Moon angle between lit and dark side and other observations

[cagey]

Looking for quite a while this evening at an amazing moon,cloud has been kind. The angle across the moon made by the light and dark halves is approximately 45 degrees with the lowest point at about 8 o’clock using an imaginary clock face. My questions are what can you deduce as a casual observer from these phenomena, I do remember being told some interpretations from looking at the moon. I got lazy in the excitement of the electronic age but I’m moving back to the traditional ways, I must keep my brain working.
Thanks
K

 

[Rhylsailer99]

Ive been reading a lot on celestial navigation and the moon is a poor choice I believe for navigation due to very complicated calculations needed.. Better to use Polaris and other stars plus the planets in the evening.
https://blog.outdoorherbivore.com/wilderness/moon-navigation/ some info

 

[davidej]

The obvious thing you can deduce is the position of the sun at the time. Where has it to be to cause the shadow you are looking at?

 

[johnalison]

The border between light and dark on the Moon, or any other curved lit surface, is called the terminator, which I think is what you are referring to. It is unfortunate that the word has acquired other connotations.

 

[Sandro]

About the border line between light and dark on the moon, I noticed a fact that I can not explain. As davidej says, the line should be at right angle with the line connecting the centers of moon and sun. But many times I observed (by eye ball, no instruments) that the angle appears to be much different from 90°. Is there an explanation? May be the air refraction, being the sun low on the horizon? Did any other one find the same?

 

[Slocumotion]

Some years ago I spent an evening in a city park in southern China . Among the many strange and rather wonderful things seen that night was the moon as it rose "laying over on it's back" ( Did I imagine that? Does it always do that? why haven't I noticed it before?....). Eventually pieced together an explanation that , at least, satisfied me. It was mid - April (Spring Equinox). Home is at 59 Degrees North, (I had never been further South than Paris before ). Kunming is virtually on the Tropic of Cancer . Sun would be descending ( I know, I know - appearing to descend ) almost vertically, changing the angle of the terminator noticeably over the course of a couple of hours.

Now I 'm wondering (prompted by this thread ) is that right? And what would I have seen if I had stayed out longer . Surely the moon could not have rolled right over backwards and come up again the other way up - that would have turned a waxing moon (as it was) into a waning moon. Would it have reached a maximum of "reclination" and then returned ?
I await eagerly , further comments from more widely travelled and/or more Astronomically learned forumites.

 

[[194224]]

Ive been reading a lot on celestial navigation and the moon is a poor choice I believe for navigation due to very complicated calculations needed.. Better to use Polaris and other stars plus the planets in the evening.....................

I may be missing something, if so I’d welcome correction, but in my experience the moon is an excellent body for use in celestial navigation. It is easy to identify and can often be seen during the day as well as during the darker hours. That gives the opportunity, given a good angle of cut, of taking near simultaneous sun and moon shots and arriving at a fix straight away in daylight hours.

As for the “very complicated calculations” suggestion then I don’t recognise that issue at all. I suspect that vast majority of navigators practising celestial navigation use either one of the many computer/calculator based options thereby avoiding the calculation problem entirely or they, like me, use sight reduction tables. These pre-computed tables have tabulated solutions to the PZX triangle problem so the practitioner does not need to do it.
Reducing a sight using tables involves the use of just two arithmetical operations; addition and subtraction – nothing more complex, no formulas to solve, no trigonometry. If you can add and subtract then you can reduce a sight for the moon, sun, stars or the four navigational planets. The moon does require a couple of corrections during the sight reduction process that a sun sight does not require but it is a matter of reading the appropriate values from the pages of the almanac.

Of course there are numerical methods of sight reduction which do add a degree of complexity. There are several primitive methods for solving problems involving the spherical triangle but their use these days is largely a curiosity, a novelty.

One method I would probably accept as falling into the “very complicated calculations” bucket is the arcane lunar distance method to determine longitude. Even with pre-computed tables it is a laborious, tortuous and potentially error prone process. However it has not been regularly used since the mid nineteenth century. The advent of readily availability accurate timepieces meant that longitude could be determined much more simply by taking the altitude of any available navigational body.

One potential difficulty in shooting the moon instead of any other body is its relatively rapid apparent movement across the sky compared to other bodies. Whilst the moon’s apparent haste is the basis of the lunar distance method it can be a bit tricky when you’re chasing it across the sky trying to take its altitude with a sextant. Anything else is, to a reasonable degree, well behaved, the navigator has time to bring the body to the horizon and take the shot. It is easy to overcome this though. If the moon I rising I would set the altitude on the sextant a little higher than the moon’s actual altitude and wait until the moon gets there and record the time. If the moon’s altitude is decreasing then of course I’d set the sextant low.

I would accept that the process of sight reduction can take some learning. You need to understand where you get all the numbers from – almanac, sight reduction tables, sextant, chronometer, but once you have them it’s just adding and subtracting.

If someone wished to learn celestial navigation and had X hours to spend then my guess would be that 0.1X hours would be spent doing the arithmetic of the sight reduction, 0.2X hours understanding the theory of the process itself (what do all the numbers mean and how do they relate to the physical relationship between the observer and the selected body, why am I adding these two numbers together and not subtracting?). The remaining 0.7X hours would be spent getting familiar with the adjustment and use of the sextant.

The first two items can be avoided using calculators or computers but the third item is crucial whatever method is chosen.