Equinoxes, Solstices, Insolation, and the Analemma

1. Equinoxes, Solstices, Insolation, and the Analemma

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 • 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

3. 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.

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 • 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

8. The first days of the seasons are solstices and equinoxes. These are key periods within Earth-Sun Relationships.

9. 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

10. 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

11. 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 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

12. Analema

13. 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.

14. 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.

15. 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.

16. Done at 8:30 AM Eastern Time

17. 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.