The Reasons for Seasons Copyright 2005 by the Rector and Visitors of the University of Virginia 4/8/2019 The Seasons

Day Night Cycle 4/8/2019 The Seasons

Rotation vs. Revolution Rotation is the spin of the Earth around its axis. The Earth rotates once a day (every 24 hours). Revolution is the orbit of one object around another. The Earth revolves around the Sun every 365 days. 4/8/2019 The Seasons

Tilt of the Earth’s Axis The axis around which the Earth rotates is tilted by 23.5 degrees This is what causes seasons. 4/8/2019 The Seasons

4/8/2019 The Seasons

Tilt of the Earth’s Axis 4/8/2019 The Seasons

Equinoxes and Solstices The Vernal (Spring) Equinox (March) Equal day and night The Summer Solstice (June) Longest day of the year The Autumnal (Fall) Equinox (Sept.) Equal day and night The Winter Solstice (December) Shortest day of the year 4/8/2019 The Seasons

Midnight Sun 4/8/2019 The Seasons

Earth on June 21 4/8/2019 The Seasons

Seasons Misconception Many people think that the seasons are due to the distance of the Earth from the Sun. However: 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. If the seasons were due to our distance from the Sun both hemispheres would have the same seasons at the same time. 4/8/2019 The Seasons

Orbit of the Earth 4/8/2019 The Seasons

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. 4/8/2019 The Seasons

Earth on June 22 4/8/2019 The Seasons

Earth on December 22 4/8/2019 The Seasons

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) 4/8/2019 The Seasons

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. 4/8/2019 The Seasons

animations Seasons use code cfp-5012 http://esminfo.prenhall.com/science/geoanimations/animations/01_EarthSun_E2.html Cosmic Map http://www.nationalgeographic.com/xpeditions/activities/07/popup/cosmic.html

Now, see what you know about the seasons! http://www.teachersdomain.org/ext/ess05_int_seasonsgame/index.html