Equinoxes, Solstices, Insolation,
and the Analemma
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
• Perihelion=when the Earth is closest to the
sun, 91.5 million miles away
• Aphelion=when the Earth is farthest away
from the sun, 94.5 million miles away
Timing of the Seasons
• It is winter in the Northern Hemisphere
when we are closest to the sun.
• It is summer in the Northern Hemisphere
when the sun is farthest away from the
sun.
– It is NOT distance from the sun that causes
seasons.
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
Circle of Illumination
• This is the border between night and day.
• It is constantly moving across the Earth.
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.
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
• The opposite hemisphere is tilting away
– Tilt and orientation do not change
• 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 cases
seasons
• This results in the migration of the subsolar point 23.5° north or
south of the equator
The first days of the seasons are solstices and
equinoxes. These are key periods within EarthSun Relationships.
• Subsolar point-the point on Earth where
the sun 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 south over the course of a year
• Equinox-when the subsolar point is at the
equator and all locations on the earth
experience equal hours of daylight and
darkness
• Solstice-when the sun angle is at 90° at
either end of the tropic boundaries
Solstices and Equinoxes
•
Spring (Vernal) Equinox
–
–
–
•
Summer Solstice
–
–
–
–
–
–
•
March 20-21
Subsolar point at Equator
Circle of illumination extends to both poles
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 22-22
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
Analema
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.
Using the Analema
• Solar Altitude= 90 degrees Arc Distance
– If the declination of the sun and the latitude of the sun
are in opposite hemispheres, add both latitudes
together to determine the arc distance.
• For example, calculate the solar altitude for Los Angeles (34
degrees N)
– From the analema, you can see that the solar altitude on July
16 is approximately 21 degrees, and this is in the same
hemisphere as the location in question.
» 34 degrees -21 degrees= 13 degrees
» Solar Altitude =90 degrees-13 degrees = 77 degrees
» So on July 16, the noon sun is 77 degrees above the
horizon in Los Angeles.
Using the Analema
• To calculate the solar altitude on
December 21 in Los Angeles, look at the
analema for that date…23.5 degrees
– Since Los Angeles and the declination of the
sun are in opposite hemispheres, add to
determine the arc distance:
• 23.5 degrees+ 34 degrees=57.5 degrees
• Then use the formula to calculate the solar
altitude: 90 degrees – 57.5 degrees=32.5 degrees
– So at noon on December 21 in Los Angeles, the sun is
32.5 degrees above the horizon.
Done at 8:30 AM Eastern Time
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.