CHANGE IN SOLAR AZIMUTH DURING SUNRISE (SUNSET) OBSERVATIONS

The azimuth of the sun is constantly changing - even while your making your daily observations.

The following data is developed to show how much time you will have to make and mark down your azimuth observations before a significant error might be induced by this change.

The data is for San Francisco at sunrise on the winter solstice on December 22, 2006 which occurs at 07:23:26 a.m.

Time

Elevation

Change

Minutes

 

Azimuth

Change

Minutes

Hr: Min: Sec

Degrees

Minutes

 

Degrees

Minutes

07:23:26

0

0

   

119

41.4

 

07:24:26

0

8.4

+8.4

 

119

51.0

+9.6

07:25:26

0

16.8

+16.8

 

120

0.0

+18.6

07:26:26

0

25.2

+25.2

 

120

9

+27.6

07:27:26

0

34.2

+34.2

 

120

18

36.6

The data shows that it takes just under 4 minutes for the complete sunrise to take place, i.e. for the sun to rise by it's visible diameter.

During that same time the Azimuth has also increased by approximately the same visible diameter, giving an upward path of about 45 degrees with the horizon instead of merely a 90 degree vertical rise.

The optimum time to observe the azimuth is when the sun is one half or more above the horizon so that the leading or trailing edge may be observed.

Since the azimuth change per day for the day before and the day after the solstice is only about 1 arc minute it is obvious that the observation must be nearly an instantaneous one.

An exception to this limit is if you are observing a sunrise past a shadowing object that has a shadowing edge which is not vertical but rather is sloped in the direction of the change in azimuth with time, such as is done with the "heel stone" at Stonehenge or the mountain peak at Machu Pichu. Under this circumstance you will have a longer time if the leading edge of the sun merely creeps along the rising edge of the shadow object.