Azimuth is the compass direction of the sun, moon, or any point in the sky, measured in degrees clockwise from true north.
Azimuth is the compass bearing of the sun or moon, measured in degrees clockwise from true north, and it is the number that turns 'the sun sets over there' into a shot you can plan for a specific date. Any composition that depends on the sun or moon lining up with a landmark, a street, or a gap in a ridge comes down to matching azimuths.
Get it wrong by ten degrees and the sun sets behind the wrong hill, or a block down the street from where you set up. The sun's setting azimuth also swings through the year, by more than 60 degrees between the summer and winter solstices at mid latitudes, so a spot where the sun drops behind a tower in June may not line up again until the following spring. Photographers check azimuth against a map bearing before committing to a location and a date.
A photographer wants the sun to set straight down the length of a dead-straight city avenue, the effect New Yorkers call Manhattanhenge. The avenue runs at a compass bearing of 116 degrees, so the shot needs an evening when the sun's setting azimuth is close to 296 degrees, its reciprocal. A sun calculator shows that alignment happening on two dates about six weeks either side of the summer solstice.
On any other evening the sun sets a few degrees off the avenue's axis and slips behind the buildings on one side rather than framing down the centre. The same method puts a full moon behind a lighthouse or the sun inside a canyon arch: find the landmark's bearing from the shooting spot, then find the date the body's azimuth matches it.
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