Tuesday, August 30, 2011

Dune Sunset

My son, Kevin, and I took a quick hike in the dunes to catch sunset last night.  We headed up into the dunes to what I call Sacred Mountain -- where the cross is located that is surrounded by the ghost forest trees.

The fall colors are becoming more evident.

The angle of the sunset is starting to be more and more south. That is another sure sign of fall. I started to think about how the position of the sunset changes on the horizon and decided to do a little research. Irrespective of where you are on the globe, the sun will always rise exactly East and set exactly West on two days: March 21 and September 21 which are the two equinoxes.

Sunrise is the time when the upper part of the sun is visible, and sunset is when the last part of the Sun is about to disappear below the horizon (in clear weather conditions. If the horizon in the direction of sunrise or sunset is at a higher altitude than that of the observer, the sunrise will be later and sunset earlier than listed (and the reverse: on a high mountain with the horizon below the observer, the sunrise will be earlier and sunset later than listed).

The Earth's atmosphere refracts the incoming light in such a way that the sun is visible longer than it would be without an atmosphere. The refraction depends on the atmospheric pressure and temperature. A higher atmospheric pressure or lower temperature than the standard means more refraction, and the sunrise will be earlier and sunset later. In most cases, however, this would affect the rising and setting times by less than a minute.

The changing position of the sun in the sky from hour to hour and from day to day is of interest to everyone, but especially to architects. They want to know the answers to such questions as, when and how far the sun will shine in the windows of different sides of a building. The diagrams below shows the sun's daily path across the sky at the latitudes of 34° and 42° North.

The sun's position with reference to the horizon is expressed by altitude and azimuth. Altitude is the angular distance above the horizon measured perpendicularly to the horizon. It has a maximum value of 90° at the zenith, which is the point overhead. It is marked on the diagram at intervals of 10° along the vertical line in the center. That line represents the celestial meridian, which the sun crosses at noon.

Azimuth is the angular distance measured along the horizon in a clockwise direction. Astronomers measure it from the south point, navigators from the north point. In the above diagrams which follow the navigators' rule, north is 0°, east is 90°, south is 180°, and west is 270°. Each degree of azimuth is shown in the circular band around the outside of the diagram, and numbers from 60 to 300 indicate the azimuth at intervals of 10°. Lines radiating from the center mark the azimuth at intervals of 5°.

The sun's daily path across the sky on or about the 21st day of each month is indicated by means of seven curved lines. The upper one is for June and the lower one is for December. Each of the other five is for two months. For instance, the path on March 21 is the same as on September 23.

Each path is divided into hours. Numbers along the upper and lower paths show the hours which would be indicated by a sundial. This is known as local apparent sun time.

It is interesting to see how much the sunrise and sunset points move during the year. The azimuths of the extreme positions are as follows:

Date Sunrise Sunset
June 21 61° 299°

Dec. 21 119° 241°

Difference 58° 58°

In other words, the sun rises 29° south of east and sets 29° south of west on December 21. This is for a latitude of 34° N. The arc of the horizon between the east point and the sunrise point is called amplitude. On June 21 it is 23-1/2° at the equator, and increases to 90° at the Arctic Circle, where the sun is up for 24 hours on that day. The value at a latitude of 42° N. is 32-1/2°.

Other facts about sunset:

Sunrise and sunset are defined as the instant when the upper limb of the sun’s disk is just touching the horizon, this corresponds to an altitude of -0.833° degrees for the sun.

Civil twilight lapse of time between sunset and when the sun reaches the elevation height of -6°, in the sky are visible only a few stars and planets particularly bright.

Nautical twilight represents the time the sun takes a pass from -6° to -12° below the horizon, in this period are distinguished horizon line and the main stars.

Astronomical twilight is the time interval between sunset and when the sun reaches 18° below the horizon, the sky is dark, is possible to distinguish the stars up to the sixth magnitude.

Noon in solar time occurs when the sun is at its highest point in the sky for the day, and it is either due south or due north of the observer depending on the latitude.

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