# Glossary of Terms

**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.

**Equidistant Projection**

This is a circular diagram where solar altitude lines are equally spaced.

**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.

**Illuminance**
A measure of the intensity of the incident light.

**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.

**Orthographic Projection**

This is a rectangular-shaped diagram where altitude is plotted on the Y-axis,
and azimuth on the X-axis. Both the altitude and azimuth scales are linear.

**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.

**Spherical Projection**

This is a circular diagram where solar altitude lines are spaced closer together as
they approach the horizon. This type of diagram increases resolution at high
solar altitudes.

**Stereographic Projection**

This is a circular diagram where the outer edge of the diagram represents the horizon,
and the centre point represents the sun vertically overhead. Solar altitude is
plotted as a series of circles around the centre, these becoming spaced wider apart
as they approach the horizon. This type of diagram increases resolution at low
solar altitudes.

**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.

**Waldram Diagram**

This is a rectangular-shaped diagram that is used for the calculation of daylight levels in buildings.
The particular feature of this diagram is that the vertical and horizontal
scales are such that equal areas on the diagram represent equal levels of illuminance from the sky.