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The Universal Decimal Classification Scheme

The classification system used at Armagh Observatory, which is endorsed by the International Astronomical Union, is known as the Universal Decimal Classification (UDC) scheme. The UDC scheme covers many subject areas, each beginning with a different initial digit. The complete schedules for astronomy, astrophysics and related subjects, e.g.\ physics, mathematics etc., may be consulted in the UDC manual which is filed under 025.45 KEMP. The division into subject areas is as follows:

Each of these main areas is sub-divided. For example, `Generalities' is divided into:

Similarly, `Pure Sciences' is sub-divided into:

Suppose we now consider UDC 52: Astronomy, Astrophysics, Space Research, Geodesy. The classification UDC 52 is sub-divided by the addition of a third digit as follows:

A further sub-division is made by placing a decimal point after the third digit, and adding a fourth digit for each sub-topic. Taking 520 as an example:

Further refinement of 520.1, say, is done by adding a fifth digit, for example taking 520.1 `Observatories':

To specialise still further, a sixth digit may be added. The maximum number of digits in use for simple numbers appears to be nine, separated by two decimal points. For example, 520.843.054 would indicate `Photoelectric Recording'.

Last Revised: 2010 March 1st