{{Short description|Colour-coded military wall clock}} {{Use dmy dates|date=March 2017}} {{Use British English|date=March 2017}} [[File:World War II RAF sector clock.jpg|thumb|Sector clock.]] A '''sector clock''' or '''colour change clock''' was a round colour-coded [[clock]] used at military airfields and observation posts in the [[United Kingdom]] to help track the movements of enemy aircraft and coordinate and control air defences.

== Overview == Developed in the United Kingdom during the [[First World War]] and originally known as "colour change clocks", the [[Royal Flying Corps]] (RFC) introduced sector clocks in 1917 to aid in the chronological monitoring and position plotting of German aircraft.

During the [[Second World War]] sector clocks played a significant role in the [[Battle of Britain]] as part of the [[Dowding system]] of air defence and continued to be used by the [[Royal Air Force]] (RAF) and [[Royal Observer Corps]] (ROC) as simple clocks and keepsakes until the end of the [[Cold War]]. Two versions of the sector clock were supplied for issue by the [[Air Ministry]]; the RAF sector clock having a "[[Tudor Crown|King's Crown]]" RAF [[Warrant Officer]]'s insignia under the "24",<ref>{{cite web |url=http://www.ww2-pathfinders.co.uk/Wyton%20sector%20clock.html |title=Wynton sector clock |work=Bennett and the Pathfinders |publisher=WW2 Pathfinders.co.uk |accessdate=19 January 2013}}</ref> the ROC issue clock having no associated insignia. Clocks were either electrical or mechanical; electric clocks usually having the coloured segments pointing inwards, mechanical clocks having the segments pointing outwards.<ref>{{cite web |url=http://www.aeroclocks.com/Catalog_pages/sector.htm |title=RAF Sector Clocks |publisher=Aeroclocks.com |accessdate=19 January 2013 |url-status=dead |archiveurl=https://web.archive.org/web/20121227075001/http://www.aeroclocks.com/Catalog_pages/sector.htm |archivedate=27 December 2012 |df=dmy-all }}</ref>

The [[United States Army Air Force]] (USAAF) adapted the RAF sector clock using a coloured block pattern in place of triangles.<ref>{{cite web |url=http://www.aeroclocks.com/Sector_pages/clock_no_2260.htm |title=Clock No. 2260 |publisher=Aeroclocks.com |accessdate=19 January 2013 |url-status=dead |archiveurl=https://web.archive.org/web/20130921112158/http://www.aeroclocks.com/Sector_pages/clock_no_2260.htm |archivedate=21 September 2013 |df=dmy-all }}</ref>

Sector clocks are sought after by collectors of [[militaria]]/aeronautica. (The clock face design also appears on wrist watches commemorating the wartime role of the sector clock).

== Function == The sector clock was a fundamental part of [[ground-controlled interception]] (GCI) before modern computerized systems were put in place for airspace control. The [[clock face]] is marked with five-minute red, yellow and blue triangular segments.<ref>{{cite web |url=http://home.earthlink.net/~modguide/opsroom.html |title=The Clock That Saved Britain |publisher= |accessdate=19 January 2013}}</ref> It has an outer 12-hour ring and an inner [[24 hour time|24-hour]] dial.

Aircraft position was recorded along with the colour of the triangle beneath the minute hand at the time of sighting. This was reported to sector headquarters, where counters of the relayed colour were used to represent each air raid on a large table with a map of the UK overlaid with a [[Ordnance Survey National Grid|British Modified Grid]]. As the plots of the raiding aircraft moved, the counters were pushed across the map by magnetic "rakes".

This system enabled "Fighter Controllers" to see very quickly where each formation was heading and allowed an estimate to be made of possible targets. The age of the information was readily apparent from the colour of the counter. Because of the simplicity of the system, decisions could be made quickly and easily.

==Gallery== <gallery perrow="5">

File:Sector clock.JPG|RAF-issue sector clock. Coloured triangles span 5 minute time periods. Outward pointing triangles indicate the model shown to have a mechanical mechanism. File:RAF Digby Ops Room.jpg|[[RAF Digby]] Operations Room with a sector clock displaying inward pointing triangles, typical of electrical models. This clock appears to show a key aperture below the dial and may be mechanical. File:The Royal Observer Corps, 1939-1945. CH11336.jpg|ROC-issue electrical and mechanical sector clocks in the No. 19 Group ([[Bromley, Kent|Bromley]]) Operations Room. Mechanical models were often paired with electrical types as a backup in the event of power outages. File:100 years of the RAF MOD 45163760.jpg|Electrical sector clock above the plotting table at headquarters [[RAF Fighter Command|Fighter Command]], [[RAF Bentley Priory]]. File:Royal Observer Corps necktie.jpg|An RAF sector clock-themed wristwatch. ROC tie with matching [[Watch strap#NATO straps|NATO watch strap]]. </gallery>

==See also== * [[Battle of Britain Bunker]] * [[Dowding system]]

==References== {{Reflist}}

==External links== {{commonscat|Sector clocks and wristwatches}}

[[Category:Clock designs]] [[Category:Battle of Britain]] [[Category:World War II military equipment of the United Kingdom]] [[Category:Military aviation]]