Download Sun Rotation and Tilt

Sun Rotation and Tilt
TEACHERS NOTES
Recommended year levels: 5-8
OBJECTIVES
1.
2.
To determine why the Sun appears to move across the sky.
To determine why seasons are reversed in the Northern and
Southern
Hemispheres.
BACKGROUND INFORMATION
Since we are moving with Earth as it rotates, it appears that the Sun is
moving across the sky form east to west but it is actually Earth that is
moving from west to east. Because of the tilt of Earth’s axis relative to its orbit around the
Sun, the Sun’s altitude at midday varies during the year, and the Sun does not rise and set
day after day in the same place.
Earth’s axis is tilted at an angle of 23.5 degrees from the perpendicular to its orbit around the
Sun. As Earth revolves around the Sun, sunlight falls more intensely on different parts of
Earth during the year, causing the seasons. The position of the Earth due to its axis results
in reversed seasons in the Northern and Southern Hemispheres. The tilt changes the
concentration of the Sun’s rays that reach certain regions of Earth and the number of hours
of sunlight each day.
In Australia our winter starts on the 1 June however in the northern hemisphere seasons
change in accordance with equinoxes and solstices.
WHAT YOU NEED
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wooden skewer
Styrofoam ball or clay sphere
Torch
Ruler
Drawing pin/thumb tac
Card with pictures of stars
Card indication sun labeled on right side ‘west’ and on left side ‘east’.
protractor
WHAT TO DO
Experiment 1 Rotation
1.
Push the skewer through the centre of the Styrofoam ball until it comes out the other
end.
2.
Draw a perpendicular to the skewer around the middle of the ball. This represents the
equator, dividing the northern and southern hemisphere.
3.
Place the drawing pin below the line to represent an observer in the Southern
Hemisphere.
4.
Place cards about 30cm apart facing each other.
5.
Stand the model Earth between the index cards (you could use a torch to represent the
sun instead of the card) so that the paper clip observer faces the
star card. Slightly tilt the model toward the Sun.
6.
Slowly rotate the model Earth counter-clockwise until the paper clip observer faces the
left edge (east side) of the Sun card.
© Gaffney, L., and Harper, A. 2009. Queensland University of Technology.
RESULTS
As the Earth model rotates away from the stars, the paper clip observer on Earth faces the
west side of the Sun first and the east side last. The Sun appears to rise above the eastern
horizon, move across the sky and set below the western horizon. If you could view Earth
from above the North Pole you would see Earth rotating counter clockwise. The drawing pin
observer first sees the western side of the Sun diagram, and then the eastern side comes
into view as the sphere rotates.
QUESTION
1.
Is every point on Earth’s orbit an equal distance from the Sun? No, the distance
between Earth and the Sun changes during Earth’s orbit around the Sun, it is an elliptical
orbit (stretched out circle or oval). The point when the Earth is closest to the sun it is called
perihelion, the point where it is furthest away this point is called aphelion. Earth reaches
perihelion during January and aphelion in July. This does not cause the seasons.
aphelion
Sun
WHAT TO DO
Experiment 2 Tilt
1.
2.
3.
4.
5.
Earth
Hold the torch about 15m from the model Earth. The flash represents the Sun.
Use a protractor to measure as you tilt the pointed end of the pencil/skewer (the North
Pole) about 23 degrees toward the torch. Observe the area of the sphere that is lit up
by the torch.
Draw your observation of the area of light and shade on the model.
Tilt the eraser end of the pencil (the Southern Hemisphere) about 23 degrees toward
the torch.
Draw your observation of the area of light and shade on the model.
RESULTS
When the top of the sphere is tilted toward the torch more light hits the top half of the sphere
than the bottom. The reverse is true when the top of the sphere is tilted away from the torch.
QUESTIONS
1.
2.
What is the special name given and the date it occurs in 2009, to the day of the year
when Earth’s Southern pole is tilted closest to the sun? Answer: Summer solstice,
December 22, 2009.
What is the special name given and the date it occurred in 2009, to the day of the year
when Earth’s Southern pole is tilted furthest away from the Sun? Answer: Winter
Solstice, June 21, 2009.
© Gaffney, L., and Harper, A. 2009. Queensland University of Technology.
perihelion
CURRICULUM CONCEPTS ADDRESSED
Essential Learnings: Earth and Beyond
By the end of year 5:
• The earth, solar system ad universe are dynamic systems eg the idea that the planets
orbit the sun can be used to explain day and night around the sun can be
By the end of year 7:
• Interactions and changes in physical systems and environments can be explained and
predicted
RESOURCES USED TO DEVELOP THIS ACTIVITY
1.
VanCleave, J. (2001). Janice VanCleave’s Teaching the Fun of Science New York:
Wiley
© Gaffney, L., and Harper, A. 2009. Queensland University of Technology.