James Turrel's DEER SHELTER SKYPACE in the Yorkshire Sculpture Park is a great place to experience the colour of the sky. Think of it a a huge cyanometer. ysp.org.uk/art-outdoors...

b #OTD 1740 Horace Bénédict de Saussure, Swiss natural philosopher famous for his ascents of Alpine peaks. He invented the cyanometer (1789), a graded scale of blues used to assess intensity of sky colour, and thus (he believed) state of weather. Used by v. Humbolt in his S.American journeys.

b #OTD 1781, René Laennec, pioneer of auscultation, diagnosis by listening to bodily sounds through a tube. Doctors in his day liked that his tube made it unnecessary to place the ear in contact with the body of an unwashed and possibly unhygienic patient and did not invade a woman’s privacy!

b OTD 1822 polymath Francis Galton. Interested in the limits of human hearing, he invented a whistle capable of producing ultrasounds up to 55kHz. He used it to test the hearing of people, usually without letting on that he was doing so, and tested every creature in London Zoo.

d OTD 1650, René Descartes, the natural philosopher who in 1637 published the first correct mathematical account of the angular dimensions of the primary & secondary arcs of the rainbow.

Died OTD, French physicist, Felix Billet, published in 1868 results of an experiment in which he illuminated a thin, vertical stream of water with sunlight and managed to see 19 rainbow orders including supernumeraries. He named it “la rose des arc-en-ciel”, known in English as Billet’s Rose.

I went the the Natural History Museum (London) In June 2019 to see Luke Jerram's splendid Museum of the Moon. Although the surface of the sphere was smooth, the shading fooled one's eye into interpreting it as embossed. My tactile acuity with Braille map didn't my match visual acuity!

Moire patterns are often noticed as one's view of the elements that create them changes. Here is a short film of a moire formed by Venetian blinds and a garden fence with open horizontal slats.

If ever you need proof that a rainbow is centred on the ye of the beholder…

This morning frost on a plastic manhole cover reveals the embedded metal grid that strengthens it.

The first systematic measurements of the speed of sound in water were made in Lake Geneva in November 1826, by Daniel Colladon a Swiss physicist, with the help of his father, some two centuries after the earliest measurements of the speed of sound in air.

L. Galvani d OTD, became interested in animal electricity when a student dissected a frog’s leg while another operated a static electricity generator, noticed that when it sparked the frog’s leg contracted involuntarily. Is it possible to claim a frog, martyr to science, had picked up a radio wave?

E.Chladni b OTD 1756, known for the patterns formed on a glass plate sprinkled with fine sand & exited with a bow, was also the first person to measure the speed of sound in gases other than air by comparing the pitch of an organ pipe filled with a gas with that filled with air.

Among his many interests, Athanasius Kircher d OTD 1680, Jesuit scholar was fascinated by architectural acoustics & unusual echoes. He found many examples of buildings that combine these things, among which was Villa Simonetta which had a remarkable flutter echo. Bombed during WW2

Now what's going on here? Why don't the primary & secondary arcs reach all the way to the ground? The short answer is that the horizontal extent of the rain cloud is not broad enough, i.e. no rainfall either to the right or left of the truncated arcs.

In 1972 astronauts in a hyperbaric chamber training for Skylab discovered they were unable to lip whistle. The atmosphere within the chamber was mixture of 70% O2 and 30% N at 1/3 atmospheric pressure. Low air density may have prevented the necessary vortices from forming.

Reflections from window panes illuminated by low sun. If air pressure within the space between glass double glazing is below atmospheric pressure the panes become concave and reflect light as shown in diagram. Red rays create bright cross due to surface distortion because glass held in square frame

We can only see a single colour emerging from any given drop which is why a #rainbow requires a multitude of drops. This is evident in sunlit drops that twinkle as one moves one's gaze across a dew laden lawn.

Whether you see a full arc or only a segment of a #rainbow depends on the height of the sun, horizontal extent of the rain shower and, as here, how close you are to it. And, of course, as you approach the shower, the position of the arc relative to the background changes.

Not every coloured arc is a rainbow! This is a very good example of a circumzenithal bow that is formed by hexagonal ice crystals. The sun is just out of view bottom right.

This photo shows a feature of #rainbows that is always present but seldom noticed, namely that the sky enclosed by the primary arc is bright. The rainbow arc is formed by light that is deviated least by its passage through a drop, and which forms a caustic, aka rainbow ray.

Great example of how a rainbow formed by the reddened sunset light is deficient in light from the blue end of sunlight.

This is the sort of rainbow that every aficionado of the phenomenon hopes to encounter, but seldom does! Note the dark band between arcs & brightness within the primary. The coloured arc is always centred on the observer's eyes, so the head's shadow marks the anti solar point & centre of the arc.

A good illustration of the progression of sun's rosy hue during a sunset. Clouds in the upper right are uncoloured because they are now within the earth's show and so no longer directly illuminated by the sun. As the sun drops further below horizon, the rosy moves west (towards right of image)

Fog bows are, as the name implies, formed in very small droplets of water such as those of which clouds and fog are composed. This one is a particularly good example of the phenomenon. For an explanation see www.atoptics.org.uk/droplets/fog...

High altitude clouds are the sine qua non of memorable sunsets. Without clouds to reflect the reddened sunlight coming from just below the horizon that light is only visible as a strip along the horizon.

When the sun is just below the horizon, the distribution of colours in sunlight reaching the ground is skewed in favour of the red end of the spectrum due to Rayleigh scattering. Rain illuminated by this light results in a rainbow in which the red arc is very prominent.

This huge spraybow shows that these eye catching arcs are not confined to rain showers.

Sunshine & showers are the sine qua non of rainbows. But always bear in mind, a rainbow only exists if it is seen. This bright rainbow exhibiting many of the classic features was visible for several minutes from a street near my home a couple of days ago. An endlessly fascinating phenomenon!

Quite apart from the aesthetic aspects of reflections from the surface of gently undulating water, I enjoy the challenge of trying to identify the various sources that give rise to them by working out how the curvature of the surface brings about the distortions.

This is a particularly striking example of a phenomenon that is often taken to be a rainbow by the uninitiated, ie by those for whom all coloured arcs are rainbows.

Faraday was a natural philosopher in the fullest sense of that calling, perhaps one of the greatest there has ever been. It was he who first suggested (1846) that light is an electromagnetic phenomenon, an idea that was taken up & mathematised by J C Maxwell in his theory of electromagnetism.

A variety of still photos from my trip to Háifoss in Sept 2022 that I posted the video of. Beautiful and impressive place :) #Iceland

Back in February I took a series of 'star squiggles' photos, where I did 3.2sec exposure while wobbling the tripod to capture scintillation in the trails and the general star colour. I included Mars and Jupiter to show that planets do not scintillate. This was just a bit of creative fun

This curious pattern is due to convection in fruit puree.

Great illustration of the relative angular dimensions of the moon & rainbow. Rainbow arc aprox 2° wide, Moon diameter aprox 0.5°.

In a letter (1632) to J. Golius, Snel’s successor in the chair of Maths at Leiden, Descartes cheerfully admitted that he had never carried out a direct experimental test of the law. Golius found that Snel had already discovered the law of refraction, but never suggested that D. had plagiarised S.

Runge & Goethe corresponded on the subject of coloured shadows, a phenomenon first described by da Vinci tinyurl.com/25b49db9 & correctly explained by G. Monge (1789). Descriptions on how to produce such shadows by Goethe tinyurl.com/476pbny6 & Rumford tinyurl.com/43w5vzj9. My flashlight version

As the author of "Now Hear This, A Book about Sound" I shall be listening on #WorldListeningDay to Radio Lento if I can't get out and about. ebook available at a generous discount here tinyurl.com/bdcud8mv

As an aficionado of sources of unusual sounds, I commend your wife’s ambition! My shop bought TT produces a large number of closely spaced harmonics, which one would not guess from its raucous sound. My T.T., 18cm long, is loudest at ~450Hz.

A sunpillar is, in fact, a glitter path in the sky, an atmospheric twin to the glitter path that one sees when looking out at a large body of water when the sun is close to the horizon. In the latter case, light is reflected from the surface of waves that crisscross the water.

This is an excellent example of a late evening rainbow, when raindrops are illuminated by reddened light from a setting sun. Hence the inner "blue" band is almost invisible. Also note contrast in brightness between the sky enclosed by the rainbow and the so called 'dark band' beyond it

Dave’s image gives the lie to the common claim that a sunpillar is due to sunlight reflected by a vertical column of ice crystals. Here clouds stretch almost to the horizon, and are closer to the eye than the sun, which would block the vertical column of crystals. This very evident in my photo.

Parhelia are formed when sunlight passes through a cloud of hexagonal plate ice crystals. If the cloud is sufficiently extensive (say 60°) and centred on the sun, there may be parhelia ~25° either side of sun. Quite common though really bright ones are rare in my experience.

Eye catching! Reflections in rippled water are amenable to the laws of reflection as you will learn from tinyurl.com/4zewy2j7 . As the diagram shows, the surface of rippled water provides several reflections of any given point on an object seen from the far side of a body of water.

This is the optical equivalent of an anechoic chamber. Most instructive! Tempted to buy a small pot of Musou Black to experience the effect it has on obliterating surface features.

The barometer was not made by Torricelli himself, but that he imagined what would happen and confided his thoughts to his great friend Viviani who had the apparatus made, procured the mercury, and was the first to make the experiment and to see the effect forecast by Torricelli.

A whispering wall behind Tate Modern; just across the river from the most famous, though by no means the only, whispering gallery in the world at the base of the dome of St.Paul’s. Had the wall been made taller and a shelf added on which one could sit, the Tate would have another "installation"

If you are interested in the science of sound & hearing, “Now Hear This, A Book About Sound” is a wide-ranging exploration of the nature of sound and hearing Employing a combination of physics, biology, philosophy, literature, history, anecdote and personal experience.

The sound of the eruption was heard 20mins later in Vancouver 330km away but not heard not heard within 100Km of Mt St Helens. A classic example of anomalous propagation of sound/zones of silence. See tinyurl.com/3v7u5zwh pp 186-8