The Reasons for Seasons

1
The Reasons for Seasons

• Edward M. Murphy
• Space Science for Teachers
• 2005

2
Day Night Cycle
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 365.26
  days.

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.

5
(No Transcript)
6
Tilt of the Earths Axis

• The axis around which the Earth rotates is tilted
  by 23.5 degrees with respect to the ecliptic.

7
(No Transcript)
8
Obliquity of the Ecliptic
9
Obliquity of the Ecliptic
June
December
10
Tilt of the Earths Axis
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

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

13
Motion on the Ecliptic
14
(No Transcript)
15
Tropics
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).

17
Earth on June 21
18
Standing on the North Pole
19
Midnight Sun
20
Standing on the Tropic of Cancer
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.

22
Standing on the Equator
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)

24
Earth on December 21
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.

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 Earths 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.

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.

28
Orbit of the Earth
29
(No Transcript)
30
The Seasons
31
The Seasons

• In fact, the seasons are due to the tilt of the
  Earths 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.

32
Solar Illumination
33
Earth on June 22
34
Earth on December 22
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.

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.

37
Sidereal Time
38
Sidereal Time
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)

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.

41
Day Night Cycle
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.
• Keplers 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.

43
Sidereal Time
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.

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.

46
Sundial
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.

48
Analemma
49
Analemma
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.

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.

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 360/1524,
  the time in each zone differs from the time in
  adjacent zones by one hour.

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.

54
Time Zones
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.

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.

57
Precession of the Earths Axis
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.

59
Tilt of the Earths Axis
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.