The first part of the Table for Washington DC on July 4 2009 is shown opposite.
Heading Title: contains the name of the location, its longitude and latitude in degrees and minutes, the date and the Time system used. Here the time system is: Eastern Standard Time.
N.B. The Title should always be be checked to ensure that
- the location and the latitude and longitude are correct, particularly N S E and W.
- the way the time is measured Local Time or GMT, UTC etc is as expected
First Column: lists the time at chosen intervals during the day. Here: 1 minute.
Second column: contains the related VERTICAL ANGLE (Altitude ) of the sun. The values are in decimal degrees. (Each 0.1 degree = 0.6 minutes ie 11.1 degrees = 11 degrees 6 minutes; 11.2 degrees = 11 degrees 12 minutes etc)
Third column: contains the horizontal angle (Azimuth) of the sun in decimal degrees as measured in an Easterly direction from North.
Thus 0.0 degrees = North, 90.0 degrees = EAST, 180.0 degrees = SOUTH and 270 degrees = WEST.
Tables can be scrolled through quickly to the relevant row for a particular TIME, ALTITUDE or DIRECTION.
Sunrise time the first time when the Altitude is positive.
When the ALTITUDE is negative the Sun is below the horizon
Morning Equi-umbra the first time when the Altitude = 45 degrees or above.
Solar Noon (Transit) in the Northern Hemisphere: the time when the Sun is directly SOUTH ie the HORIZONTAL ANGLE(Azimuth) = 180 degrees: in the Southern Hemisphere when the Sun is exactly North ie the HORIZONTAL ANGLE(Azimuth) = 0 degrees.
Afternoon Equi-umbra the last time when the Altitude = 45 degrees or above.
as the last time when the Altitude is positive.
Example: shown below are later parts of the USNO Altitude-Azimuth table for the sun over Washington DC, on 2009 July 4.
Sunrise at 04:50 hrs EST when the altitude is first positive
04:48 -0.9 59.3
04:49 -0.1 59.4
04:50 0.0 59.6
04:51 0.2 59.8
Morning Equiumbra at 08:54 hrs EST when altitude = 45 degrees
08:52 44.7 95.6
08:53 44.9 95.7
08:54 45.1 95.9
08:55 45.3 96.1
Solar Noon at 12:13 hrs EST when Azimuth = 180 degrees
12:12 73.9 179.5
12:13 73.9 180.3
12:14 73.9 181.2
Afternoon Equiumbra at 15:31 hrs EST when Altitude = 45 degrees
15:30 45.3 263.8
15:31 45.1 264.0
15:32 44.9 264.2
15:33 44.7 264.4
Sunset at 19:35 hrs EST when Altitude is last positive
19:34 0.2 300.2
19:35 0.0 300.3
19:36 -0.1 300.5
19:37 -0.9 300.6
