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: [rad] the primary unit of angular measurement, it is the central angle subtended by an arc of a circle equal in length to its radius. 2p radians = 360ºor 1 rad
˜ 57.3º.


A single line or narrow beam of light.


A term sometimes used for the angle by which a reclining dial leans away from the observer (i.e. the complement of the inclination). It is more consistent, however, to translate this into the equivalent inclination.


An old term for a right-angled triangle where the other two angles are the latitude and co-latitude. Used particularly in the graphical construction of dials.

reflex angle

An angle of greater than 180º.


The “bending” of light at the interface of two materials of different refractive indices. It accounts for the focusing action of lenses. In dialling, use is made of ~ in dials which use a clear liquid in a solid cup to compress the hour lines, or which use a cylindrical lens to focus sunlight onto a curved dial plate. Atmospheric refraction (due to the curve of the Earth’s surface and the variation of atmospheric density with height, in turn dependent on meteorological conditions) is the effect which makes the Sun (or other celestial body) look slightly higher in the sky than its true astronomical position. It is only significant when the Sun is within a few degrees of the horizon. At 0º altitude, the bending is equivalent to approximately 34 arc-minutes, so that it is possible to see the Sun when it has actually just sunk below the horizon. See Equations. This effect is not generally included in normal sundials but it must be allowed for when calculating solar parameters from observations using meridian lines.

The refractive index [µ
, MU]
of a medium (or its index of refraction), needed to calculate these effects, is defined as the ratio of the speed of light in a vacuum to its speed in the medium.

Regiomontanus houses

An astrological division of the celestial sphere into 12 segments or houses. The division is performed in equal segments around the celestial equator, rather than around the ecliptic as is done for the normal signs of the zodiac. The houses are numbered I to XII, beginning at the east point of the horizon and are, confusingly, associated with the standard zodiac signs with I corresponding to Aries. Only the last 6 of the signs appear above the horizon. They are shown as domifying circles on some old dials.

resolution (of a dial)

The smallest time increment to which the scale on a dial can be read. Contrast with accuracy. See also precision.


Been used as a graduated scale fixed to an astronomical telescope.


Fine lines or scales on an optical element in a sighting device (e.g. a telescope) to aid in alignment or measurement of an object.


To orbit around another body, e.g. the Earth revolves around the Sun. Contrast to rotate.

right ascension:

[a, RA] a co-ordinate used by astronomers, as part of the equatorial co-ordinate system, (together with declination) to define the position of a celestial body. It is the angular distance measured along the celestial equator (positive to the east) from the vernal equinox to the intersection of the celestial equator to with the hour circle through the point in question. Usually measured from 0 to 24 hours, but sometimes 0º to 360º.

root (of a gnomon)

The fixing between the gnomon and the dial plate. See Figure 1


To spin on its own axis, e.g. the Earth rotates on its polar axis. Contrast to revolve..

Pages: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Introduction    Dial types    Hours (types of)    Time (types of)    Illustration
Symbols    Equations    Biographies    Chronology    Sources    Appendices