1. The Reasons for Seasons
Copyright 2005 by the Rector and Visitors of the University of Virginia
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2. Day Night Cycle
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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 days.
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4. 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.
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6. Tilt of the Earth’s Axis
The axis around which the Earth rotates is tilted by 23.5 degrees with respect to the ecliptic.
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8. Tilt of the Earth’s Axis
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9. 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.
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10. 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.
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11. 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).
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12. Midnight Sun
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13. Earth on June 21
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14. 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.
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15. 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.
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16. Orbit of the Earth
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18. The Seasons
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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.
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20. Earth on June 22
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21. Earth on December 22
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22. 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.
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23. 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.
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24. 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)
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25. Day Night Cycle
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26. 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.
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27. Tilt of the Earth’s Axis
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28. 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.
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