Proof that there is indeed a hole in the ozone layer? That a divine geometry is indeed at work in the universe? Or for the Sunday poets (sic) the “bullet hole of God”. In Normandy, in the Oise, in the Loire Valley or in Toulouse, many of you this weekend were ecstatic about this spectacular perfect circle enveloping the full moon.
This appearance of a “lunar halo” is actually quite simple to explain. “It’s an optical phenomenon which is due to particles, mainly ice crystals, suspended in the air,” says Olivier Sanguy, head of space news at the Space City. It is a refraction of the light which arrives in the atmosphere, in this case that which arrives from the Moon. It is deflected by these particles.” With, luckily this time, a complete halo when it happens more often that there is only a simple arc of a circle.
The physical explanation is the same for diurnal solar halos which appear in foggy weather, or for rainbows.
Concerning this weekend, it was a “small halo, of 22 degrees”, or the measurement of the angle between the famous circle and the line which connects the spectator to the star of the night. “There are also large halos of 46 degrees,” emphasizes Olivier Sanguy.
An unpredictable appearance
Simple to explain, the lunar halo is not frequently visible. “All atmospheric conditions must be met,” says the man who is also an amateur astronomer. The night must be clear enough to see the Moon clearly, but it must also be a bit hazy, with “at five or ten kilometers altitude” cirrus clouds, these filamentous high-altitude clouds that are official suppliers of ice crystals. .
The lunar halo, unlike an eclipse, is “unpredictable”, which adds to the poetry of the moment.