Lab six earth and sun relationships

Earth and Sun Relationships

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

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.

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

Axial Tilt

• The opposite hemisphere is tilting away– Tilt and orientation do not change

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

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

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.

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

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

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

AnalemmaWHAT IS AN ANALEMMA?

An analemma is a natural pattern traced out annually in the sky by the Sun.

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.

• 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

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.

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.

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.

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.

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.

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.

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.

Done at 8:30 AM Eastern Time

http://vrum.chat.ru/Photo/Astro/analema.htmIt 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.

Assignment

• Earth Sun Relationships Packet– Complete ALL problems