This page describes the fundamental components that make up a Sun-Path Diagram. Understanding these components is important if you are to be able to accurately read off solar positions and properly appreciate overshadowing information that may be overlaid on such diagrams.
Azimuth Lines
Azimuth angles run around the edge of the diagram in 15° increments. A point's azimuth from the reference position is measured in a clockwise direction from True North on the horizontal plane. True North on the stereographic diagram is the positive Y axis (straight up) and is marked with an N.
Altitude Lines
Altitude angles are represented as concentric circular dotted lines that run from the centre of the diagram out, in 10° increments from 90 to 0. A point's altitude from the reference position is measured from the horizontal plane up.
Date Lines
Date lines represent the path of the sun through the sky on one particular day of the year. They start on the eastern side of the graph and run to the western side. There are twelve of these lines shown, for the 1st day of each month. The first six months are shown as solid lines (Jan-Jun) whilst the last six months are shown as dotted (Jul-Dec), to allow a clear distinction even though the path of the Sun is cyclical.
Hour Lines
Hour lines represent the position of the sun at a specific hour of the day, throughout the year. They are shown as figure-8 style lines that intersect the date lines. The intersection points between date and hour lines gives the position of the sun. Half of each hour line is shown as dotted, to indicate that this is during the latter six months of the year.
This characteristic figure-8 shape results from what is termed the Analemma, an effect resulting from the elliptical orbit of the Earth areound the Sun and the slight tilt of the Earth's axis of rotation relative to its orbital plane. This simply means that there is some seasonal variation in the difference between local and solar time.