SeasonsSeasonsEarth – Sun RelationshipsEarth – Sun Relationships

Why do we have seasons?Why do we have seasons? Seasons are the result of Earth’s tilt, in relation Seasons are the result of Earth’s tilt, in relation

to our position around the sun.to our position around the sun.

Key terms:Key terms:

RotationRotation: Earth : Earth spinningspinning on its axis on its axis RevolutionRevolution: Earth : Earth revolvingrevolving around the sun. around the sun. Plane of the EclipticPlane of the Ecliptic: An imaginary path of the Earth : An imaginary path of the Earth

around the sun.around the sun.

Parallelism of the EclipticParallelism of the Ecliptic: Earth’s position is always : Earth’s position is always parallel to itself throughout its orbit.parallel to itself throughout its orbit.

SolsticesSolstices: When the sun’s vertical rays are farthest : When the sun’s vertical rays are farthest from the equator (summer and winter)from the equator (summer and winter)

EquinoxEquinox: When sun’s rays are vertical at the equator; : When sun’s rays are vertical at the equator; everywhere on Earth has 12 hours of day/night. everywhere on Earth has 12 hours of day/night. (Spring/Fall)(Spring/Fall)

Circle of IlluminationCircle of Illumination: Boundary between day and : Boundary between day and night ~ ** Percentage lit is always the same!!! **night ~ ** Percentage lit is always the same!!! **

Solstice & EquinoxSolstice & Equinox

On the solstices, the sun’s On the solstices, the sun’s rays are direct on either the rays are direct on either the tropic of Cancer (summer) tropic of Cancer (summer) or the tropic of Capricorn or the tropic of Capricorn (winter)(winter)

On the equinoxes, the sun’s On the equinoxes, the sun’s rays are direct on the rays are direct on the equator (Spring and Fall)equator (Spring and Fall)

*(PS: Look at the circle of *(PS: Look at the circle of illumination . . .)illumination . . .)

Hours of Daylight- The number of daylight hours (i.e. duration of insolation) is affected by our position.

Winter solstice – 12 hours at Winter solstice – 12 hours at the equator, 8 hours for us, 0 the equator, 8 hours for us, 0 hours at the North Pole, 24 hours at the North Pole, 24 hours at the South Pole.hours at the South Pole.

Summer solstice –12 hours Summer solstice –12 hours at the equator, 16 hours for at the equator, 16 hours for us, 24 hours at the North us, 24 hours at the North Pole, 0 hours at the South Pole, 0 hours at the South Pole.Pole.

WinterWinter

SummerSummer

Equinoxes – same Equinoxes – same number of daylight number of daylight hours everywhere: 12 hours everywhere: 12 hours at the equator, for hours at the equator, for us and us and bothboth poles. poles.

The Fall Equinox and The Fall Equinox and the Spring Equinox are the Spring Equinox are identical in terms of identical in terms of position and duration – position and duration – it’s just the it’s just the directiondirection that’s different.that’s different.

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

Each of the poles will Each of the poles will experience 24 hours of experience 24 hours of darkness and 24 hours darkness and 24 hours of light, based on the of light, based on the season.season.

The North Pole has The North Pole has darkness in the winter darkness in the winter and light in the summer and light in the summer and the opposite is true and the opposite is true for the South Pole.for the South Pole.

Our seasons are Our seasons are reversed with the reversed with the southern hemisphere southern hemisphere because of our tilt and because of our tilt and position. position.

In the summer, we’re In the summer, we’re facing the sun, in the facing the sun, in the winter, they’re facing winter, they’re facing the sun.the sun.

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

23.523.5º North: Tropic of Cancerº North: Tropic of Cancer 23.523.5º South: Tropic of Capricornº South: Tropic of Capricorn

The tilt of the Earth and our orbit around the sun determine the The tilt of the Earth and our orbit around the sun determine the duration of insolation (length of daylight hours) and seasons.duration of insolation (length of daylight hours) and seasons.

On the winter solstice, we’re facing away from the sun and On the winter solstice, we’re facing away from the sun and have short/cold days.have short/cold days.

On the spring/fall equinoxes we are in line with the sun and 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 experience 12 hours of day and 12 hours of night. Earth is warming towards the sun in the spring and cooling away from warming towards the sun in the spring and cooling away from the sun in fall.the sun in fall.

On the summer solstice we’re facing the sun and have On the summer solstice we’re facing the sun and have long/warm days.long/warm days.