Imagine a meridian that moves over the Earth's surface such that it remains at the longitude that corresponds to a particular solar time (e.g. midday).

This meridian would remain stationary relative to the Sun as the Earth revolves under it.

Is there a name for such a meridian?

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    $\begingroup$ This is an interesting question! I think that a sun-synchronous orbit's sub-satellite point would likely follow this meridian, whatever it might turn out to be called. $\endgroup$ – uhoh Sep 23 '19 at 12:51
  • $\begingroup$ The only line I can think of is the terminator, which is the line that separates day from night. $\endgroup$ – Fred Sep 23 '19 at 16:58
  • $\begingroup$ I think I'm missing something here. The Earth itself isn't stationary with respect to the Sun, so a meridian that is stationary wouldn't keep up with the Earth's revolution? $\endgroup$ – user967 Sep 23 '19 at 17:06
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    $\begingroup$ solar noon is effectively that kind of meridian $\endgroup$ – haresfur Sep 24 '19 at 0:37

This is better suited for English Language &Usage SE, but the word you're looking for is


an imaginary line or a line on a chart connecting points at which an event occurs simultaneously or which represents the same time or time difference

(Source: Merriam-Webster)

Isochrons are typically used in diagrams rather than maps, for techniques like rock dating, where correlating things by time is more important.

But cartographers are used to putting all types of isarithms on maps, and would understand what an isochron was meant on a map.

The trick is to define "same time" as "same solar time", giving you the "moving meridian" you're looking for.

You could contrive a more specific term like isoheliochron but it hardly seems worth the effort, because they're pretty boring on a map.

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  • $\begingroup$ Would this just be the longitude of the subsolar point then? $\endgroup$ – user967 Sep 24 '19 at 16:23
  • $\begingroup$ @BarryCarter That would be a specific value, and in the rotating frame of reference OP is referring to, the longitude value changes. OP seems to be looking for some sort of invariant within the frame, and "isochron" appears to give enough abstraction to be useful. $\endgroup$ – Spencer Sep 25 '19 at 21:27

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