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John Naylor @johncnaylor.bsky.social · 28/09/2026

You couldn’t get a finer view of the fabled ‘seven colours’ of a rainbow than this, always bearing in mind that it’s colours blend into one another, unlike those that can be produced with a prism or diffraction grating

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John Naylor @johncnaylor.bsky.social · 23/09/2026

Sound is a wave so diffracts wherever it encounters an object or an opening. With few exceptions, almost every sound we hear consist of a collection of wavelengths. Short wavelengths are diffracted much less than longer ones, so low frequency sounds remain audible when source is blocked.

$Waves diffract around an object. (A) Longer wavelengths wrap around an object. (B) shorter wavelengths create a shadow beyond an object$

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John Naylor @johncnaylor.bsky.social · 23/09/2026

Effects of diffraction first noted by F.M.Grimaldi c.1665. Thoroughly investigated by Newton c 1675 with his usual phenomenal attention to detail and measurement. He accepted that water waves diffract but refused to accept that the phenomenon is due to wave nature of light.

$Newton’s illustration of diffraction of waves in water traveling from A to PQ, after passing through a narrow opening, (Principia Mathematica: Motion propagated through fluids, Prop 41)$

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John Naylor @johncnaylor.bsky.social · 22/09/2026

Lunar & solar coronas are not rare but they are seldom as complete and well defined as this one. Note to self : when out & about, look up at the sky not down at my shoes!

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John Naylor @johncnaylor.bsky.social · 31/08/2026

Marin Mersenne, Minim friar, died 1 Sept 1648. He was the first person to discover experimentally how the frequency of a vibrating wire is related to its physical characteristics, to measure the speed of sound (by echoes & gunfire), and to determine the least pitch audible to humans is 16Hz.

Marin Mesenne
8 Sept, 1588 - 1 Sept, 1648

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John Naylor @johncnaylor.bsky.social · 31/08/2026

Hermann v Helmholtz, born 31 Aug, 1821, not only the foremost authority on the optics of the human eye during latter half of 19th C., he was also the leading authority on the perception of sound and its relationship to music. Author of “On The Sensations Of Tone” & “Treatise on physiological optics”

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John Naylor @johncnaylor.bsky.social · 28/08/2026

This is a very clear example of a rainbow feature often ignored by casual observers: the sky enclosed by the arc is filled by light reflected by rain drops. Very obvious here because the rain is illuminated by the rosy light of a setting sun, light that is deficient in blues & greens

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John Naylor @johncnaylor.bsky.social · 28/08/2026

The bluish fringe just beyond the reddish umbra may be due to sunlight that has passed though the earth’s ozone layer. Ozone strongly absorbs longer wavelength light, which gives the gas its characteristic bluish tinge. Often Noticeable at zenith during twilight.

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John Naylor @johncnaylor.bsky.social · 25/08/2026

As a variation on the hackneyed theme of the sound of falling trees in the absence of a listener, have you ever asked yourself the same question about rainbows? Can there be a rainbow if there is no one to see it?

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John Naylor @johncnaylor.bsky.social · 22/08/2026

An excellent example of a double rainbow that shows very clearly important features of a rainbow that one should look out for: well defined colours in primary arc, together with supernumerary arcs, the dark band between primary and secondary and colour reversal of secondary arc.

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John Naylor @johncnaylor.bsky.social · 14/08/2026

Colanders are so passé. The shape of the hole is immaterial as long as the distance to the screen is a least 110 times hole diameter. First explained by J.Keppler in 1600. He named these pinhole images "figurationes lucis ".

When a screen with holes of different shapes is close to a screen, the images match those of the holes.

At a suitable distance the images match the source, here the crescent sun during eclipse. They are not perfectly in focus because the holes in this case are approximately 0.75cm.

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John Naylor @johncnaylor.bsky.social · 14/08/2026

Gringos on eclipse safari in the Wimbledon Common savannah

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John Naylor @johncnaylor.bsky.social · 12/08/2026

I imagine that a fair number of novice astronomers, having purchased eclipse spectacles, will be dismayed at how tiny the solar disc is, no larger than the finger nails of one’ little finger held at arm’s length.

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John Naylor @johncnaylor.bsky.social · 11/08/2026

Important to note that (a) the distance between hole & screen must be at least 110 hole diameter for a clear image, and (b) under those conditions the shape of the hole plays no part. Any shape projects clear image of the sun.

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John Naylor @johncnaylor.bsky.social · 10/08/2026

For all the recommendations to view the upcoming eclipse using colander, the use of a sieve or crossed fingers to project eclipse image is nothing new. See "Problemata" Book 15, sec 11, a work attributed to Aristotle c. 350BC, but likely by his followers.

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John Naylor @johncnaylor.bsky.social · 09/08/2026

Bees are sensitive to near u.v. and insensitive to the red end of the spectrum, so in theory can see uv rainbows (but not rosy sunsets?). Diagram from p. 274 of my vade mecum of optics "Seeing the Light, Optics in Nature, Photography, Colour, Vision and Holography" by D.Falk, D.Bril. D.Stork.

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John Naylor @johncnaylor.bsky.social · 09/08/2026

Multispectral rainbow photos by Andrew Steele. These three images are of the same bow using i.r. and u.v. filters. The central portion is of the visible light bow, below this is the infrared bow and above it is the ultraviolet bow. Newton, of course was unaware of these invisible radiations.

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John Naylor @johncnaylor.bsky.social · 09/08/2026

The reason that one can't see indigo is the spectrum is because it isn't there! Newton added indigo & orange to his original description of 5 spectral colours in order to make the number up to 7 in accordance with his belief that Pythagorean musical numerology applies to light.

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John Naylor @johncnaylor.bsky.social · 09/08/2026

SOLAR ECLIPSE: Earth and Moon both orbit the Sun in the same direction, but at this point of their orbits Earth is travelling faster than the Moon. Hence the Moon’s shadow moves across Earth’s surface from west to east as the Earth overtakes the Moon.

To understand the mechanics of a solar eclipse bear in mind (1) both Earth & Moon orbit the Sun in the same distraction & (2) at this point in their orbits, Earth moves faster than the Moon

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Reposted by John Naylor

Mary McIntyre @marymcintyreastro.bsky.social · 02/08/2026

I've just published a detailed video covering how to observe and photograph the deep #PartialsolarEclipse and the #Perseids #MeteorShower on 12th August 2026. Timestamps for each part are in the description box. Watch here: youtu.be/ZTwRaAbYvag

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John Naylor @johncnaylor.bsky.social · 02/08/2026

If you are interested in rainbows, mirages, halos etc then you will find Stanley Gedzelman's "Wonders of the atmosphere" www.stanrenaissanceman.com/BOOKS/WONDER...

stanrenaissanceman.com

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John Naylor @johncnaylor.bsky.social · 01/08/2026

For a thorough explanation of mirages see aty.sdsu.edu/mirages/miri...

aty.sdsu.edu

Mirages and Green Flashes

Mirages

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John Naylor @johncnaylor.bsky.social · 01/08/2026

An inferior mirage. Requires air layer next to the ground warmer than that further up (here a hot runway). This causes light that skims the ground to refract upwards reaching observer from below eye level. What appears water is refracted skylight, that can be mistaken for a pool.

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John Naylor @johncnaylor.bsky.social · 20/07/2026

Shadows among the most fascinating optical phenomena you are likely to encounter daily. A shadowless world might be very disorientating and certainly less interesting.

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John Naylor @johncnaylor.bsky.social · 20/07/2026

Shadows are an important clue to the position of an object with relation to the ground. Cover up the “shadow” in each of these photos.

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John Naylor @johncnaylor.bsky.social · 20/07/2026

For an early account of the phenomenon see American astronomer D Rittenhouse in 1786 “Explanation of an Optical Deception” tinyurl.com/2p86y8eh

tinyurl.com

Explanation of an Optical Deception | JSTOR

D. Rittenhouse, Explanation of an Optical Deception, Transactions of the American Philosophical Society, Vol. 2, 1786

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John Naylor @johncnaylor.bsky.social · 20/07/2026

It seems that our visual system is primed from the very start to assume that the world is lit from above. Hence this compelling illusion.

Shading creates the illusion of convexity & concavity even though the surface is perfectly flat

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John Naylor @johncnaylor.bsky.social · 19/07/2026

Can the MetOffice really want us to believe that “Warmer air can hold more water vapour than cooler air…” Of course not. So why mislead us with cod science? Fear that the correct explanation can’t be understood by the public? weather.metoffice.gov.uk/learn-about/...

weather.metoffice.gov.uk

Understanding humidity

Humidity refers to the amount of water vapour present in the air. It has an important influence on our atmosphere, affecting weather, climate, and even human and animal health.

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John Naylor @johncnaylor.bsky.social · 18/07/2026

How interesting. Images of the Sun’s spectra don’t usually include uv and ir. Note that absorption lines are present in both uv and ir. The atmosphere in transparent to visible light but much less so to uv & ir, which is why this spectrum was made beyond the atmosphere

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John Naylor @johncnaylor.bsky.social · 08/07/2026

A couple of unusual things here. Both the full moon and and the arc of a rainbow have a fixed angular: rainbow 2degrees, moon 1/2 degrees. As a result, both exhibit the ‘moon illusion’. Indeed the illusion is much more obvious with rainbows because you can make your own & compare with another one.

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John Naylor @johncnaylor.bsky.social · 05/07/2026

Great image. Not ply a bright primary arc, the secondary is also brighter that most such arcs, which emphasizes the dark band between the arcs. And there is at least one supernumerary arc inside the primary.

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Reposted by John Naylor

Frank Swain @frank-swain.bsky.social · 24/06/2026

It's probably time European journalists wrapped their heads around Heat Index (temp + relative humidity). In high humidity, sweating is less efficient, making heat worse, e.g.: Experiencing 35°C in London UK (70% RH) is more dangerous than 50°C in Phoenix Arizona (15% RH)

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Reposted by John Naylor

Nick Grant @ecominimalnick.bsky.social · 25/06/2026

Healthy humans can survive high temperatures but die if we can't dump the heat we generate. Over 100F/37C the only mechanism is evaporation which slows as RH rises. We can't cheat physics. (OK, we radiate to a clear sky but gain heat from hot surfaces & solar). www.bbc.co.uk/news/health-...

Graphs showing danger zone for combination of high temperature and RH

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Reposted by John Naylor

Astronomy Picture of the Day 🪐 @apod.shinyakato.dev · 25/06/2026

🔭 Anticrepuscular Rays over Sicily Image Credit & Copyright: Marcella Giulia Pace Text: Cecilia Chirenti (NASA GSFC, UMCP, CRESST II) ap260625

A landscape with an open horizon shows
sunset color and coverging rays of light.

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John Naylor @johncnaylor.bsky.social · 08/06/2026

Lots to see here! "All the colours of the rainbow"? I see only 4 clearly. Note that the sky appears darker beyond the red edge (aka Alexander's dark band) and brighter beyond the blue one.

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John Naylor @johncnaylor.bsky.social · 05/06/2026

An excellent example of a a double bow.

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John Naylor @johncnaylor.bsky.social · 25/05/2026

Great example of crepuscular rays. They are formed by sunlight that is not blocked by tall clouds below the horizon. The dark gaps are the shadows of cloud tops. They appear to spread out due to perspective.

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John Naylor @johncnaylor.bsky.social · 19/05/2026

To see a circular rainbow, one has to be able to see drops below the anti solar point. This can be achieved either by being able to look down on a rain shower from well above the ground (say using a drone) or being very close to it (say the spray from a garden hose).

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John Naylor @johncnaylor.bsky.social · 16/05/2026

Why do the most distant reflections of the perfectly circular holes appear to be hexagonal?

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John Naylor @johncnaylor.bsky.social · 16/05/2026

Daliesque reflections in water

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John Naylor @johncnaylor.bsky.social · 10/05/2026

The wave theory of light was originally proposed by T. Young. Fresnel developed the necessary mathematical underpinnings of the theory. Despite their cordial relations,Young always considered the wave theory as his brainchild and maintained that Fresnel had merely extended his ideas.

Despite their cordial relations, Young always considered the wave theory as his brainchild and maintained that Fresnel had merely extended his ideas. Late in 1824, an exasperated Fresnel felt it necessary to put him right.

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John Naylor @johncnaylor.bsky.social · 30/04/2026

And there’s a challenge. Can you identify something just from the sound it makes when it is lashed by rain or blasted by wind? It’s not impossible, though it takes not a little practice. Thomas Hardy learned to do so, as is obvious from the opening paragraph of his "Under the Greenwood Tree"

Under the Greenwood Tree by Thomas Hardy

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John Naylor @johncnaylor.bsky.social · 12/04/2026

Not only a spectacular image of a fog bow, but also of at least a couple of supernumarary arcs.

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John Naylor @johncnaylor.bsky.social · 02/04/2026

Born OTD, F.M.Grimaldi, c. 1640 carried out a wide ranging experimental investigation into the properties of light as they were then known, with a view to investigating the Peripatetic idea that light is an accident, i.e. a quality, rather than a substance & in the process discovered diffraction.

Francesco Maria Grimaldi, Jesuit mathematician, 2 April 1618 – 28 December 1663

$(a) Sunlight enters a darkened room through a narrow opening AB. FE is a small disc casting an umbra GH and penumbra IL. Grimaldi noticed that the penumbra is surrounded by faint, coloured bands extending to M & I. (b) Grimaldi’s diffraction. Sunlight enters a darkened room through a narrow hole CD. EF is a board, GF a hole larger than CD. The geometric edge of the spot of light should be NO. But the spot itself extends beyond this to I and K$

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Reposted by John Naylor

Oliver Morton, Eater of Sun @eaterofsun.bsky.social · 31/03/2026

If the impending launch of Artemis II has you think "whats this whole Moon mullarkey about anyway", could I recommend my book on the topic. "The Moon: A History for the Future"? I think its pretty good and I am reliably assured that others do to. www.google.co.uk/books/editio...

google.co.uk

The Moon

An intimate portrait of the Earth's closest neighbor--the Moon--that explores the history and future of humankind's relationship with itEvery generation has looked towards the heavens and wondered at ...

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John Naylor @johncnaylor.bsky.social · 31/03/2026

I have long liked to stand and stare (shadows, reflections in water, rainbows, blue skies etc), but all too often I failed to linger and listen.So I have belatedly resolved to make up for lost time and listen to the world as attentively as I look at it,

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