The Reasons for Seasons Edward M. Murphy Space Science for Teachers 2005 Copyright 2005 by the Rector and Visitors of the University of Virginia 11/14/2018 The Seasons

Day Night Cycle 11/14/2018 The Seasons

Rotation vs. Revolution Rotation is the spin of an object about 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. 11/14/2018 The Seasons

The Zodiac As the Earth revolves around the sun the different constellations of the Zodiac appear in the sky What are the 12 signs of the zodiac? Pieces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius 11/14/2018 The Seasons

11/14/2018 The Seasons

Tilt of the Earth’s Axis The axis around which the Earth rotates is tilted by 23.5 degrees with respect to the ecliptic. 11/14/2018 The Seasons

11/14/2018 The Seasons

Tilt of the Earth’s Axis 11/14/2018 The Seasons

Tropics 11/14/2018 The Seasons

Equinoxes and Solstices The Vernal (Spring) Equinox (about March 21): 1st day of spring Every place on earth has 12 hrs day light Sun is directly over equator at 12 noon 11/14/2018 The Seasons

Equinoxes and Solstices The Summer Solstice (about June 21): 1st day of summer Longest day of the year Sun is over tropic of Cancer Sun does not set above arctic circle 11/14/2018 The Seasons

Earth on June 21 11/14/2018 The Seasons

Equinoxes and Solstices The Autumnal (Fall) Equinox (about September 21): 1st day of Fall Every place on earth has 12 hrs day light Sun is directly over equator at 12 noon 11/14/2018 The Seasons

Equinoxes and Solstices The Winter Solstice (about December 21): 1st day of winter shortest day of the year Sun is over tropic of Capricorn 11/14/2018 The Seasons

Earth on December 21 11/14/2018 The Seasons

Motion on the Ecliptic 11/14/2018 The Seasons

11/14/2018 The Seasons

Standing on the North Pole 11/14/2018 The Seasons

Midnight Sun 11/14/2018 The Seasons

Standing on the Tropic of Cancer 11/14/2018 The Seasons

Standing on the Equator 11/14/2018 The Seasons

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. Perihelion (closest to the Sun) is around January 3 when Earth is about 91,405,436 miles from the Sun. Aphelion (farthest from the Sun) is around July 4 when Earth is about 94,511,989 miles from the Sun. 11/14/2018 The Seasons

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. 11/14/2018 The Seasons

Orbit of the Earth 11/14/2018 The Seasons

11/14/2018 The Seasons

The Seasons 11/14/2018 The Seasons

The Seasons Seasons are due to the tilt of the Earth’s axis. Summer: The sunlight is spread out over less ground and heats the ground better. The Sun is above the horizon for a longer period of time. 11/14/2018 The Seasons

Solar Illumination 11/14/2018 The Seasons

Earth on June 22 11/14/2018 The Seasons

Earth on December 22 11/14/2018 The Seasons

Time: 11/14/2018 The Seasons

Day Night Cycle 11/14/2018 The Seasons

Why is there Time Zones? Problem: If the sun travels slowly across the US technically each city will have a different time Imagine starting in Charlottesville at exactly noon. As you travel to the west, the Sun will appear further east in the sky (i.e. lower and further from the sun). Even if you travel only a few miles west, the Sun gets lower in the horizon. Each city would have its own time. 11/14/2018 The Seasons

Time Zones With the advent of rapid travel by trains in the 19th century, it became necessary to standardize the time for all cities within a certain region. In November 1883, the railroad companies divided the United States into four time zones. Everyone in a time zone set their clocks to the same standard time. 11/14/2018 The Seasons

Time Zones In 1884, an international conference was held in Washington D.C. by 26 countries. The world was divided into 24 times zones, with each zone being roughly 15 degrees wide in longitude. Divided into 24 time zones one for each hour of the day. Since there are 24 hours in a day, and 360/15=24, the time in each zone differs from the time in adjacent zones by one hour. 11/14/2018 The Seasons

International Date Line Standard time gets earlier as you travel to the west. The International Date Line line was established in the middle of the Pacific Ocean. As you go from east to west, you gain a day as you cross the line. As you go from west to east, you lose a day as you cross the line. 11/14/2018 The Seasons

Time Zones 11/14/2018 The Seasons

Daylight Saving Time.. Why? 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. Now is used so people can make better use of time in the afternoons 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. 11/14/2018 The Seasons

Tilt of the Earth’s Axis 11/14/2018 The Seasons