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forrestguice / SuntimesWidget

Android app (and widget collection) that displays sunlight and moonlight times.
GNU General Public License v3.0
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Clarify "Actual Time" in the Help #432

Closed ericjs closed 4 years ago

ericjs commented 4 years ago

Since the sun is not a point source, "on the horizon" is not clear enough. Touching the horizon at the bottom? Bisected by the horizon? It also would be nice to define "horizon". I assume you must mean "standard sea level".

forrestguice commented 4 years ago

Well, technically sunrise is "the moment at which the upper limb of the Sun appears tangent to the horizon", but I'm not sure there is actually any value pointing that out. I think "on the horizon" is already accurate enough without splitting hairs.

The help is intentionally brief (intended for the layman) and I'd rather not fill it with standard definitions, beyond simple descriptions of the twilight periods (which many have never heard of).

forrestguice commented 4 years ago

Perhaps just adding the phrase "upper limb" is enough to clarify things without complicating the definition. "Actual Time: the upper limb of the sun is on to the horizon. This is the approximate time of sunrise and sunset."

forrestguice commented 4 years ago

This is a nice article; https://www.nationalgeographic.org/encyclopedia/horizon/ and https://glossary.ametsoc.org/wiki/Horizon

Again, to get technical with it, "horizon" refers to the "geographic horizon" and not necessarily "sea level horizon" (unless actually at sea level), and definitely not the "local horizon". The app can make adjustments for altitude but not topography.

We could probably add that, 'The local topography (such as mountains, buildings, etc) is not taken into account', to dissuade people from thinking adjustments have been made for the local horizon (which would really be quite a feat).

Actual Time: The upper limb of the sun is on the geographic horizon. This is the approximate time of sunrise and sunset. Local topography (mountains, buildings, etc) is not taken into account.

ericjs commented 4 years ago

I think "upper limb" isn't going to be very clear to a layman.

Looking at the pages you linked, it's still a little unclear to me what "geographic horizon" is. It's not clear to me if both "geographic horizon" and "sea-level horizon" both use what the horizon would be like if the surrounding topography were flat and at sea-level, and differ only in taking viewer altitude into account or not, or if "geographic horizon" uses some other way of ignoring "local prominences". Given natural irregularity of surface elevation variation I don't know how you draw a clear line between what is topographic prominence to be ignored and local elevation above sea-level to be counted.

forrestguice commented 4 years ago

I was kind of thinking the same, "what's an "upper limb".. This is why I think less is more.. Its better not to confuse people. I guess a simple way of putting it is.. sunrise is the moment the top of the sun becomes visible above the horizon, and sunset the moment the top of the sun disappears below it.

Given natural irregularity of surface elevation variation I don't know how you draw a clear line between what is topographic prominence to be ignored and local elevation above sea-level to be counted.

Your understanding of geographic horizon sounds fine to me. Local prominence is ignored. We have no knowledge of that. What we have instead are different models of the curvature of the earth (not a perfect sphere), and an adjustment for altitude above it. This is literally the only difference between the time4a-cc and time4a-4j calculators.

From the time4j docs..

4J

Based mainly on the astronomical calculations published by Jean Meeus in his book "Astronomical Algorithms" (second edition). This calculation offers high precision with the general limitation that the local topology or special weather conditions cannot be calculated. The altitude of the observer is taken into account using a spheroid (WGS84) and the assumption of a standard atmosphere (for the refraction).

CC

Follows closely the algorithms published by Dershowitz/Reingold in their book "Calendrical Calculations" (third edition). The altitude of the observer is taken into account by an approximated geodetic model.

ericjs commented 4 years ago

sunrise is the moment the top of the sun becomes visible above the horizon, and sunset the moment the top of the sun disappears below it.

I think that wording is clear and understandable by any layman.

forrestguice commented 4 years ago

Another wrinkle - the twilight events are measured from the center. Only sunrise/sunset uses the upper limb.

In the last patch I removed the details. I think what is essential is knowing that "Actual Time" is a synonymous for sunrise/sunset (as it appears in the UI). I was considering reversing that and putting a technical definition there (like "the upper limb of the sun is on the geographic horizon"), but I also think the in-app help needs to be as concise as possible (one or two sentences per topic max).

So maybe instead the help dialog should link to some online resource. I think the project wiki could be used to host online help. That would also allow others to contribute (since this sounds like a ton of work).