The Reasons for Seasons Copyright 2005 by the Rector and Visitors of the University of Virginia 4/18/2019 The Seasons

Day Night Cycle 4/18/2019 The Seasons

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. 4/18/2019 The Seasons

The Constellations on the Ecliptic As the Earth revolves about the Sun, the Sun appears to move through a set of constellations called the zodiac. The path of the Sun through the sky is called the ecliptic. The sun travels through a set of “12” constellations (13 actually) that are called the zodiac. 4/18/2019 The Seasons

4/18/2019 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. 4/18/2019 The Seasons

4/18/2019 The Seasons

Tilt of the Earth’s Axis 4/18/2019 The Seasons

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. 4/18/2019 The Seasons

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. 4/18/2019 The Seasons

The Annual Path of the Sun On the summer solstice: the Sun will appear directly overhead to someone at 23.5 degrees north latitude. This latitude is called the Tropic of Cancer. The Sun does not set for people within 23.5 degrees of the North pole (above the Artic circle) The Sun does not rise for people within 23.5 degrees of the South pole (below the Antarctic Circle). 4/18/2019 The Seasons

Midnight Sun 4/18/2019 The Seasons

Earth on June 21 4/18/2019 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. 4/18/2019 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. 4/18/2019 The Seasons

Orbit of the Earth 4/18/2019 The Seasons

4/18/2019 The Seasons

The Seasons 4/18/2019 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. 4/18/2019 The Seasons

Earth on June 22 4/18/2019 The Seasons

Earth on December 22 4/18/2019 The Seasons

The Length of the Day A day is defined as the time that it takes the Earth to rotate on its axis. However, there is more than one way to define a day: A sidereal day is the time that it takes for the Earth to rotate with respect to the distant stars. A solar day is the time that it takes to rotate with respect to the Sun. 4/18/2019 The Seasons

The Length of the Day A solar day is slightly longer than a sidereal day. A sidereal day is 23h 56m 4.091s. We set our watches according to the solar day. Astronomers use sidereal time because we are mostly interested in distant celestial objects. 4/18/2019 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) 4/18/2019 The Seasons

Day Night Cycle 4/18/2019 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. 4/18/2019 The Seasons

Tilt of the Earth’s Axis 4/18/2019 The Seasons

Length of a Year It takes Earth one year to orbit the Sun A sidereal year is the length of time it takes for the Earth to return to the same position with respect to the stars. It is 365d 6h 9m 9.5s A tropical year is the length of time it takes the Sun to go from one vernal equinox to another. It is 365d 5h 48m 45.51s The difference is due to precession. 4/18/2019 The Seasons