The Sun’s Daily Path Across the Sky
Dinsmore Alter and Clarence Cleminshaw
Chapter 9 of Pictorial Astronomy published by the Griffith Observatory, Los Angeles, © 1948
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 accompanying diagrams at the bottom of this page show the sun’s daily path across the sky at the latitudes of 34° and 42° North. From these two diagrams, one can determine the approximate position of the sun with respect to the horizon of any place in the United States at any hour of the year.
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. An altitude can be read at any time of the day by means of concentric circles which are 2° apart.
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 our 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. Standard time will differ from this, depending on the equation of time and the longitude. However, this uncertainty is not important for our purpose, which is to show the general course of the sun across the sky and not its exact position at any particular instant of 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:
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°.
On March 21 and September 23 the sun is on the celestial equator, which intersects the celestial meridian at a distance from the zenith equal to the latitude. At a latitude of 34° N. On these dates the noon sun is 34° south of the zenith. Its altitude is 56°, which is found by subtracting 34° from 90°.
On June 21 the sun is 23-1/2° north of the celestial equator and its maximum altitude at noon is 79-1/2°, which is the sum of 56° and 23-1/2°. On December 21 the sun is 23-1/2° south of the celestial equator and its noon altitude is 32-1/2°, which is the difference between 56° and 23-1/2°. Thus the sun’s meridian altitude varies by 47°. This range is twice 23-1/2° and is the same for all latitudes.
The approximate duration of sunlight can be estimated from the diagram. The upper curve for June 21 shows that the sun rises at about 4:45 A.M. and sets at about 7:15 P.M., giving a duration of 14-1/2 hours. On May 21 and July 21 the sun is up for 14 hours. The number for March and September is 12 hours. There are nearly 10 hours of sunshine on the shortest day.
The sun’s daily path across the sky on or about the 21st of each month at latitude 34° N. All times are standard time (adjust for daylight time when in effect; remember, 9:00 standard time = 10:00 daylight time).
The sun’s daily path across the sky on or about the 21st of each month at latitude 42° N. All times are standard time (adjust for daylight time when in effect; remember, 9:00 standard time = 10:00 daylight time).