2. Earth-Sun Relationship
• 1 year= 365.24 days
It takes Earth 1 year to revolve around the
sun. We have leap year every four years
to make up for the .24
3. Like all planets in our solar system, the Earth is in an elliptical orbit around our Sun.
In Earth's case, its orbit is nearly circular, so that the difference between Earth's
farthest point from the Sun and its closest point is very small. Earth's orbit defines
a two-dimensional plane which we call the ecliptic.
It takes roughly 365 days for the Earth to go around the Sun once. This means that
the Earth is rushing through space around the Sun at a rate of about 67,000 miles
per hour! The time it takes for the Earth to go around the Sun one full time is what
we call a year.
The combined effect of the Earth's orbital motion and the tilt of its rotation axis
result in the seasons.
4. Rotation and Revolution
• Earth rotates on its axis (counter clockwise)
– It takes one day, 24 hours to complete one rotation
– As Earth rotates, half of the Earth is always
illuminated by the sun and half of the Earth is always
dark.
• Earth revolves around the sun (also counter
clockwise)
– It takes one year, 365 days, to complete one
revolution
5. Circle of Illumination
• This is the border between night and day.
• It is constantly moving across the Earth.
6. Earth’s Axial Tilt=23.5°
• The tilt of Earth’s axis one of the two reasons for the seasons
– Imagine if Earth was not tilted. The sun’s rays would always
strike the Earth most directly at the equator, and the subsolar
point would always be the equator. Earth would receive a
consistent intensity of solar radiation and there would be no
seasons.
The earth's tilt determines the angle that the sun's rays strike the surface.
7. Axial Tilt
• One hemisphere is
always in the
process of tilting
towards the sun
– In June, the northern
hemisphere is tilted
towards the sun
– In December, it is
tilted away, and it is
the opposite for the
southern hemisphere
8. Axial Tilt
• The opposite hemisphere is tilting away
– Tilt and orientation do not change
9. Axial Tilt
• The position of the Earth relative
to the sun changes as its orbit
progresses
– Has the effect of moving each
hemisphere either towards or away
from the sun’s rays
– Movement of hemispheres towards
or away from the sun causes
seasons
• This results in the migration of the
subsolar point 23.5° north or south of
the equator
10. The first days of the seasons are solstices and
equinoxes. These are key periods within Earth-
Sun Relationships.
11. Subsolar Point
• This is the place on Earth where the suns’
angle is 90° and solar radiation strikes the
surface most directly.
– Earth’s axial tilt and it’s orbit cause the
subsolar point to move between 23.5° north
and 23.5° south over the course of a year.
12. Equinox and Solstice Conditions
• Equinox-when the subsolar point is at the
equator and all locations on the earth
experience equal hours of daylight and
darkness
13. Equinox and Solstice Conditions
• Solstice-when the sun angle is at 90° at
either end of the tropic boundaries.
– Topic of Cancer 23.5° N
– Tropic of Capricorn 23.5° S
14. Solstices and Equinoxes
• Spring (Vernal) Equinox
– March 20-21
– Subsolar point at Equator
– Circle of illumination extends to
both poles
• Summer Solstice
– June 20-21
– Northern hemisphere tilts towards
the sun
– Southern hemisphere tilts away
– Subsolar point=Tropic of Cancer
23.5° N
– Above 66.5 ° N=24 hours of
daylight (Land of the Midnight
Sun)
– 66.5 ° S to 90 ° S= 0 hours of
sunlight (tilted away from the sun)
• Fall (Autumnal) Equinox
– September 22-23
– Subsolar point at the equator
again
– Equal hours of day and light at all
locations
• N or S hemisphere not tilted
towards the sun
• Winter Solstice
– December 21-22
– Northern hemisphere tilted away
from the sun
– Southern Hemisphere tilted
towards the sun
– Subsolar point at 23.5 ° S, Tropic
of Capricorn
– Above 66.5 ° N, 24 hours of
darkness
15. Analemma
WHAT IS AN ANALEMMA?
An analemma is a natural pattern
traced out annually in the sky by the Sun.
16. Analema
• The analema is the geographers tool used
to locate the subsolar point, or the point on
Earth’s surface where the sun is directly
overhead at noon.
• The analema can be used for any place
on earth, and any day of the year.
17. • Due to the earth's tilt
on its axis (23.5°) and
its elliptical orbit
around the sun, the
relative location of the
sun above the horizon
is not constant from
day to day when
observed at the same
time on each day.
http://en.wikipedia.org/wiki/Analema
18. Using the Analemma
• The analemma can be
used to determine the
sun’s subsolar point for
any given date.
– For example: find October
10th
on the analemma,
follow that point on the
analemma out to the right
edge of the grid and notice
that it is at 6° south.
• This means that on
October 10th
, the subsolar
point is 6° south, in other
words 6° south is the
place on the Earth where
the sun’s rays are striking
at a 90° angle.
19. Using the Analemma
• The analemma is also uses to
determine what time the sun
reaches its zenith, or what time
noon is.
• Again, look at October 10th
.
Follow that point to the top of
the grid.
• Notice that for October 10th
, the
sun’s zenith is 12 minutes fast.
• This means that noon will be
12 minutes early on October
10th
, so the sun will reach its
zenith at 11:48 AM.
20. Using the Analemma
• The analemma can also be used to
determine the angle that the sun is hitting
ANY location on earth for any given date.
• This is known as the solar altitude.
21. Using the Analemma to Calculate
Solar Altitude
• First you must determine arc distance.
– Where are you calculating from? What is your
location?
• Second you must determine the subsolar point
for that date.
• If your two locations (your location and the
subsolar point) are in the same hemisphere, you
will minus those two latitudes.
• If your two locations are in opposite
hemispheres, then you will add those two
latitudes together.
• The end result is your arc distance.
22. Using the Analemma to Calculate
Solar Altitude
• Once you have determined your arc
distance, you simply minus it by 90° in
order to calculate the solar altitude at your
location.
23. Using the Analema to Calculate
Solar Altitude
For example, calculate the solar altitude for Los Angeles (34°N) on
July 16.
From the analemma, you can see that the solar altitude on July 16
is approximately 21° north, and this is in the same hemisphere as
the location in question.
34° -21°= 13° Arc Distance
90°-13° = 77° (Solar Altitude-Arc Distance=solar altitude for a
particular location)
So on July 16, the noon sun is 77° above the horizon in Los
Angeles.
24. Using the Analema to Calculate
Solar Altitude
To calculate the solar altitude on December 21 in Los Angeles, look
at the analema for that date…23.5° in the SOUTHERN
HEMISPHERE
Since Los Angeles and the declination of the sun are in opposite
hemispheres, add to determine the arc distance:
23.5°+ 34°=57.5°
Then use the formula to calculate the solar altitude: 90° –
57.5°=32.5°
So at noon on December 21 in Los Angeles, the sun is 32.5° above
the horizon.
26. http://vrum.chat.ru/Photo/Astro/analema.htm
It shows position of the Sun on the sky in the same time of a day during one year. Analemma - a trace of the annual
movement of the Sun on the sky - is well known among experts of sun-dials and old Earth's globes as a diagram of change of
seasons and an equation of time. Between August 30th 1998 and August 19th 1999 I have photographed the Sun 36 times
on a single frame of 60-mm film. The pictures were taken exactly at 5:45 UT (Universal time) of every tenth day.
