1. Earth – Sun Relationships
Seasons
Earth – Sun Relationships

2. Why do we have seasons? Key terms:
Seasons are the result of Earth’s tilt, in relation to our position around the sun.
Key terms:
Rotation: Earth spinning on its axis
Revolution: Earth revolving around the sun.
Plane of the Ecliptic: An imaginary path of the Earth around the sun.

3. Parallelism of the Ecliptic: Earth’s position is always parallel to itself throughout its orbit.
Solstices: When the sun’s vertical rays are farthest from the equator (summer and winter)

4. Equinox: When sun’s rays are vertical at the equator; everywhere on Earth has 12 hours of day/night. (Spring/Fall)
Circle of Illumination: Boundary between day and night ~ ** Percentage lit is always the same!!! **

5. Solstice & Equinox
On the solstices, the sun’s rays are direct on either the tropic of Cancer (summer) or the tropic of Capricorn (winter)
On the equinoxes, the sun’s rays are direct on the equator (Spring and Fall)
*(PS: Look at the circle of illumination . . .)

6. Hours of Daylight - The number of daylight hours (i. e
Hours of Daylight - The number of daylight hours (i.e. duration of insolation) is affected by our position.
Winter solstice – 12 hours at the equator, 8 hours for us, 0 hours at the North Pole, 24 hours at the South Pole.
Summer solstice –12 hours at the equator, 16 hours for us, 24 hours at the North Pole, 0 hours at the South Pole.
Winter
Summer

7. Equinoxes – same number of daylight hours everywhere: 12 hours at the equator, for us and both poles.
The Fall Equinox and the Spring Equinox are identical in terms of position and duration – it’s just the direction that’s different.

8. The sun is never directly overhead at our latitude because we are too far north of the equator. It will only be direct over the tropics.

9. Each of the poles will experience 24 hours of darkness and 24 hours of light, based on the season.
The North Pole has darkness in the winter and light in the summer and the opposite is true for the South Pole.

10. Our seasons are reversed with the southern hemisphere because of our tilt and position.
In the summer, we’re facing the sun, in the winter, they’re facing the sun.

11. The tropic zone is bounded by 23
The tropic zone is bounded by 23.5 º because that is the position of our tilt ~ the sun’s rays are always direct between the Tropic of Cancer and the Tropic of Capricorn.
23.5º North: Tropic of Cancer
23.5º South: Tropic of Capricorn

12. The tilt of the Earth and our orbit around the sun determine the duration of insolation (length of daylight hours) and seasons.
On the winter solstice, we’re facing away from the sun and have short/cold days.
On the spring/fall equinoxes we are in line with the sun and experience 12 hours of day and 12 hours of night. Earth is warming towards the sun in the spring and cooling away from the sun in fall.
On the summer solstice we’re facing the sun and have long/warm days.