by Rex Parker, PhD director@princetonastronomers.org

October 10 Meeting at Peyton Hall on Campus.  We will meet once again in person at Peyton Hall for our next monthly meeting.  Thanks to Ira and Dave, we’ll also be running a hybrid meeting through Zoom.  For information about the guest speakers, please see Victor’s section below. We hope to see you there in person or on Zoom! 

Red Sky in Morning, Astronomers Take Warning.  Sailors and astronomers alike have witnessed an unwelcome physics experiment over the past summer.  Our G-type star, the sun,  normally appears white or yellowish, a blending of all the colors it emits.  This summer it turned red, as if it were an M-type red star such as those we sometimes observe through telescopes.  The cause was huge flaming wildfires in the boreal forests of Canada, with unimaginable volumes of smoke released into the atmosphere and spread by currents across the northern US skies.  Combustion of forest biomass produces a complex mixture of carbon-based particles and volatile organic compounds, nitrogen oxides, and other trace minerals.  It’s been estimated that over a million tons of smoke particles were released from the fires burning in Quebec alone this summer. 

The sun in its red costume as seen from southeast Alaska, July 2023. Photo taken without filters using a Canon Powershot SX70, by RA Parker.

So just why did smoke in the air this summer cause our sun to appear red, even at noon?  Of course, you should never look directly at the sun without proper filters even when it’s red!  This is a different physics phenomenon than the Rayleigh scattering that explains why the daytime sky is blue and turns red at sunset and sunrise.  Rayleigh scattering involves an interaction of air gas molecules with electromagnetic waves, where the molecules act as radiating dipoles whose radiation appears as scattered light.  The interaction occurs only when the molecular diameters of gases (particles) are much smaller than the wavelengths of light.  The shorter wavelength blue light is scattered much more by this mechanism, which we perceive as a blue sky.  When the Sun is lower in the sky the path length is greater, resulting in even more blue scattering, and thus red penetration to our eyes.

However, a very different type of light scattering results when the size range of particulates suspended in air is comparable to the wavelength of light.  The diameter of wildfire smoke particles typically ranges from ~0.5 to 1 micrometers, overlapping the wavelength range of red light but greater than blue light.  The scattering of light waves by particles of comparable size is known as Mie scattering, after the German physicist Gustav Mie who solved the physics of light scattering and absorption in 1908. Mie derived his solution from the famous Maxwell equations (which few could understand then and even today), where he quantified the scattering of an electromagnetic wave by spheres of defined radii.  One feature of Mie scattering is that certain sizes of particles scatter light especially strongly, known as the Mie resonances.  Wildfire smoke particles fall into such a resonance, so that Mie scattering dominates and results in a red sun and sky background. 

Gustav Mie’s papers were hardly noticed for 50 years, apparently because computers hadn’t been invented yet and the equations were quite difficult!  But more recently his works on scattering and absorption have been referenced over 4000 times in astronomy, meteorology, and fluid dynamics.  If you are interested in a deeper dive into the story, see this reference:  H. Horvath, Journal of Quantitative Spectroscopy & Radiative Transfer 110: 787-799 (2009).

Update:  Lunar South Pole Observing Challenge.  The members’ observing challenge which I presented last winter is still open, as we’ve had no reports of a successful observation yet.  The challenge is to observe with your telescope (and image, if possible) the extreme southern polar region of the moon around Shackleton Crater.  This region is hugely important to the future of lunar exploration for many reasons, with data from orbiting instruments strongly indicating the presence of water ice in permanently shadowed regions of Shackleton and nearby south pole craters.

It may sound simple, but observing the moon’s south pole is quite difficult.  The trick for earthly astronomers is to observe the moon at precisely the right time in the monthly lunar libration sequence when the extreme south pole craters become visible, perhaps one night a month.  This is described further in Sky & Telescope, March 2022, “Meet Shackleton Crater:  Future Moon Landing Site”.  Observing the south pole from an off-our-planet vantage point is another way to do it.  Indeed, this happened twice in the past couple of months!  So, here are two challenge “winners”:  the Indian Space Research Organization (ISRO) Chandrayaan 3 lunar lander, and NASA’s Lunar Reconnaissance Orbiter (LRO) together with the Korea Aerospace Research Institute (KARI)’s Korea Pathfinder Lunar Orbiter (KPLO) mission, also known as Danuri.   

On Aug 23, ISRO’s Chandrayaan 3 became the first spacecraft to land near the lunar south pole.  The Chandrayaan-3 mission is unfolding at a time when the US and China are planning future crewed missions to the region.  NASA aims to land astronauts and eventually build a base near the lunar south pole in perhaps 2026 on its Artemis 3 mission and beyond. This is essential to developing the Lunar Gateway, the first planned extraterrestrial space station in lunar orbit, eventually providing a path to Mars and beyond.

Last week NASA released the first-ever detailed photograph of the permanently shadowed Shackleton Crater located very close to the south pole last week.  This photo was an innovative mosaic of data from two sources:  the Lunar Reconnaissance Orbiter Camera (LROC) and a new NASA-developed instrument called ShadowCam aboard KARI’s Danuri orbiter. The super high resolution of LROC is half-a-meter per pixel, but requires bright illumination such as Shackleton Crater’s rim.  ShadowCam, about 200-times more light sensitive than LROC, was able to reveal details in the permanently shaded interior of the crater.  The resulting composite picture shows the 2 mile deep and 12 mile wide Shackleton Crater as never before seen by human eyes.  For more info, see this link: NASA shares unprecedented view of moon’s south pole region (axios.com)

The first detailed image of Shackleton Crater at the south pole of the moon, in a mosaic photo from two lunar orbiters.  See the text above for explanation. Credit:  NASA