Lab two 2011

Earth and Sun Relationships and Topographic Maps Lab Two

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

• 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

Analemma

WHAT 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.

Using the Analemma to Calculate the Sun’s Declination or Angle of Incidence1. Where are you calculating from? What is your

location?2. Second you must determine the subsolar point for that

date.3. If your two locations (your location and the subsolar

point) are in the same hemisphere, you will minus those two latitudes.

4. If your two locations are in opposite hemispheres, then you will add those two latitudes together.

5. The end result is your arc distance.6. Once you have determined your arc distance, you

simply minus it by 90° in order to calculate the solar altitude at your location.

Example

• Use the analemma to find the sun’s declination (angle) for Los Angeles (34°N) on July 16.– 1. Where: 34°N– 2. Subsolar point July 16 = 21°N– 3. Hemisphere: SAME!– 4. 34° - 21° = 13°– 5. 13° is your arc distance– 6. 90 – 13 = 77°

• SO THAT MEANS THE SUN’S DECLINATION (ANGLE) IN LOS ANGELES ON JULY 16TH IS 77°

Example

• Use the analemma to find the sun’s declination (angle) for Los Angeles (34°N) on November 20.– 1. Where: 34°N– 2. Subsolar point Nov. 20 = 20°S– 3. Hemisphere: OPPOSITE– 4. 34° + 20° = 54°– 5. 54° is your arc distance– 6. 90 – 54 = 36°

• SO THAT MEANS THE SUN’S DECLINATION (ANGLE) IN LOS ANGELES ON November 20TH IS 36°

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.

Topographic Maps• Topographic maps are large-

scale maps that use contour lines to portray the elevation and shape of the topography.

• Topographic maps show and name both natural and human-made features.

• The US Geological Survey (USGS) is the principle government agency that provides topographic maps for the United States.– USGS topographic maps

cover the entire United States at several different scales.

Computing Distances with Fractional Scales

• To determine distances represented on a map by using the fractional scale:

1. Use a ruler to measure the distance on the map in inches (or centimeters). This is the measured distance.

2. Multiply the measured distance by the map’s fractional scale denominator. This will give you the actual distance in inches (or centimeters).

3. To convert your actual distance in inches (or centimeters) to other units, use the following formulas:

Measuring Road Distance

• Look at the map scale. – In the lower left or right

corner there will be a small graph that shows a unit that corresponds to the distance on the map.

– For example, if the scale is one inch long and is labeled five miles, then one inch on the map is equal to five miles on the ground.

– Hold a ruler next to it and measure it.

• Lay one end of string on one end of the road so that it follows every curve as accurately as possible. If you don’t have a string, tear a strip of paper and bend it along the curves of the road.

• Hold the string or paper so that you pinch it where the road begins and ends.

• Measure it with a ruler.

• Multiply the length of the string by the scale.

• For example, if the string was 7 inches long and each inch represents 5 miles, the road is approximately 35 miles long.

• Look for small sequential numbers next to the road on the map.

• These numbers indicate miles.

• On detailed maps, there may be a marker every mile, but on less detailed maps it could be every 10 miles, or some other scale.

• Find the number at the beginning of the section of the road you’re driving and the number at the end.

• If the numbers only show every several miles, estimate the location.

• For example, if the road ends between 50 and 60 miles, call it 55.

• Subtract the lower number from the higher number to get the total distance.

• For example, if the road starts a the 25 mile marker and ends at the 55 mile marker, the total is 55 – 25 = 30 miles.