The instrument

An eye is not a lens. Naked-eye viewing maps an angle to an angle; a camera maps it to f·tan θ, with a sensor, a focal length, an aperture and an exposure that draws star trails and dots a flashing light. Switch the device and the whole frame changes — the same sighting through three of them.

Only what that device could actually have been set to is offered: an Instamatic had one aperture and one shutter speed, so there is nothing to choose, and a camera that did not exist yet is flagged against the observation's own date.

And the instrument decides how faint a thing could be recorded at all. That same Instamatic stops at magnitude 4.2 — two short of the observer holding it, which is half of why so many “the sky was full of stars” accounts come with an empty black photograph. A 50 mm at f/2 for twenty seconds reaches 9.7, three magnitudes past that observer. Aperture, shutter and focal length settle it against the sky's own brightness.

A longer pose stops helping once the sky has slid further than the lens can resolve, which is why an hour on a tripod is no deeper than five seconds on one — only longer trails.

Something can also be held in front of the instrument. A solar filter — eclipse glasses, or the film that goes over a lens — cuts everything by a hundred-thousandfold: the sky goes black, no star is drawn, and the Sun is left alone, which is the only way the shape of a partly eclipsed Sun can be seen at all (the partial phases of 1999, through one). It changes no angle, only the light, and it is put on and taken off along the recording: the glasses come off for totality.

Every source is named where its data is reported, with the attribution its licence requires — and can be swapped for another. The picker is the credit. What is still missing, and what each item is waiting on, is on the roadmap.