Altitude
The angle up from the horizon.

Angle of Incidence
The angle between the normal to a surface and the direction of the sun. The sun will be perpendicular to the surface if the angle of incidence is zero.

Azimuth
The angle from North measured on the horizon in the sense NESW. Thus, North is zero degrees, East is 90 degrees.

Horizon
The apparent intersection of the sky with the Earth’s surface. For rise/set computations, the observer is assumed to be at sea level, so that the horizon is geometrically 90 degrees from the local vertical direction.

Inclination
Surface tilt expressed as an angle to the horizontal plane. Horizontal is zero degrees, vertical is 90 degrees.

Local Civil Time (LCT)
A locally agreed time scale. The time given out on the radio or television, and the time by which we usually set our clocks. LCT depends on the time of year and your position on earth. LCT can be defined as the time at the Greenwich Meridian + Time Zone Correction + Daylight Saving.

Orientation
The angle of a structure or surface plane relative to North in the sense NESW. Thus, North is zero degrees, East is 90 degrees.

Shadow Angles
Shadow angles refer to the azimuth and altitude of the sun, taken relative to the orientation of a particular surface.

Horizontal Shadow Angle (HSA)
The angle between the orientation of a surface and the solar azimuth.

Vertical Shadow Angle (VSA)
The angle between the HSA and the solar altitude, measured as a normal to the surface plane.

Solar Noon
The time when the sun crosses the observer’s meridian. The sun has its greatest elevation at solar noon.

Sunrise/ Sunset
The times when the upper edge of the disk of the sun is on the horizon. It is assumed that the observer is at sea level and that there are no obstructions to the horizon.

Twilight
The intervals of time before sunrise and after sunset when there is natural light provided by the upper atmosphere.

Civil Twilight is defined as beginning in the morning and ending in the evening when the centre of the sun is geometrically 6 degrees below the horizon.

Nautical Twilight is defined as beginning in the morning and ending in the evening when the centre of the sun is geometrically 12 degrees below the horizon.

Astronomical Twilight is defined as beginning in the morning and ending in the evening when the centre of the sun is geometrically 18 degrees below the horizon.