## G

: the point on the Earth’s surface directly beneath a celestial body (i.e. where a line to the body from the centre of the Earth intersects the surface).

**geoid**

The Earth’s shape, formed by the mean sea level and its supposed extension under the land masses. It cannot sensibly be represented mathematically, and is often approximated by one of many ellipsoids. Those most likely to be encountered are the Airy 1836 ellipsoid, used to define Ordnance Survey maps, and the WGS 84 ellipsoid, used in the GPS system.

**globe**

A spherical chart of the Earth. Note that although the Earth’s geoid is actually a flattened ellipsoid (i.e. slightly melon-shaped with an equatorial radius of 6378 km, and a polar radius of 6357 km) the spherical representation is used for all dialling activities with the exception, for example, of the model used by the GPS system.

**gnomon**

Gnomon: the physical structure of a sundial which casts the shadow. See Figure 1.

Hence **gnomonics**: the science of sundialling and **gnomonist** (seldom used): a person who practices gnomonics.

**Golden Number**: a number sequence (1-19) used to describe the year number in the metonic cycle. It was

used, together with the Dominical letters, to find Easter, and is often found on portable dials.

**grade**

[grad] a unit of angular measurement, equal to 1/100th of a right angle, or

p

/200 radians. Used particularly in France in the 18th and 19th centuries.

**great circle**

A circle on the surface of a sphere whose diameter is equal to the diameter of that sphere. Thus the circle has the same centre as the sphere. The shortest route between two points on the surface of a (solid) sphere lies on the circumference of the great circle connecting them.

**GPS (Global Positioning System)**

A system of polar-orbiting satellites, run by the US Dept. of Defense, which allows hand-held radio receivers to provide accurate 3-D location information anywhere on (or near) the Earth’s surface. It also provides a highly accurate clock, based on UTC. The system uses the WGS84 co-ordinate system and description of the Earth’s geoid.

**grazing incidence**: a term used to describe illumination in which the rays are parallel to the receiving

surface. The shadow of a point above the surface falls at infinity.

**green flash**

An atmospheric phenomenon occasionally observed during the final phase of sunset, when the upper limb of the Sun shows as a green flash due to the complex wavelength-dependence of atmospheric refraction.

Greenwich Mean Time (GMT

): see GMT, *time (types of)*

**Greenwich Meridian**

The line of longitude (or half a great circle) passing through the centre of the Airy transit circle at the old Royal Greenwich Observatory in London, and which defines the origin of Longitude (º

= 0º). It is now designated the Prime Meridian. Note: prior to 1884, there was no single fixed prime meridian, and hence early sundials sometimes refer to different origins, notably Paris.

**Gregorian calendar**

The calendar first introduced by Pope Gregory XIII in 1582 AD and now the accepted calendar throughout the vast majority of the world. It introduced the modern system of leap years which results in an error of only 3 days in 10,000 years. Note that adoption of this calendar throughout Europe took an extended period. Its introduction in Britain in 1752 produced a step change of 11 days which can be seen in the difference between EoT tables on dials earlier than this and those on later ones.