The Moonlight Batteries were Searchlight units of Britain's Royal Artillery that specialised in providing 'artificial moonlight', otherwise known as 'movement light' or 'Monty's moonlight', for ground operations during the latter stages of World War II.
Early uses
Sea
Navies were the first military forces to adopt the newly developed electric Arc lamp Searchlights (S/Ls) in the later 19th Century, to illuminate potential targets at night. Before the Bombardment of Alexandria in 1882, the Royal Navy force employed its searchlights to prove that Ahmed ‘Urabi's troops were strengthening the fortifications at night in defiance of the British ultimatum. Similarly, searchlights were also used to defend Ports against incursions by warships at night: in Britain this role became the responsibility of the fortress engineers and electrical engineers of the Royal Engineers (RE).
Land
Early in the Second Boer War, Colonel Robert Baden-Powell improvised searchlights to deter night attacks on his lines during the Siege of Mafeking. Soon afterward Major Rookes Crompton led a detachment of the Electrical Engineers Volunteers to South Africa where they operated electric arc lamp S/Ls of his own design, the first use of such equipment by the RE on campaign. They provided a primitive 'artificial moonlight' by reflecting the searchlight beams from clouds. After the onset of trench warfare in World War I, the Tyne Electrical Engineers provided detachments to operate oxy-acetylene S/Ls in forward areas to assist in night defence, but these proved a failure.
Air
World War I saw massive expansion of searchlight use in Anti-Aircraft (AA) defence to illuminate Zeppelins and bomber aircraft at night so that they could be engaged by AA guns and fighter aircraft. This became the predominant military use for searchlights between the World Wars, and in the 1930s the threat from Luftwaffe bombers in the event of war with Germany led to rapid expansion in the number of AA S/L units in Britain's part-time Territorial Army. These were frequently converted infantry battalions transferred to the RE or the RA. They formed part of Anti-Aircraft Command in Air Defence of Great Britain, but some units were also sent overseas after the outbreak of war to supplement the small number of Regular Army S/L units in defending ports, bases, airfields and other vital points.
World War II
All the Army's AA S/L units were transferred to the Royal Artillery during 1940. Their primary role was to illuminate enemy aircraft for AA guns or night fighters to engage, although providing directional beacons for friendly aircraft was a valuable secondary role. Searchlight units scored occasional successes by dazzling enemy pilots, but attempts to set up dazzle barrages (eg a 'Cardiff–Newport Dazzle Area') were not particularly successful. Other uses for S/Ls were slower to emerge.
Alamein
At the time of Battle of Alamein, Eighth Army had two Troops of 27th (London Electrical Engineers) S/L Rgt under command for area AA protection, but five of these lights were used to assist assaulting troops in direction-keeping during the night attack that launched the battle (Operation Lightfoot). No attempt was made to illuminate the sky (the battle was fought in full moonlight, though this was much obscured by dust).
Sicily & Italy
Battlefield illumination was also used in the campaigns in Sicily and Italy up to and including the Gothic line. It was used to illuminate the battlefield for not only infantry attack but also, because of the ridge nature of the terrain, catching out German Artillery in the full glare of light on the opposite slopes. Careful reconnoitring of the area and individual placement achieved excellent results. The 1st Canadian Group had with them 422nd S/L Battery who undertook this task successfully.
Normandy
By the time of Operation Overlord and the Normandy campaign in the summer of 1944 the Luftwaffe was greatly weakened, and the use of radar by AA guns and night-fighters was widespread, so the S/L units in 21st Army Group were under-employed. Some lights from 474th S/L Battery were given a subsidiary coast defence role to protect the anchorages from E-Boat attacks. By late August, as 21st Army Group broke out of the beachhead, enemy night air raids were rare and 557th S/L Bty was used to provide floodlighting for round-the-clock bridge-building by the Royal Engineers; this soon became a routine task for S/L units. As the advance across France gathered momentum, newly arrived batteries of 41st and 42nd S/L Rgts found themselves providing security lighting for Prisoner of war (PoW) camps.