EARTH-SUN RELATIONS (Energy of the Sun to Earth).ppt

4/25/2024 (c) Vicki Drake, SMC 1
EARTH-SUN
RELATIONS
Rotation, Revolution, Seasons

EARTH’S ROTATION
 The Earth rotates on
its axis
 One complete rotation
(3600) takes
approximately 24
hours
• Rotation is from West to
East
 Sun appears to ‘rise’ in
East and ‘set’ in West
• Rotation speed is
variable
 Fastest at the equator

EARTH’S REVOLUTION ABOUT
THE SUN
 The Earth revolves about the Sun
• One complete revolution takes
365.2422 days
 365 days, 5 hours, 48 minutes, 36 seconds
 Approximately 365 ¼ Earth days
 The Earth’s revolution is slightly
elliptical, not circular
• Direction of revolution is counter-
clockwise from an outer space
perspective

AXIS TILT AND REVOLUTION
 Earth moves in a constant
plane – Plane of the
Ecliptic – in its revolution
about the Sun
• All the planets (and even
the sun) are moving in the
Plane of the Ecliptic
 Earth’s axis is tilted about
23.50 from perpendicular
to Plane of Ecliptic
 Earth’s tilt has two
characteristics:
• Angle of inclination
• Parallelism

ANGLE OF INCLINATION AND
PARALLELISM
 The angle of
inclination, the tilt
of 23½ degrees, is
a constant.
• The angle does not
change throughout
the entire revolution
 Parallelism means
the axis is always
pointed in the
same direction
• The axis does not
point in different
directions as the
Earth moves in its
orbit

EARTH’S ELLIPTICAL
REVOLUTION
 The Earth, in its elliptical revolution,
has an average distance of
approximately 93,000,000 miles
from the Sun
 At two points in the revolution, the
distance varies
• Perihelion: Earth is closest to Sun,
~91.5 million miles
• Aphelion: Earth is farthest from Sun,
~95.5 million miles

PERIHELION AND APHELION
91,500,000
miles
95,500,000
miles

PERHELION AND APHELION -
DATES
 Perihelion occurs on, or about,
January 3
• Northern Hemisphere Winter
 Aphelion occurs on, or about, July 4
• Northern Hemisphere Summer

PERIHELION
 At Perihelion, the Earth’s orbit is the
closest to the Sun .
 The Northern Hemisphere is ‘tilted
away’ from the sun, receiving less
solar radiation, with shorter daylight
hours.
 This is the Winter period for the
Northern Hemisphere.

PERIHELION

APHELION
 At Aphelion, the Earth’s orbit is
furthest away from the Sun.
 The Northern Hemisphere is ‘tilted
toward’ the Sun, resulting in more
solar radiation, and longer daylight
hours.
 This is the Northern Hemisphere
Summer period.

APHELION

Changes in Axis Orientation, Tilt
and Revolution
 Orientation of Earth’s axis changes during a
23,000-year cycle called precession
 The Earth’s degree of tilt (obliquity) changes
through a 41,000-year cycle – ranging between
22.5 and 24 degrees
 Earth’s orbit (revolution) about the Sun changes
from nearly circular to elliptical and back every
100,000 years – this process is called eccentricity
 Milankovitch Theory: these changes can be linked
to long-term climate changes based on latitudinal
differences in insolation (incoming solar radiation)

CIRCLE OF ILLUMINATION
 During rotation, at any given time, half of the Earth is
receiving solar radiation – daylight
 The other half of the Earth is in darkness – night
 The ‘line’ separating day from night is the Circle of
Illumination
 The image below illustrates the Circle of Illumination
without the tilt of the axis

INSOLATION AND LATITUDES
 Insolation: solar radiation received by
the Earth (incoming solar radiation)
 Seasons: Variations of insolation due to
spherical surface of Earth
 Some latitudes receive more insolation:
• Angle of incidence
• Duration

INSOLATION AND LATITUDES
 Only one latitude, at any
time during Earth’s
revolution, receives
insolation at right angles at
noon
• The subsolar point on the
Earth
• Zenith Angle for Sun
 Intensity of insolation
measured by using Sun’s
zenith angle
• Sun’s angle above horizon
at local noon
 The angle at which Sun’s
rays strike Earth’s surface
determines amount of
insolation
• More direct angle =
greater insolation
Subsolar
point

LATITUDES and SUN RELATIONS
 The following three latitudes are important because of their significance
to seasons on the Earth
• On certain days of the year (Equinoxes and Solstices), the Sun’s Zenith Angle, at
local noon, will be 900 above one of these latitudes
 Equator: 00
• an imaginary line on the Earth's surface equidistant from the North Pole and South Pole
that divides the Earth into a Northern Hemisphere and a Southern Hemisphere
• Two days per year (Autumnal Equinox: September 21,22 and Vernal Equinox: March 20)
the Sun’s location, at local noon is directly over the Equator
 Tropic of Capricorn: 23½0 South
• One day per year (Winter Solstice: December 21, 22) the Sun’s location, at local noon, is
in the Capricorn constellation
 Tropic of Cancer: 23½0 North
• One day per year (Summer Solstice: June 21,22) the sun’s location, at local noon, is in
the Cancer constellation
-------------------------------------------------------------------------------
 Arctic Circle: 66½0 North
• marking the southern limit of the area where the sun does not rise on the Northern
Hemisphere winter solstice (December 21) or set on the summer solstice (June 21)
 Antarctic Circle: 66½0 South
• marks the northern limit of the area where the Sun does not set on the Southern
Hemisphere summer solstice (December 21) or rise on the winter solstice (June 21)

ZENITH ANGLE AND LATITUDES
– WITHOUT TILT
66 1/20 N
23 1/20 N
00
23 1/20 S
66 1/20 S

SOLSTICES, EQUINOXES, AND LATITUDES:
NORTHERN HEMISPHERE BIAS!
 Summer Solstice: Sun’s Zenith Angle of
900, at noon, is located at Tropic of
Cancer, 23.50 (23 ½0) North
• On or about June 21, 22
 Winter Solstice: Sun’s Zenith Angle of 900,
at noon, is located at Tropic of Capricorn,
23.50 (23 ½0) South
• On or about December 21, 22
 Vernal Equinox and Autumnal Equinox:
Sun’s Zenith Angle of 900, at noon, is
located at the Equator, 00
• On or about March 20 and September 21, 22
respectively

SUMMER SOLSTICE
 Summer Solstice, June
21, 22
 Northern Hemisphere is
tilted towards the Sun
 Latitudes higher than 66.50
North receive 24 hours of
sunlight
South receive 24 hours of
night
 Longest period of daylight
for one day in year for
Northern Hemisphere
latitudes
• First day of Summer:
Northern Hemisphere
Vertical rays of
Sun at noon

WINTER SOLSTICE
 Winter Solstice,
December 22
 Northern Hemisphere tilted
away from the Sun
North receive 24 hours of
night
South receive 24 hours of
daylight
 Longest period of night for
one day for Northern
Hemisphere latitudes
• First day of Winter:
Northern Hemisphere
Vertical rays
of sun at
noon

EQUINOXES: VERNAL, AUTUMNAL
Sun’s
vertical rays
at noon

VERNAL (SPRING) EQUINOX
 Vernal Equinox, March 20
 Zenith Angle of Sun at noon is 900 above Equator
 Day and night are of equal length at all locations
on the Earth
 First day of Spring, Northern Hemisphere
 Calendar (including specific dates) and even
monuments based on Vernal Equinox
• For example: the Council of Nice decreed in 325 A.D.
that "Easter was to fall upon the first Sunday after the
first full moon on or after the Vernal Equinox”
• Julian and Gregorian Calendar
• Early Egyptians built the Great Sphinx so that it points
directly toward the rising Sun on the day of the Vernal
Equinox.

AUTUMNAL (FALL) EQUINOX
 Autumnal Equinox, September 22
 Zenith Angle of Sun at noon is 900
above Equator
 Day and night are of equal length at
all locations on the Earth
 First day of Fall, Northern
Hemisphere

SEASONS AND EARTH’S REVOLUTION
Direct Rays
23 1/20 N Direct Rays
23 1/20 S

CALENDARS AND SEASONS
 Julian Calendar:
• Introduced in 46 BC
• A regular year of 365 days divided into
12 months, and a leap day is added to
February every four years.
 The Julian year is, on average, 365.25 days
long.

CALENDARS AND SEASONS
 Gregorian Calendar
 Decreed in 1582 by Pope Gregory
• Equinox and solstices almost two weeks early
on Julian Calendar
• Pope Gregory dropped 10 days from calendar
to put equinoxes and solstices back on track.
 October 4 followed by October 15
 Changes in Gregorian Calendar
• Add extra day to month of February every four
years: “Leap Year”
• Exception - only century years divisible by 400
become leap years

SEASONS
 Distance between Earth and Sun NOT a
determinant of seasons
• Perihelion occurs during Northern Hemisphere winter
 Determinant # 1: Angle of Incidence of Sun’s
rays striking Earth’s surface
• Latitudes receiving more perpendicular rays receive
more insolation for heating
 Determinant # 2: Length of daylight hours
• Longer daylight hours means more insolation
 Determinant # 3: Angle of Incidence and length
of daylight hours directly affected by tilt of
Earth’s axis

ANGLE OF INCIDENCE -
INSOLATION
The more
vertical the
rays of Sun
means a more
concentrated
amount of
solar radiation
for a location.

ANALEMMA – MAPPING THE
SUN’S MOVEMENT
 An analemma traces the annual
movement of the Sun on the sky.
• It illustrates the positions of the Sun at the
same time of day (at approximately 24 hour
intervals) and from the same location on Earth
on successive days through the calendar year.
• This apparent shift of Sun’s position is due to
the Earth’s orbit about the Sun
• An analemma appears as a ‘loopy’ figure eight
 the highest point is Summer
 the lowest point, Winter

ANALEMMA

ANALEMMA
 The Analemma has a
calendar printed on it
• This calendar indicates
which latitude (subsolar
point) receives the Sun’s
direct rays at noon
(“Zenith Angle”) on any
day of the year.
 The most northern
latitude is 23.50 North
 The most southern
latitude is 23.50 South

EARTH-SUN RELATIONS (Energy of the Sun to Earth).ppt

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