103 points•dima55•3 days ago•68 comments•

68 comments

j1mr10rd4n3 days ago
To me, this article conflates lunar elevation and phase. Together with the use of ambiguous terms "up", "down", "above", and "below" makes it difficult to understand the author's intent and what their software is meant to visualise.

Lunar elevation (aka "altitude" in celestial coordinates) is sidereal, so the maximum range is determined by the viewer's latitude. The moon's orbital plane is not perfectly aligned with that of the sun-earth so there's another component there, but this only varies +/- 5 degrees of the sun's declination.

Lunar phase is synodic, so there's no significant variation in the observed illuminated portion with the moon's elevation over one night.

The full moon occurs when the moon is on the opposite side of the earth from the sun, so it rises at sunset, transits the meridian at midnight and sets at sunrise, (+/- a few minutes for seasonal variation).

Here's a nice visualisation of the lunar analemna - the figure described by the apparent position of the moon over a month - from Mt. Laguna at 32.8 degrees north. https://www.hpwren.ucsd.edu/news/20250212/

I also found this page with a decent explanation of the apparent effects of orbital cycles. https://www.cyclecalcs.com/learn/synodic-sidereal.html#faq

pjc503 days ago
The strange thing is that there's only visualizations and no photographs! It's not like it's difficult to find the moon to take photos of, nor the sunset.

Or is this a variant of the "counterintuitive" long shadows of mountains at sunrise/sunset? e.g. https://www.fox13seattle.com/weather/skies-erupt-in-color-du... shadow from underneath the cloud layer.

I'm sure I've seen a picture of the long downwards shadow of a mountain on a desert or plain taken from the top with the sun behind the photographer, but the search seems to be sufficiently AI poisoned now :(

j1mr10rd4n3 days ago
I think you're looking for crepuscular rays.

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

A spectacular phenomenon for sure, and I suppose loosely related to lunar phases by virtue of the fact that they both require a very distant source of illumination.

kurthr2 days ago
Beautiful picture of Mt Rainier!

I'm sure there are other places that have similar phenomenon, but Mauna Kea often casts great shadows. Here's one with the moon in the cast shadow on the cloud layer below.

https://twanight.org/gallery/inside-the-shadow-of-mauna-kea/

rob743 days ago
The only place where the illuminated part of the moon points straight down, as the article expects, is also the only place where the sun moves perpendicular to the horizon when setting: if you're at the equator.
thombat2 days ago
Doesn't the sun do this at all latitudes at the equinoxes? (Rises due east, sets due west)
ncruces3 days ago
In that time lapse, when the moon is visible, the illuminated portion is always facing “down,” i.e. the horizon?
foobarbecue3 days ago
*analemma
fizzbuzzbarbazz3 days ago
You walk into a dark basketball court through a doorway with a single bright light above it. as you approach the free-throw line, you realize someone has left a basketball at center court. part of it is lit by the light of the doorway.

even though you are very drunk, you don't assume that spinning around will allow you to se more of the basketball than you currently do. even leaning left and right while looking at the ball doesn't give you much of a different view.

the sun is the door. the moon is the ball. you are a person looking at the ball from earth. leaning shifting left a step might give you a perspective that may correlate roughly with moon-at-dusk. shifting right a step might give you a perspective that is roughly moon-at-dawn.

leaning forward or backward, left or right, drunkenly, and spinning around might give you different angles (relative to the line drawn from your ass to your head) that the shiny-side of the basketball seems to be pointing. but you're just drunk. the shiny part of the basketball always points towards the light above the door. it is just your local perspective that is changing.

throwup2383 days ago
Is that the Alien version of the Voight-Kampff test?
a3w2 days ago
More like the Moneyball rational fiction with robots version of it. Or whichever movie was about basketball.
dvh3 days ago
> Now, what happens if the moon is not at the horizon, but higher up at the sky?

Moon follows the sun to the west so to move moon "higher" up in the sky you need to rewind time (15deg/hour). In e.g. 4 hours moon barely moves in it's 28 day cycle so the moon will look exactly the same (you can take a picture and compare). It's the earth rotation that make moon "go down" or up. I've read your article 2 times and I still don't understand what is apparent problem, no wonder AI had trouble. Also it's hard to reason about up and down when moon move along ecliptic which is curve. Down at noon points elsewhere as down at sunset.

Joker_vD3 days ago
> Also it's hard to reason about up and down

The moon is above the horizon. The sun is below the horizon. Seems pretty straightforward, no?

The trouble is that shouldn't the Earth block the sun's light from reaching the moon then? And the answer is no, because the space is three-dimensional, so the Earth isn't on the straight line(s) connecting the sun and the moon (sometimes it is, but that's what causes lunar eclipses, not the lunar phases).

MarkusQ3 days ago
Was this known as the "Lunar Terminator Paradox" or called that anywhere before this article? So far as I can tell this seems to be entirely made up and the only references I can find are to TFA or related.

As others have noted, there doesn't even seem to be an actual paradox other than a possible confusion about how a half-lit sphere looks from various perspectives.

nkrisc3 days ago
There’s no paradox present, just one person’s misunderstanding of the arrangement of celestial bodies in 3D space.
Sharlin3 days ago
The sky is spherical, so straight lines in the sky aren’t straight lines on its 2D projection on the retina. Rather, straight lines are great circles. So if you trace the great circle that connects the sun and the moon, the moon should "point" along that great circle – which can be considerably off of what we think of as a straight line (which is curved in reality).
dima553 days ago
Well sure. Yeah. Do you find this explanation clear and satisfying though? Can you use it to answer a question like "at sunset, during a half-moon, can you see an upward-facing moon"? Maybe you're much smarter than me, but I had to write the simulator to figure out the answer. Claude and gemini never wrote the simulator, so they consistently answer that question incorrectly :)
MarkusQ3 days ago
No, you will not see an "upward facing" moon. At sunset, the west side of a half the moon will be illuminated. I think you are getting your self tangled with terms like "upward" and "downward" and "facing"
while_true_3 days ago
Makes perfect sense to me. When starting the shortest path for a flight from San Francisco to Tokyo, you don't point the plane toward Japan, you point it towards Alaska.
flymasterv3 days ago
Well, you also point it at Japan.

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