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16th of September 2020
A Lunar Soap Bubble
by Emil Lenc
Observing with radio light can provide astronomers with a different view of the Universe compared to the optical view we are more familiar with. For example, regardless of the actual phase of the Moon, the moon appears as a featureless disk when observed in radio light with the Australian Square Kilometre Array Pathfinder (ASKAP). However, ASKAP has another trick up its sleeve - it can "see" both the intensity of the radio light and its polarisation i.e. the orientation of the light. Astonishingly, when we look at the moon in polarisation it looks like a cosmic soap bubble (see image above) - why is that? When light reflects off a surface at a critical angle (known "Brewster's angle") it becomes highly polarised. Fishermen exploit this effect by wearing sunglasses that filter out the polarised sunlight (glare) reflected off the water's surface, allowing them to more easily see the fish underneath. In the case of the moon, the orientation of the polarised light from a point on the moon will depend on the orientation of the lunar surface at that point as seen from ASKAP's location. The overall effect is that the orientation of the polarisation will always point towards the centre of the moon, with the intensity being greatest at the limb. Since colour was used to represent the orientation of the polarisation in the image above, we see this as a rainbow effect along the lunar limb with opposite sides of the moon having the same colour. Since the effect is strongest at the limb this gives an overall "soap bubble" effect. (Image credit: E. Lenc, V.A. Moss, & K.W. Bannister (CSIRO) and D.C. Price (Curtin/Berkeley))

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