I've been reading some books on the Japanese invasion of Malaya and Singapore during the Second World War. A mention of the sighting of the Southern Cross by the advancing forces into Singapore caught my eye, and I decided to find out what the night sky was like on that historic night.
The area of the landings
According to the history books, the initial main Japanese landings took place on the night and early morning of 8-9 February 1942 (Sunday/Monday), by motorised craft launched from the Sungei Skudai river estuary in southern Johor state. This estuary is west of Johor Bahru and is now commonly known as the Danga Bay area; it is due north of (opposite) the present day Sungei Buloh Wetlands Reserve in Singapore. From there, the Japanese troops headed south-west, crossing the Johor Straits, and landed at the north-western mangrove marshes of Singapore island at the Sungei Buloh - Lim Chu Kang - Sungei Sarimbun area.
A check with a planetarium app shows that the Moon was then 48% illuminated (22 days old, Last Quarter, waning, in Libra). Taking Singapore local time then (UTC +7.5 hrs), the Moon rose at 12:42am on Mon 9 Feb 1942; taking present Singapore Standard Time (UTC +8.0 hrs), the Moon rose at 1:12am; and taking Tokyo time then (UTC + 9.0hrs), which I believe was the time used by Japanese forces, the Moon rose at 2:12am.
Last Quarter Moon was on the night of Sunday 8 February 1942 itself. The next New Moon was on Sunday 15 February 1942, Chinese New Year's day (in the Year of the Horse); the surrender of British forces took place that day at the Ford Motor Factory in Upper Bukit Timah. So invasion to surrender took just one quarter of a Moon phase, over the period from Last Quarter to New Moon (7 days, Monday to Sunday).
According to the book, "Guns of February - Ordinary Japanese Soldiers' Views of the Malayan Campaign & the Fall of Singapore 1941-1942", by Henry Frei, Singapore University Press 2004, the Japanese launched their main sea invasion forces immediately after midnight, according to the Japanese's watches.
I think it probably was with reference to Tokyo time, and so it meant that the start of the invasion (i.e. crossing of the 1.5km wide Johor Straits) took place in the cover of lunar darkness as it took place a full two hours before the last quarter Moon rose. I believe the exact timing was therefore probably deliberately chosen to avoid ground/sea illumination by the Moon. Sea tides were probably also a factor taken into consideration (see further below).
The Night Sky: 12.01am Monday 9 February 1942. Really the Southern Cross?
The following are extracts from the above-mentioned book "Guns of February", Chapter 7 ("Crossing of the Straits in the Face of the Enemy"). Note the references to the time, darkness and tides:
"In the evening Ochi's machine-gun company assembled in the rubber plantation, squatting in the dark,...
Engineers furthermore had miscalculated the timings of the tide. They knew about the Straits two metre-tidal difference over 24 hours, but had not taken into account the finer three hour-difference, from actual boarding time to departure time, between nine in the evening and midnight. ...
The launches drifted sideways close to the Malayan coast to await the massed crossing at midnight sharp.
The Southern Cross shone left above them. It was almost time. Ochi moistened his lips... He peered into the darkness... "Look! The Southern Cross." Ochi unscrewed his water bottle and held it up. ...
Ochi glanced at his watch. It was one minute to midnight. "It's midnight," Ochi turned to Lt. Oue. ... To their right, a flurry of boats dashed into the open Straits. ... Like the massive start of a grand marathon, the 300 boats sped into the middle of the Straits. The short journey across seemed like an eternity. ..."
From the above extract, it's apparent that the boats left the Skudai estuary into the Johor Straits at the stroke of midnight. However, in my view, one detail mentioned might have been a mistake. The Southern Cross was spoken of by the Japanese as being sighted. I've done a check with a planetarium app, the excellent Sky Safari Plus (for Android), which I highly recommend.
Assuming that the Japanese forces operated on Tokyo time (UTC +9.0hrs, and indeed Singapore switched to Tokyo time during the Occupation), the Southern Cross (in the constellation Crux, at around R.A. 12hr 30m) was then too low at only 5 degrees above the south-eastern horizon to be seen (pls see below screenshot). Every amateur astronomer knows that in our humid tropical skies, with its persistent low-level cloud cover, and even from rural dark sky locations, it would have been very unlikely that the constellation stars could have been seen at that altitude, even assuming no light pollution there in 1942. It seems to me that what the Japanese actually sighted was the nearby False Cross (in the constellations Vela and Carina), which is further west, about 3.5 hrs earlier/preceding, at around R.A. 8hr 50m. The False Cross was then at a much higher altitude of between 25 to 30 degrees above the SSE horizon (i.e. left of due South), and would therefore have been much more prominent. Further, given that the Japanese are from a northern latitude (35 deg north) where the Southern Cross (declination 60 deg south) cannot be seen, they would not have been familiar with the difference between the famous Southern Cross and the lesser-known, and thus well-named, slightly larger False Cross.
Below are screenshots from Sky Safari Plus of the night sky at 12:01am in the early morning of Monday 9 February 1942 (Tokyo time, UTC +9.0). I've set it to precession epoch 1942. From the Japanese forces' point of view, looking south, they would have seen the two brightest stars in the sky: Sirius (in Canis Major) high at almost due south, with Canopus (in Carina) directly below it. The False Cross (in Vela/Carina) would have been just to the left of Canopus, giving it prominence: this reinforces my belief that they had seen the False Cross and mistook it for the Southern Cross. This is indeed a fairly common mistake, especially amongst first time northerners.
From the defending British-Commonwealth point of view, looking northwards, they would have seen the three brightest outer planets Jupiter, Saturn and Mars in a line in the NW, with Castor and Pollux (Gemini) in the north, and Leo in the northeast. The Winter Milky stretched from Perseus in the NW, to Vela in the SE, and was then culminating at the zenith in the Canis Minor - Monoceros region. Given the relative lack of light pollution then, it was likely possible for them to have seen the stretch of the Winter Milky, and indeed it seems to have been a relatively clear night.
So there we have it: a look back at the celestial sky over Singapore on that fateful night.
Pls see the planetarium screenshots below from Sky Safari Plus. At 12.01am on Monday 9 February 1942 (at UTC +9.0). Precession epoch 1942. Location set at the Johor Straits area between Skudai and Kranji.
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