Exact local time?
November 18, 2021 2:42 PM   Subscribe

Is there a term or concept for an exact local time? Regular time-zone local time is granular. The measurement I’m thinking of, someone standing 10 miles east of me would be at 3:01pm when I’m at 3:00pm. It’s easy to calculate, I’m just curious if there’s a term for it. Thanks!

It seems like it would be useful for plotting, say, average temperature at 3pm on November 18 across the US - if you do this in “normal” local time you get ugly lines at time zone barriers.
posted by ftm to Science & Nature (8 answers total) 5 users marked this as a favorite
Best answer: It's local solar time.
posted by Fiasco da Gama at 2:49 PM on November 18, 2021 [19 favorites]

See also sidereal time
posted by Cogito at 3:32 PM on November 18, 2021 [8 favorites]

An adjacent term is culmination when the sun is at its highest from the horizon at noon. That will vary incrementally through the time zone.
posted by BobTheScientist at 10:50 PM on November 18, 2021 [2 favorites]

Best answer: It’s easy to calculate...

Then you mean Local Mean Time. True Local Solar Time is the time on a sundial and that is not so easy to calculate as it varies throughout the year.
posted by vacapinta at 3:31 AM on November 19, 2021 [4 favorites]

You need an almanac.

A Nautical Almanac is made for sailors doing celestial navigation with a sextant. It has daily pages with tables for the location of the sun. With a bit of interpolation, you can calculate the longitude of the sun to within a mile at any time.

Surely by now, there is an online almanac for backyard astronomy.
posted by SemiSalt at 4:46 AM on November 19, 2021 [1 favorite]

I’ve seen this picturesquely referred to as ‘God’s time’.
posted by bq at 7:41 AM on November 19, 2021

Just some fun trivia to add to this question... The kind of time you are referring to was the way humans measured time until the 19th century when railroads knitted together places fast enough that a consistent single time measure was best.

The best-known (or only?) example of solar time being officially used in the modern era is Riyadh Solar Time, which was in place 1987-1989. I can't find any good links on this now, sorry! It's a topic of amusement for software nerds who care about timekeeping; here's a table of daily offsets you can use to calculate it.

The mean vs. solar time thing vacapinta mentions is summed up in the equation of time. The effect is significant, up to 15 minutes on the extreme days. I think for your purposes you'd be more interested in solar time than mean time; you want to measure vs when the sun is directly overhead, and that's solar time. It is complicated but well understood and calculating either with a computer is pretty straightforward.
posted by Nelson at 8:49 AM on November 19, 2021 [4 favorites]

temperature at 3pm on November 18 across the US

This is known as a synoptic measurement, and these are all measured at the same time. Not local time, simultaneously: often the first or last ten minutes of the hour are averaged.
posted by scruss at 2:27 PM on November 19, 2021 [1 favorite]

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