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What is sunset? The modern definition (34 minutes of arc for refraction and 16' for Sun semi-diameter but no correction for height of eye and no mention of local topography) is largely nonsensical and arbitrary. The older definition --when the altitude of the true Sun is zero (Ho = Hc = 0.00°)-- has some nice properties, and is arbitrary but in a reasonable way (*). Why can't we use the exact astronomical event of sunset? Because there's no such thing.

Judging from some of the contemporary comments in 1898/99, it seems that part of the problem is that folks were not yet entirely comfortable with the idea of GMT as a zone time, distinct from local mean times, at that date. Of course by local mean time, sunset occurs at the same time throughout the UK on the equinoxes, which would have been very close on the date in question. So should we calculate the sunset time at Greenwich and then declare that as the time throughout Britain as measured by GMT? Or should we separately calculate the GMT of sunset for each location? It wasn't an easy choice back then, and I'm not surprised that it was considered legally undecidable at the time (so "case dismissed").

Such a quaint old story of a naughty bicycle rider, isn't it? No relevance to the modern world, of course... Oh yeah?? Then answer this: when are you required to turn on your headlights in your car as darkness falls in the modern world. Naturally there are different laws in different countries, but the laws are surprisingly variable and naturally arbitrary and ill-defined even in the different fifty states of the USA.

I made a map (attached to this message) of US headlight laws for evening "light up" time. Not fit for navigation and not fit for legal argument! In many states headlights should be turned on at sunset. But what time is sunset? In other states headlights should be turned on by 30 minutes after sunset. Nice that they inform us that it's "exactly" 30 minutes after sunset, regardless of the duration of civil twilight which depends on both driver's latitude and also the Sun's declination. And again, what time is sunset? 

There seems to be one exception to the two options for the law among US states. Various compilations list the rule for the state of Maryland, but they seemed a bit contradictory. I found a statement of the rule from the Maryland government, and it appears they require headlights "at night" (good for them; they punted!). What does "night" mean legally? It may mean (in evening hours) after the end of civil twilight, which is a legally accepted time, or does it just mean whatever the police officer says that it means when you're ticketed??

I do suspect that in the modern world, in the US at least, the cop always wins when a driver gets a ticket for no headlights even if the times are ambiguous. Usually ticketing for lack of headlights is of no interest to police unless there's an obvious threat to safety or they've been told to run out the ticket book "or else". I was ticketed once (c.2005) for no headlights in the heart of Chicago right next to Wrigley Field just after a night baseball game. The neighborhood was a sea of mayhem as usual, and cars were crawling along at 2-3 mph. Of course the streets were awash in brilliant lights from neighboring businesses as well as the towering night game lights on the baseball field. I couldn't tell without checking carefully whether my headlights were on or off. But let's just say the cop was grumpy that night...

Frank Reed

* Two things in favor of the old definition of sunset:
1) if we define sunset by calculating when the Sun's true geocentric altitude is 0° (equivalently when the true, unrefracted zenith distance of the Sun's center, ignoring parallax, is 90°) then this is more consistent with the definition of twilight. Astronomical twilight at Hc=-18.0°, nautical at -12.0°, civil at -6.0°, and then sunset at 0.0°.
2) if we define sunset as above, then the equinoxes recover their original meaning (very nearly), and we won't have to put up with the "everything they taught you in school was wrong" memes twice a year when the Sun crosses the equator.

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