Earth revolves around the sun. How long does it take for Earth to make one revolution around the Sun? 1 year or 365 days

AXIS Earth has a North Pole and a South Pole. These are the north and south ends of an imaginary axle, called an axis, on which Earth rotates.

ROTATION Earth turns like a top, and this turning motion is called rotation. Earth does not spin straight up and down, but is tipped over at a significant angle of 23.5 degrees.

LET’S THINK… How long does it take for Earth to rotate (turn around once) on its axis? 1 day, or 24 hours How do we know where it is day and where it is night on these globes? The side illuminated by the Sun is day, the dark side is night. How much of the globe is in daylight at any given time? The globe is always exactly half light, and half dark. ALWAYS!

NORTH STAR There is a star in the night sky called Polaris, or the North Star. It is positioned directly over Earth’s North Pole. Earth’s North Pole always points to the North Star- summer, fall, winter, spring, day, and night.

NORTH STAR That’s why the North Star has been a navigation aid to seafarers and explorers for centuries.

NORTH STAR In our Sun-Earth system, we need a North Star. You will have to visualize the North Star through this wall and way, way off in the distance. The North Pole of our model Earth will always point at the North Star as it revolves around our model Sun.

KEY QUESTION Why do hours of daylight differ depending on time of year and location on Earth? Use our model to help us figure it out.

KEY QUESTION Position Earth and the Sun on the same plane. Keep Earth’s North Pole pointed at the North Star at all times. Place Earth at various positions around the Sun. Rotate Earth on its axis at each position, and observe the time in the light and the time out of the light.

DEMONSTRATION! For each stop, think about these questions. Which region of Earth is getting the greatest number of hours of daylight now? Which is experiencing the most hours of darkness? When Earth is at this point in its revolution, what season is it in the Northern Hemisphere (our town)?

SUMMARY Is there a part of Earth that experiences only daylight or only darkness? Yes, above the Arctic Circle and below the Antarctic Circle Is there ever a time when the day and night are equal everywhere on Earth? Yes How many times does this happen during a revolution? Twice!

EQUINOX There are 2 days during the year that day and night hours are equal. These days are called equinoxes. Equinox comes from Latin, meaning “equal night”. One equinox, the spring or vernal equinox, occurs around March 21. When do you think the other equinox occurs? Around September 21, in the fall. This equinox is the fall or autumnal equinox.

SOLSTICE When is the longest day of the year in the Northern Hemisphere? Which way is the North Pole tilted, toward the Sun or away from the Sun?

SOLSTICE The longest day of the year is the summer solstice. Solstice means “Sun stands still”. When is the shortest day of the year in the Northern Hemisphere? The shortest day of the year is the winter solstice.

SUN-EARTH SYSTEM Turn to this page in your lab books.

READING: “Seasons” page 17

Rotation- Earth turning around on its axis. One rotation=24 hours. WORD BANK Rotation- Earth turning around on its axis. One rotation=24 hours. Revolution- One object moving around another. One Earth revolution around the Sun=365 ¼ days. Axis- Imaginary line from North to South pole on which Earth rotates. Tilted at 23.5 degrees.

Equinox- Day and night hours are equal. WORD BANK Polaris (North Star)- Star directly over Earth’s North Pole. Always points North. Equinox- Day and night hours are equal. Spring Equinox=March 21 Fall Equinox=September 21 Summer Solstice- Longest day of the year. June 21 Winter Solstice- Shortest day of the year. December 21