2/25/2016the seasons1 the reasons for seasons. 2/25/2016the seasons2 day night cycle

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The Reasons for Seasons

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Day Night Cycle

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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 365.26 days.

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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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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.

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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.

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Midnight Sun

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Earth on June 21

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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.

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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.

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Orbit of the Earth

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The Seasons

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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.

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Earth on June 22

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Earth on December 22

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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)

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Day Night Cycle

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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.