2/25/2016The Seasons1 The Reasons for Seasons

2/25/2016The Seasons2 Day Night Cycle

2/25/2016The Seasons3 Rotation vs. Revolution Rotation is the spin of an object around its axis. –The Earth rotates once a day (once every 24 hours). Revolution is the orbit of one object around another. –The Earth revolves around the Sun every days.

2/25/2016The Seasons4 Tilt of the Earth’s Axis The axis around which the Earth rotates is tilted by 23.5 degrees with respect to the ecliptic.

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2/25/2016The Seasons6 Equinoxes and Solstices The Vernal (Spring) Equinox (about March 21): The location where the Sun crosses the equator when going from south of the equator to north of the equator. The Summer Solstice (about June 21): The location where the Sun is at its furthest north.

2/25/2016The Seasons7 Equinoxes and Solstices The Autumnal (Fall) Equinox (about September 21): Where the Sun crosses the equator when going from north to south. The Winter Solstice (about December 21): The location where the Sun is at its furthest south.

2/25/2016The Seasons8 Midnight Sun

2/25/2016The Seasons9 Earth on June 21

2/25/2016The Seasons10 Seasons Misconception Many people carry the misconception that the seasons are due to the distance of the Earth from the Sun. However, consider the following facts: –The Earth’s orbit around the Sun is nearly a perfect circle. The Earth is slightly closer to the Sun in January and farther from the Sun in July.

2/25/2016The Seasons11 Seasons Misconception –While it is winter in the Northern hemisphere it is summer in the Southern hemisphere. If the seasons were due to our distance from the Sun both hemispheres would have the same seasons at the same time.

2/25/2016The Seasons12 Orbit of the Earth

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2/25/2016The Seasons14 The Seasons

2/25/2016The Seasons15 The Seasons In fact, the seasons are due to the tilt of the Earth’s axis. Consider what happens on June 21 when the northern hemisphere of the Earth is tilted toward the Sun: The sunlight strikes the ground more vertically than in December. The light is spread out over less ground and heats the ground better. The Sun is above the horizon for a longer period of time.

2/25/2016The Seasons16 Earth on June 22

2/25/2016The Seasons17 Earth on December 22

2/25/2016The Seasons18 A.M. and P.M. At midday, the Sun is on your meridian. –This occurs close to, or at, noon. –A.M. comes from ante meridiem (before midday) –P.M. comes from post meridiem (after midday)

2/25/2016The Seasons19 Day Night Cycle

2/25/2016The Seasons20 Daylight Saving Time During the late spring, summer, and early fall, we set out clocks ahead to have an extra hour of daylight at the end of the day. –This change in time is called Daylight Saving Time. –The idea of changing our clocks was first used in the United States during World War I to conserve energy. Since 1986, the United States has set our clocks one hour ahead on the first Sunday in April and one hour behind on the last Sunday in October. –Spring ahead, fall back.