The Reasons for Seasons

The Reasons for Seasons Edward M. Murphy Space Science for Teachers 2005 The Seasons

Day Night Cycle 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. 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. The Seasons

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. The Seasons

The Seasons

Obliquity of the Ecliptic The Seasons

Obliquity of the Ecliptic June December The Seasons

Tilt of the Earth’s Axis 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. • Position of the Sun: R.A. 0h, Dec 0o • The Summer Solstice(about June 21): The location where the Sun is at its furthest north. • Position of the Sun: R.A. 6h, Dec +23.5o The Seasons

The Autumnal (Fall) Equinox(about September 21): Where the Sun crosses the equator when going from north to south. Position of the Sun: R.A. 12h, Dec 0o The Winter Solstice(about December 21): The location where the Sun is at its furthest south. Position of the Sun: R.A. 18h, Dec –23.5o Equinoxes and Solstices The Seasons

Motion on the Ecliptic The Seasons

The Seasons

Tropics 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). The Seasons

Earth on June 21 The Seasons

Standing on the North Pole The Seasons

Midnight Sun The Seasons

Standing on the Tropic of Cancer The Seasons

The Annual Path of the Sun • On the autumnal equinox: • The Sun will appear directly overhead to someone on the equator at 0 degress latitude. • The Sun sets at the North Pole, ending 6 straight months of day and beginning 6 straight months of night. • The Sun rises at the South Pole ending 6 straight months of night and beginning 6 straight months of day. The Seasons

Standing on the Equator The Seasons

The Annual Path of the Sun • On the winter solstice: • The Sun will appear directly overhead to someone at 23.5 degress south latitude, the Tropic of Capricorn. • The Sun does not set for people within 23.5 degrees of the South Pole (below the Antarctic Circle) • The Sun does not rise for people within 23.5 degrees of the North Pole (above the Arctic Circle) The Seasons

Earth on December 21 The Seasons

The Annual Path of the Sun • On the vernal equinox: • The Sun will appear directly overhead to someone on the equator at 0 degress latitude. • The Sun rises at the North Pole, ending 6 straight months of night and beginning 6 straight months of day. • The Sun sets at the South Pole ending 6 straight months of day and beginning 6 straight months of night. 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. 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. The Seasons

Orbit of the Earth The Seasons

The Seasons

The Seasons 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. The Seasons

Solar Illumination The Seasons

Earth on June 22 The Seasons

Earth on December 22 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. 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. The Seasons

Sidereal Time 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) The Seasons

Apparent Solar Time • Apparent solar time is the time measured with respect to the actual position of the Sun. • At noon, the Sun would be exactly on the meridian. • 1 P.M. would be exactly one hour after the Sun was on the meridian. • 9 A.M. would be exactly 3 hours before the Sun was on the meridian. • The apparent solar time depends on your longitude. The Seasons

Day Night Cycle The Seasons

Apparent Solar Time • The length of an apparent solar day varies throughout the year. • Although the rotation of Earth is fairly constant, the revolution speed of Earth in orbit around the Sun is not. • Kepler’s Second Law tells us that Earth moves faster in January when it is close to the Sun and slower in July when it is further from the Sun. • In one day in January, Earth must rotate a little bit more than one day in July in order to bring the Sun back to the meridian because Earth has moved further in its orbit during that one day. The Seasons

Sidereal Time The Seasons

Mean Solar Time • Therefore, the length of an apparent solar day is variable. • Rather than constantly reseting our watches as the length of a solar day varies, we keep time using mean solar time. • A mean solar day is the average length of a solar day during the year. • Mean solar time is the time kept by a fictitious “Sun” moving at a uniform rate along the equator. The Seasons

Mean Solar Time • A sundial keeps apparent solar time and it will differ from the time on your watch during the course of a year. • This means that the true Sun is not always on the meridian at exactly noon. • Sometimes the Sun is on the meridian before noon and sometimes after noon. • The difference, called the equation of time, can be as much as 17 minutes. The Seasons

Sundial The Seasons

Apparent Solar Time • The path of the Sun at noon during the year makes a figure 8 shape called the analemma. • The north-south motion is due to the 23.5 degree tilt of the celestial sphere with respect to the ecliptic. • The east-west motion is primarily caused by the varying speed of Earth in its orbit around the Sun. The Seasons

Analemma The Seasons

Time Zones • Both the mean solar time and the apparent solar time differ with longitude. • 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 meridian). • Even if you travel only a few miles west, the Sun moves off the meridian. • Each city would have its own time. The Seasons

The Reasons for Seasons