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05/06/23 The Seasons 1
The Reasons for Seasons
05/06/23 The Seasons 2
Day Night Cycle
05/06/23 The Seasons 3
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.
05/06/23 The Seasons 4
Tilt of the Earth’s Axis• The axis around which the Earth
rotates is tilted by 23.5 degrees with respect to the ecliptic.
05/06/23 The Seasons 5
05/06/23 The Seasons 6
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.
05/06/23 The Seasons 7
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.
05/06/23 The Seasons 8
Midnight Sun
05/06/23 The Seasons 9
Earth on June 21
05/06/23 The Seasons 10
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.
05/06/23 The Seasons 11
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.
05/06/23 The Seasons 12
Orbit of the Earth
05/06/23 The Seasons 13
05/06/23 The Seasons 14
The Seasons
05/06/23 The Seasons 15
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.
05/06/23 The Seasons 16
Earth on June 22
05/06/23 The Seasons 17
Earth on December 22
05/06/23 The Seasons 18
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)
05/06/23 The Seasons 19
Day Night Cycle
05/06/23 The Seasons 20
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.