Moon Periods

The need for moonlight

One restriction was obvious from the earliest experiments in 1914: to fly at low level to a particular field behind enemy lines, the pilot needed moonlight and plenty of it. A generation later in 1940 the principle remained the same. Radio navigation aids were too imprecise, even if they had been made available. While bomber crews hated the Moon’s light for revealing them to enemy flak and fighters, clandestine air operations could only be flown this way.

The Lunar cycle

The Moon takes about 29½ days to orbit the Earth. For about two nights the Moon’s bright sunlit side faces away from the Earth and is invisible to us; this is the New Moon. For another six or seven nights its bright side faces less than half-way towards us; we see it as a crescent, but it generates too little light to be useful. At First Quarter (i.e. a quarter though the entire cycle of light and darkness) the moon is a bright half-disc, and generates enough light for about the first half of the night. For the next fourteen or fifteen nights it shows at more than half its sunlit side to us as it waxes from First Quarter towards Full Moon, before gradually waning to Last Quarter. Then it thins to a crescent before vanishing with the next New Moon, and the cycle re-starts.

The ‘Moon Period’

Only during those fifteen or so nights between First and Last Quarter did the Moon reflect enough light to be useful for an aircraft crew to navigate across country by its cold light, flying low enough to pick out individual features on the landscape below. The Special Duties units, and everyone involved with them — planning staff, ground crews and agents — knew these periods of bright moonlight as ‘Moon Periods’.

The Moon periods up to April 1942 are as follows:

Moon Periods August 1940 – April 1942

 

Moon Period Start Date (1st Qtr) Full Moon End Date (Last Qtr)
1940
August 10/8/40 17/8/40 26/8/40
September 8/9/40 16/9/40 24/9/40
October 8/10/40 16/10/40 24/10/40
November 6/11/40 15/11/40 22/11/40
December 6/12/40 14/12/40 22/12/40
1941
January 5/1/41 13/1/41 20/1/41
February 4/2/41 12/2/41 18/2/41
March 6/3/41 13/3/41 20/3/41
April 5/4/41 15/4/41 18/4/41
May 2/5/41 9/5/41 16/5/41
June 2/6/41 9/6/41 16/6/41
July 2/7/41 8/7/41 16/7/41

From 31 July 1941 the moon period straddled each month-end, though no SD operations were flown at the end of July:

Moon Period Start Date (1st Qtr) Full Moon End Date (Last Qtr)
July – August 31/7/41 7/8/41 15/8/41
August – September 29/8/41 5/9/41 13/9/41
September – October 27/9/41 5/10/41 13/10/41
October – November 27/10/41 4/11/41 12/11/41
November – December 25/11/41 3/12/41 11/12/41
December’41 – January’42 25/12/41 2/1/42 10/1/42
1942
January – February 24/1/42 1/2/42 8/2/42
February – March 23/2/42 3/3/42 9/3/42
March – April 25/3/42 1/4/42 8/4/42

Operations might be carried out on one or two nights either side of the moon period if (a) the weather was unusually clear and (b) the flight time was short. There was only one confirmed attempt during the pre-Tempsford era to fly an operation during the ‘dark period’, a Lysander operation flown by S/Ldr Alan ‘Sticky’ Murphy in August 1941. It was not successful.

There’s more to it than brightness. The nearly-new Moon stays up for only a short time after sunset, soon following the sun over the horizon. With each night the moon sets a little later. At First Quarter, the start of the Moon Period, the Moon sets at about midnight, but it was bright enough, and stayed up for long enough, for a crew to find their way to a target and drop the agent before it set. Full Moon was best: the land beneath lay bathed in light for almost all the night, cloud permitting. As the moon wanes the first part of the night becomes moonless until, by the end of the Moon Period at Last Quarter, the moon doesn’t rise until midnight. Then the most important factor was to get clear of the enemy coast before daybreak and the Luftwaffe’s dawn patrols.

The essential value of moonlight

Accurate navigation to the precise target was the main problem, without which everything failed, but it wasn’t the only one: aircrew needed light to verify that parachute canopies had opened, and in especially bright moonlight agents could be seen to have landed. The agents themselves needed to have enough light after landing to conceal their parachutes and harness before getting clear of the area. Reception committees needed the moon’s ambient light to get to the landing ground and set out the lamps (which were only lit at the sound of an aircraft), and after the drop to locate agents and containers that had fallen wide.

While it may be tempting to look up at the night sky of 2016 and try to equate it with what the aircrew and agents would have seen 75 years ago, in 2016 the Moon’s phases are nearly eleven days behind those for 1941.