Earth and Sun Relationships and Topographic Maps

Lab Two

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

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

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.

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:

Topographic Maps

• A map is a representation of the Earth, or part of it. • The distinctive characteristic of a topographic map is that

the shape of the Earth's surface is shown by contour lines.

• Contours are imaginary lines that join points of equal elevation on the surface of the land above or below a reference surface, such as mean sea level.

• Contours make it possible to measure the height of mountains, depths of the ocean bottom, and steepness of slopes.

• A topographic map shows more than contours. • The map includes symbols that represent such features

as streets, buildings, streams, and vegetation.

http://erg.usgs.gov/isb/pubs/booklets/symbols/

Contour Lines• Contour lines placed on the map

represent lines of equal elevation above (or below) a reference datum (or a known and constant surface)

• To visualize what a contour line represents, picture a mountain and imagine slicing through it with a perfectly flat, horizontal piece of glass. 

– The intersection of the mountain with the glass is a line of constant elevation on the surface of the mountain and could be put on a map as a contour line for the elevation of the slice above a reference datum.

http://geology.isu.edu/geostac/Field_Exercise/topomaps/topo_map.htm

Compass

Compass Directions