DOCS: Update solar

[yvonnefroehlich]

Description of proposed changes

Relate to GenericMappingTools/pygmt#3857 (comment)

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[joa-quim]

Don't know why you want to change it. It's day/night, not day/light.

[yvonnefroehlich]

Don't know why you want to change it. It's day/night, not day/light.

We worked on the PyGMT docs and came cross that both "day/night" and "day/light" are used (GenericMappingTools/pygmt#3857 (comment)) in the GMT docs, please see https://docs.generic-mapping-tools.org/dev/solar.html.
My interpretation was that "day/light" refers to all four options, i.e. day-night terminator and the civil, nautical and astronomical twilights. But I am not sure if this is an official definition or formulation and, if this os directly clear for everybody. At least we should handle it consistent across the documentation and examples, which is the aim of this PR.

[joa-quim]

I don't really case if it is day/night or day-night, but please do not changes day nights by day lights.

[yvonnefroehlich]

I don't really case if it is day/night or day-night, but please do not changes day nights by day lights.

Hm. I aggree that, using "day-night" and "day-light" may be confusing for users. However, at the beginning of the GMT docs (https://docs.generic-mapping-tools.org/dev/solar.html) there is currently written:

Plot day-light terminators and other sunlight parameters

So, I am a unsure how to continue here (or close this PR?). At least the formulations should be consistent across the GMT and PyGMT docs.

[seisman]

Maybe we should just go with day-night?

[joa-quim]

Hm. I aggree that, using "day-night" and "day-light" may be confusing for users. However, at the beginning of the GMT docs (https://docs.generic-mapping-tools.org/dev/solar.html) there is currently written:

Plot day-light terminators and other sunlight parameters

Honestly, I don't find it so confusing (and that I know, no one complained) but yes, that entry line is better stated as day-night

[yvonnefroehlich]

Now I changed "day-light" consistently to "day-night".