Moonlighting: What's Wrong with My 3D Perception?

The Earth's axis and rotation is tilted is relation to the Moon's orbit...

Thanks Solar, doesn't really help a lot: all the pictures of the moon are shown as if observed from the centre of the earth ie. the axis is vertical, and, the sun can clearly be on a line perpendicular to that axis.

https://earthsky.org/astronomy-essentials/do-we-all-see-the-same-moon-phase

..."This diagram shows typical paths for the Moon during the year (and the resulting look of the waxing crescent Moon) for a specific latutude in the northern hemisphere. Notice that during winter, the Moon sets north of west and follows a path almost straight down to the horizon. During the summer, the Moon sets south of west and follows a slanted path down to the horizon"...

https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question43.html

moon nothing.

what about that antenna?

The TV aerial is just included to establish a vertical (to show that I didn't have the camera "tilted").

"You, the observer, are also in this plane."

I can't see how that can be: I'm standing well below and roughly eastish of the TV aerial, the sun is setting in the North West.

To simplify things let's do away with the Earth (or make me the observer and the earth the same thing). Then clearly I can draw a line from the Moon to the Sun (which is low and to the right outside the picture), and define a plane. But then why is the axis of the visible moon not at right angles to that plane.

This is really weird: I seem to be arguing even though I know I must be wrong. Still looking for guidance/insight.

Sorry, but I was always under the impression that the Earth was blocking the Sun from illuminating the Moon, as in an eclipse of the Moon, which clearly can't be based on the position in SE's #1 post at Full Moon. I'll have to rethink that.

Some 46 million miles to the right of where it says Horizon Plane, there is a black hole, which bends the sun's rays on the way to the moon.

This is a great explanation. I wouldn't be worried so much with the moon as I would be with the radiator that is tilted 5-10 * right of horizontal, unless the radiator is straight and the mast is at an angle which unfortunately askews all of Rixter's measurements.

Part of the problem may be in semantics. Both you and Rixter (Post 3) have used the term "axis". The axis of the moon (that imaginary line around which the moon rotates) is never visible, period!

In your image, the sun is well above horizontal and to the right, illuminating the upper right portion of the moon. The moon itself blocks sunlight from reaching the lower left side of the moon, so it is dark and not visible during the daytime. The curve separating the sunlit side of the moon from the shadowed side is called the terminator, and the line that Rixter has labelled "axis" is a line passing through the ends of the terminator and the center of the moon. The line perpendicular to that line must point its right end at the sun. You are looking edge-on at a plane defined by the centers of the Moon, the Sun, and you, and the line pointing to the sun is in that plane

I don't know what seems wrong to you about the photo. I get the impression that you think because the sun is "low in the sky", the illuminated portion should be down and right, but that can only happen after dark. At sunset, the illuminated portion would be directly on the right.

Perhaps it would help to observe the shadow of the top of the antenna mast on the antenna reflector. That shadow (or the lack of the shadow directly left of the mast top) clearly shows that the sun is still well above horizontal.

Rixter effectively defines his use of "axis" as the line drawn through the ends of the terminator line, and whilst it may be confusing and may not be strictly correct, for the purposes of this discussion it's very useful. Clearly the line about which the moon rotates is completely irrelevant for this discussion.

"In your image, the sun is well above horizontal"

That's just it I took the photo 36 minutes before sunset (I've checked all the details subsequently), so, the sun must be only 9^o above the horizontal. The moon on the other hand is very high in the sky: the top of the TV aerial is about 30 feet high and I was standing about 30 feet away from it horizontally, so the moon must be about 45^o above the horizontal.

That is fantastic: thank you.

Clearly the big clue is that the "Horizontal for Antenna" does not look horizontal from the observation point.

I've just measured the angle between the "Horizontal for Antenna" and the "to Sun" line from the moon at 14^o, but I guess that could be corrected by rotating the antenna slightly so that it is pointing directly towards the direction of the sun.

Good job of measuring! CAD says 13.6°, properly rounded to 14°. The lines were drawn with care, bur are certainly no more precise than within about a half degree. ...and you're right about rotating the antenna.

Incidentally the sun was 59^o West of North when I took the photo; it set at 51^o West of North (21:10 31 May 2020).

Theres a calculator here https://www.esrl.noaa.gov/gmd/grad/solcalc/

Thanks for the calculator link! That's great! Many years ago, I spent most of a day making a fairly accurate drawing to determine a single value of sun location!

I find it interesting that you gave your values in degrees from North. Since I always think of the sun as setting generally in the West, I'd think in terms of deviation from West, as 31° N of W and 39° N of W. On that day, our 5PM sun was 10° S of W, and our sunset was at 31° N of W. Quite a difference due to latitude!

Clever using the antenna as coordinate axes for the "horizon plane". GA.

The problem is, your perspective of the antenna is making you think that the moon's rotational axis should also be in the same orientation. Here's a diagram (below) from Wikipedia that shows the relative angles between the ecliptic (earth's orbit around the sun), the moon's orbital plane, and the earth's rotational axis. Unless you have them all aligned properly, you will always see some angle between your "vertical" and the vertical axis of the moon's illuminated side.

https://en.wikipedia.org/wiki/Orbit_of_the_Moon#/media/File:Lunar_Orbit_and_Orientation_with_respect_to_the_Ecliptic.tif

Why do you think the typical pictures of a crescent moon usually show it with a "hook" at the bottom?

Why do you think the typical pictures of a crescent moon usually show it with a "hook" at the bottom?

Good question. The terminator is a circle separating the lighted and unlighted parts of the moon, and as a circle seen at an angle should be a section of an ellipse with the far side hidden.

It's probably artistic license.

This is the one I want someone to explain...

Thanks for pointing this out, I've never noticed before. I looked at the moon on my walk tonight and saw the same phenomena where the sun was not where you would expect it to be illuminating the moon at that angle. I don't have an explanation, but it will give me something to think about for awhile. Have you ever noticed light beams streaming through dark clouds that don't line up with the angle of the sun? That puzzled me for a while until I realized the light was coming from illuminated clouds above the dark ones, not the sun itself.

I've seen that explanation, that sunbeams are from an illuminated cloud above broken clouds where the sunbeams occur. The reasoning behind this is that the sun is very far away and its rays should be parallel, so the source must be nearby.

The illusion is due to the fact that, without any reference, the brain assumes that all the rays are equidistant. Actually the rays are from the sun and are parallel, but slanted with the bottom toward the observer. The apparent divergence is the same perspective effect that the edges of a straight road would appear to diverge if there were not other distance cues.

https://en.wikipedia.org/wiki/Sunbeam

I think both explanations can be true (not at the same time), depending on the situation. I've seen sunbeams clearly not lined up with the sun angle. Next time I see it I'll try and grab a photo.

The only time I can imagine light reflected off a cloud being bright enough to form "beams" through spaces between lower clouds would be around dusk, when the lower clouds and the observer are in the Earth's shadow (and therefore very dark), and a very tall thunderstorm has its cloud top still in the sunlight. I do not recall ever having seen such a phenomenon.

Three other people on this thread need to "GA" this to make it the good answer it deserves.

DKW's post 17 should also get more GAs.

_{I wonder if petitioning for GAs is against the unwritten rules?}