1. The Reasons for Seasons
Edward M. Murphy
Space Science for Teachers
2005
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 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 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. Obliquity of the Ecliptic
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9. Obliquity of the Ecliptic
June
December
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10. Tilt of the Earth’s Axis
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11. 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
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12. Equinoxes and Solstices
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
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13. Motion on the Ecliptic
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15. Tropics
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16. 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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17. Earth on June 21
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18. Standing on the North Pole
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19. Midnight Sun
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20. Standing on the Tropic of Cancer
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21. 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.
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22. Standing on the Equator
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23. 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)
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24. Earth on December 21
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25. 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.
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26. 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.
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27. 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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28. Orbit of the Earth
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30. The Seasons
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31. 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.
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32. Solar Illumination
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33. Earth on June 22
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34. Earth on December 22
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35. 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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36. 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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37. Sidereal Time
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38. Sidereal Time
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39. 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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40. 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.
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41. Day Night Cycle
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42. 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.
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43. Sidereal Time
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44. 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.
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45. 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.
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46. Sundial
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47. 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.
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48. Analemma
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49. Analemma
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50. 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.
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51. 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.
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52. 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.
Time zones have been modified for political, social and economic reasons.
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.
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53. 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.
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54. Time Zones
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55. 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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56. Precession The Earth not only spins like a top, but it wobbles.
The period of the wobble is 25,725 years.
This wobble causes the North and South celestial poles to move through the sky.
Discovered in 129 B.C by Hipparchus.
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57. Precession of the` Earth’s Axis
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58. Precession
The vernal equinox moves through the constellations of the zodiac.
Currently, the vernal equinox is in Pisces.
It was in Aries about 2500 years ago, and the solstices were in Cancer and Capricornus and the autumnal equinox was in Libra (balance).
About 4000 years ago the vernal equinox was in Taurus (bull associated with fertility).
In 2700 A.D., the vernal equinox moves into the constellation of Aquarius.
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59. Tilt of the Earth’s Axis
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60. 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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