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Light pollution

How dark is the sky over this place?

Under the clock, Celeris says how bright the night sky over the selected place is: a brightness in magnitudes per square arcsecond, a swatch in the colour scale of the light-pollution atlases and the Bortle class it comes to. The same reading stands beside every saved location and on every plan.

Celeris models that number itself — from the VIIRS night lights of the earth, with the Garstang scattering model the world atlases of artificial night sky brightness were computed with, and its scale fitted to sky quality meter readings. It covers the world from 65° south to 75° north; where nothing has been modelled, no line is drawn.

• Full version: After dusk the 3D sky shows what the number means — the background lifts, the faintest stars fade out along the limiting magnitude of the class, light domes stand on the horizon towards the towns. One switch in the settings takes it away again.
• Full version: one tap washes the map in the colours of the atlases
• Full version: “Dark places nearby” lists the darkest places worth standing on within 10, 25, 50 or 100 km — with distance, direction, the sky brightness there and how many magnitudes darker it is than here

A model, not a measurement: the satellite records the light going up, and the model turns it into the sky straight overhead on a clear night. The Bortle class beside it is an approximation — that scale judges the whole sky by eye, this figure is the zenith alone. This product was made utilizing VIIRS Nighttime Lights (VNL) data produced by the Earth Observation Group, Payne Institute for Public Policy, Colorado School of Mines, CC BY 4.0; the scale is fitted to sky quality meter readings collected by Globe at Night (NSF NOIRLab, CC BY 4.0).

Celeris · Apple Watch

A companion at the tripod, not a small Celeris.

Seven pages, one above the other, because a watch is read by flicking rather than by navigating in and out — eight on a day this place has an eclipse. Each of them names itself at the head of the screen.

• Tonight — the whole night drawn as a ring: sunset at twelve o’clock, clockwise through the twilights to the stretch of real dark opposite and back to sunrise, with the dark-night window written in the middle of it, a gold hand on the moment, a thin arc for the moon over the ridge and a tick for the station. Under it the sun’s and the moon’s times and how much of the moon is lit, for the position on the wrist or for the place the phone last looked at. Where the phone has measured the land, the ridge time stands beside the almanac one.
• The eclipse day — a page that is there only on a day this place has an eclipse, and stands second while it is: the countdown to the next contact in gold, the two discs sliding across each other, and under them first contact, totality, the maximum and last contact as times, the ones already past in grey, with how much of the disc goes at the deepest moment. On every other day of the decade it is not there at all.
• Where it stands — the compass, and the one page that is an instrument rather than an almanac: a dial filling the screen, faint stars inside it, a machined ring around it and the measured land as a band of hills at its edge. The sun, the moon, the galactic centre, any of the seven planets or the object chosen on the phone stands on the sky itself — the bearing is the angle, the height is the distance in from the rim — with a needle out to it, gold where it can be seen and dashed where it cannot, and two marks on the ring for where it comes up and where it goes down. The Digital Crown scrubs the time twelve hours either way. Aimed at the next plan’s standpoint it becomes navigation to the tripod hole, and in the last thirty metres the plan’s own frame stands on the dial as a gold wedge, exactly as wide as the lens it was composed with, to line the wrist up on.
• Galactic centre — the core’s climb from sunset to sunrise as a curve, with the hills it has to clear drawn under it and the stretch of real dark behind it: when it clears the ridge, when it stands highest and how high, when it goes, and the best moonless window of the night.
• Moon — the phase drawn as a disc, lit exactly as much as it really is tonight and on the side it really is, moonrise and moonset with the ridge time beside them, the dates of the next new and full moon, and how many hours of tonight’s real dark the moon leaves alone.
• Next plan — the frame you framed as a card, with the countdown standing on it in gold, and under it the name, the place, the time and the lens. Three taps on the wrist announce it: ten minutes before, one minute before, and at the moment itself. Four more are for the night itself, saved or not — before the sun reaches the measured ridge, when the dark night begins, when the moon clears the land, and before the station comes over — and on an eclipse day two more, long enough to get a filter on the lens and then to take it off again.
• ISS — the next pass that can really be seen from here, counted down to and written out from when to when and how high it gets, and the next crossing of the sun or the moon within three days, to the second.
• Sequence timer — exposure and frame count set with the Digital Crown, then a gold ring filling as the run goes, the frame it is on, the time left, and a tap on the wrist as each frame lands.

Three complications put the same night on the watch face: Main Events in the rectangular, circular, inline and corner slots, Next plan in the rectangular and inline ones, and the Night ring in the two round ones — the Tonight page’s ring, with the hours of real dark in the middle of it.

Everything arrives from the iPhone whenever the two are near — the place, the terrain skyline, the plans and their pictures. The watch asks for nothing on its own and never goes online.

Every page, the eclipse day included, is part of the free app; the three complications belong to the full version, like the other widgets. There is no map, no 3D sky and no purchase on the wrist: the watch follows what the phone has bought.