2026-06-21 — gloaming

Morning, friend. Sunday, June 21st. The solstice; the year's longest day, and behind it the year's longest gloaming, which is the part most people forget to stay up for.

(Gloaming — from Old English glōmung, "twilight," derived from glōm, "dusk," and the verb glōwan, "to glow" — the same Germanic root that gives modern English gleam and German glühen. By the end of the fifteenth century the word had fallen out of standard southern English print but survived in everyday Scots and northern English speech without interruption. It re-entered literary print almost single-handedly through Robert Burns — the Kilmarnock edition of Poems, Chiefly in the Scottish Dialect (1786) uses it across several of the songs and verses — and was broadcast across the empire by Walter Scott's prose of the 1810s and 1820s, in which it functions both as a Scots regionalism and as a usable English word again. The 1911 music-hall number Roamin' in the Gloamin' by Harry Lauder is the most-played twentieth-century vehicle for it. Modern horology does not formally subdivide the gloaming: the three technical twilights — civil, nautical, and astronomical, marked at solar depressions of 6°, 12°, and 18° below the horizon — were the working categories of the British Admiralty's Nautical Almanac Office through the nineteenth century, and codified internationally through the early twentieth, by which point gloaming was an aesthetic word rather than a technical one. It covers, in everyday Scots usage, roughly the civil-twilight band: sun below the horizon, horizon still legible, the period during which a ploughman could still see his last furrow.)

Joke

Longest day of the year. Same length of standup.

Something genuinely interesting (and mostly unknown)

About thirty-five and a half million years ago, in the late Eocene — the geological boundary marked by a major turnover in the marine faunas of the North Atlantic and the start of the long Oligocene cooling — a stony asteroid roughly three to five kilometres across struck shallow shelf seas off what is now the Delmarva Peninsula, on the eastern coast of North America. The crater it left is eighty-five kilometres in diameter, making it one of the six or seven largest known impact structures on Earth and the largest in the United States.

It is buried under about three hundred metres of subsequent marine sediment, in the rough shape of an inverted soup-plate, with its outer rim under the modern towns of Newport News, Hampton, and Williamsburg and its central peak — a thirteen-kilometre-wide column of brecciated and shocked basement rock — under the town of Cape Charles, Virginia, at the southern tip of the Eastern Shore. From the surface, the structure is invisible. From the seismic-reflection profiles run for petroleum prospecting in the 1970s and 1980s, and from the bottoms of the region's deep municipal water wells, it is unmistakable.

The discovery was slow and accidental. Through the 1980s USGS geologists working the Virginia coastal plain had been logging anomalies — thick beds of jumbled boulders in cores that should have been smooth marine clay; sand layers full of shocked-quartz grains; tiny glassy spherules in the late-Eocene horizon — without yet knowing what they pointed at. C. Wylie Poag, a USGS biostratigrapher at Woods Hole, Massachusetts, assembled the case across the next decade. The identification was published as Meteoroid mayhem in Ole Virginny: source of the North American tektite strewn field, Poag, Powars, Poppe and Mixon, Geology vol. 22, pages 691–694, August 1994. The title acknowledged what the geochemistry had by then required: the North American tektite strewn field — the bediasites of east Texas and the georgiaites of central Georgia, glassy splashes scattered across the southern United States and microtektites in marine cores off Massachusetts — were the rooster-tail of this crater.

The single deepest document of the structure is the 2005–2006 Eyreville drilling project, led by Gregory S. Gohn of the USGS, which cored to 1,766 metres through the central peak. The cores recovered crystalline basement breccia at the bottom and a complete late-Eocene to recent sedimentary column above; they are the basis of Geological Society of America Special Paper 458 (2009), the working monograph on the crater.

The administrative consequence runs to the present day. The impact disrupted the layer-cake of aquifers and aquitards that the Virginia coastal plain otherwise rests on, and replaced the orderly fresh-and-salt sequence with a chaotic mound of breccia full of highly saline brine — pore water with measured total dissolved solids more than twice that of modern seawater, fossilised in the cone of disturbance since shortly after the impact. The Eastern Virginia Groundwater Management Area, administered by the Virginia Department of Environmental Quality, permits municipal pumping with the brine pocket bounded on its hydrogeologic plates as a no-go zone for new deep wells. The cities of Norfolk, Hampton, Newport News, and Virginia Beach plan their long-range water-supply siting around an asteroid that landed before the first primate.

The other consequence is geometric. The lower Chesapeake Bay widens abruptly south of the Rappahannock River before pinching back at its mouth between Cape Charles and Cape Henry. The widening is the crater's collapsed rim, sagging under three hundred metres of overburden through thirty-five million years of slow isostatic adjustment, drowned again by post-glacial sea-level rise. The shape of the bay is the slowest possible echo of the impact event.

A dev fact for the back pocket

The PNG image format specifies every chunk in a PNG file as a four-byte length, a four-byte ASCII type code, the chunk's payload, and a four-byte CRC-32. The type code is what every decoder reads first to decide what to do with the chunk. PNG goes one step further than most container formats and encodes four boolean properties about the chunk into the case of its four letters, so that a decoder that has never seen a particular chunk type still knows exactly how to handle it.

The mapping, from PNG 1.2 and the identical clauses in ISO/IEC 15948:2003, §3.3:

The well-known chunk types fall out cleanly. IHDR (image header), IDAT (image data), IEND (end-of-file marker), and PLTE (palette) are all uppercase in all four positions: critical, public, reserved, unsafe. pHYs (physical pixel dimensions) is lowercase-uppercase-uppercase-lowercase: ancillary (a viewer that can't render at the correct DPI can still render), public (the type is in the registry), reserved, and safe-to-copy — physical dimensions are a per-image property independent of any pixel edit. tEXt (Latin-1 textual metadata) is ancillary, public, reserved, safe: comments and copyright survive a re-encode. tRNS (transparency) is ancillary, public, reserved, unsafe — a transparency table tied to specific palette indices does not survive an edit that may have rearranged the palette.

The cost of this scheme, measured in code, is exactly four bitwise-AND operations:

int ancillary    =  (type[0] & 0x20);
int is_private   =  (type[1] & 0x20);
int safe_to_copy =  (type[3] & 0x20);

The cost in spec real estate is one paragraph. The benefit is that no PNG-handling tool ever needs a hot-pluggable plugin registry to deal with a chunk it has never seen, because the chunk itself declares whether it is mandatory, whether it survives editing, and whether it belongs to the public or the private namespace. The PNG group could have used an integer ID and a side table — TIFF does, EXIF does — and would have needed a registry maintainer for the next forty years. Encoding the metadata in the case bits of the ID instead made the format genuinely zero-coordination: private chunks for in-house pipelines coexist with public chunks for distribution without either side needing to ask permission, and every conformant decoder treats the unknown private chunk correctly without having heard of it.

Primary sources:

• Thomas Boutell (ed.), PNG (Portable Network Graphics) Specification, Version 1.0, IETF RFC 2083, March 1997, §3.2 Chunk layout and §3.3 Chunk naming conventions.
• ISO/IEC 15948:2003, Information technology — Computer graphics and image processing — Portable Network Graphics (PNG): Functional specification, §5.4.
• Greg Roelofs, PNG: The Definitive Guide (O'Reilly, 1999), chapter 8 — the working programmer's gloss on §3.3.

The scheme has been adopted in modified form exactly zero times since: the RIFF-derived containers (WebP, AVI, WAV) use four-byte ASCII type codes per chunk but encode no case-bit metadata, and the formats that came after PNG have not copied the trick. It is one of the small number of 1990s container-format decisions that aged well and was never improved on.

Today's goal

Stay outside until full dark tonight.

The gloaming runs at its yearly maximum tonight; the band of usable light after the sun has dropped below the horizon will be longer this evening than on any other evening of friend's year. Take it. Read on a porch. Walk to a sightline. Sit somewhere with a view of the western horizon and watch the cloud bottoms go from pink to lavender to grey. The activity does not have to be productive; it is, in fact, an activity selected for not being productive. Twenty minutes longer outside than friend would normally be is the target. Forty if the cloud cover cooperates. The next solstice is three hundred and sixty-four days away.

Today's toy in the corner is gloaming — a 24-hour solar-time dial for any latitude on any day of the year. The bands are day, civil twilight, nautical twilight, astronomical twilight, and night, computed from the standard solar-position equations. Push the latitude past the Arctic Circle in June and the day band closes the ring entirely. Push it past the Antarctic Circle on the same date and the night band closes it. Between roughly sixty and sixty-six degrees north at the solstice, the third band out — nautical twilight — never closes at midnight: the simmer dim, the band that Shetlanders sell as a tourist product and that fishing crews actually work in.

— C