Hello.

I can see the moon high in the northern sky. It is close to the meridian. The left hand side of it is brightly illuminated by the sun. The terminator is slightly clockwise of vertical. Yet the sun has already set, i.e. the sun is much, much lower than the moon as I look at it. Why isn't the terminator at right-angles to the line of the sun?

I've noticed this puzzling thing many times. It is most obvious when both the sun and moon are in the sky together, and the moon is visible in daylight. Often the terminator is totally skewed to the direction of the sun. The only time I've seen the terminator orthogonal to the sun was when I"ve been near the equator and the sun and moon are passing directly overhead.

What is the explanation please?

clop

I've always noticed that it is orthogonal. Next time you see the Moon and Sun together in the sky, hold a round ball in front of the Moon. You'll find that the phase of the ball matches that of the Moon. And the terminator on the ball will be orthogonal to the Sun line.

Do you live near the equator then?

It is certainly not orthogonal for me, round ball or no round ball. If the sun is below the horizon to my left, and the moon's terminator is slanted slightly to the right, which would imply that the sun were slightly higher than the moon in the sky, which it isn't.

clop

Which phase is it in? If its a waning gibbous phase, and the Moon is near the horizon after sunset you need to draw the line the other direction, from the Moon to its closest horizon, beneath the Earth, to the Sun, rather than across the sky to the western horizon. Just guessing, but I suspect you're not drawing the shortest line possible.

I live in San Francisco, at latitude 38, but even when I was in Antarctica I noticed the Moon's terminator was perpendicular to the Moon / Sun line.

See if you can take a picture to post here next time you notice the terminator pointing the wrong direction. Try to zoom out as much as possible, and include horizon in the shot. It should be pretty easy to figure it out from that.

Here's a picture I took in Antarctica. The Sun is not in the picture, but it is exactly where you'd expect to find it based on the Moon's terminator. It is about 90 degrees to my left, and slightly higher than the Moon.

Click here for the bigger version:

http://orbitsimulator.com/Antarctica...G_71891024.jpg
http://orbitsimulator.com/Antarctica...G_72191024.jpg

Can you provide a photo?

__________________
"The truth may be out there, but lies are inside your head" Terry Pratchett

I've noticed it for many years, too. It's caused by an
unconcious impression that the Sun and Moon are about the
same distance away from you, even though the Sun is actually
400 times farther. That huge error in judging the distance
makes the angle look wrong.

Here is a pair of diagrams I made to show the geometry:

http://www.freemars.org/jeff2/SunMoon.png

They are not to scale, of course. In both diagrams, the
observer on Earth is at the "top" of the globe, with the Sun
near the horizon on the left, as shortly after sunrise, and the
Moon high overhead.

The diagram at upper-right shows the Sun and Moon as the same
size, since that is how they appear. In reality, the Sun is
400 times larger than the Moon.

The thing that makes the angle of the terminator look wrong
is the direction that sunlight appears to be coming from,
compared to the direction it is actually coming from.

-- Jeff, in Minneapolis

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http://www.FreeMars.org/jeff/

"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

When you say orthogonal are you projecting the line between the Moon and Sun as a straight line? If so that may be the source of youir confusion.

Remember, the Moon and the sun follow a path that appears from our point of view to be an arc. In the attached jpeg I've drawn a rough diagram that I believe represents the situation. The Moon's terminator is perpendicular to the path it traces. The arrow represents the direction you assume the sun to be in by projecting a line perpendicular to the terminator.

Attached Thumbnails

 

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"The very powerful and the very stupid have one thing in common: They don't alter their views to fit the facts, they alter the facts to fit their views." The Doctor, Doctor Who: The Face of Evil.

In your diagram, shouldn't the side of the moon facing the sun be illuminated?

As Jeff Root says, others have noticed this illusion before.

shouldn't the side of the moon facing the sun be illuminated?

Nope. Go out and observe the progress of the Moon as it moves across the sky. You will find that the terminator is perpendicular to the path it traces.

Jeff's diagram explains phases nicely, but I can't see how it explains why the angle of the terminator is aparently off.

__________________
"The very powerful and the very stupid have one thing in common: They don't alter their views to fit the facts, they alter the facts to fit their views." The Doctor, Doctor Who: The Face of Evil.

Yes I understand that the moon and sun follow an arc across the sky (roughly the ecliptic) and that the terminator on the moon is orthogonal to the tangent of the ecliptic at the point where the moon is located. But it makes for a really puzzling optical illusion when you can see the sun below the moon and yet the terminator indicates the sun is above the moon.

clop

I've always wanted to illustrate that so I can visualize the effect. Thanks, that's a great diagram.

Again, good representation, but I had a hard time at first figuring it out. I'm not sure how I would make it clearer.

Maybe some combo of the two. I see the perception of distance and the location relative to the horizon as two seperate factors here.

I have a question about the ecliptic and didn't want to necessarily start a new thread:

I understand the ecliptic (the apparent paths of the sun, moon, and planets when viewed from a particular place on earth) will change seasonally as the earth progresses in its orbit and due to the tilt of the earth's rotational axis in relation to the so called plane of our solar system. Sometimes it will be higher in the sky with a steep angle to the horizons, sometimes lower with shallower angles.

Last month for me (40 degrees N lattitude) the Moon and at least a couple planets were very low in the sky. I tried figuring it out holding apples and oranges and thought I understood the seasonal change. However, less than a month later, the moon is extremely high in the sky for me, so much so that it looks like it rises north of east. I didn't think the change would happen so fast. Is there something about the orbit of the Moon I'm not understanding that makes its path across my sky vary so quickly? (Or am I nuts? I can accept that. )

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When in doubt, stand on it - Stroker Ace

Thump

I've accepted mine a long time ago.
The moon's orbit has its own tilt in relation to the sun, or the equator, and none of them match up.

The seasonal change you're talking about happens once a year for the Sun as it moves around the ecliptic. When the Moon or planets are near the Sun, they'll be in the same seasonal position as the Sun: low in winter, high in summer. But when they're opposite the Sun, they'll be appearing around midnight, and their seasonal position will be reversed: high in winter, low in summer.
Since the Moon passes right around the ecliptic in a single month, you'll see it shift from low to high and back again in that time frame. In winter, it will be low in the sky during crescent phase, but high around the time of the full moon.

Grant Hutchison

When considering the Moon/Sun line, I always follow the shortest arc across the sky, rather than the straight line as depicted in Jason's diagram. That probably explains why I've never percieved the effect of the terminator pointing in the wrong direction.

btw. Today's a good day to see the Moon in the daytime sky if you have good weather. It's a waxing gibbous, just past 1st quarter.

I know that light travels in a more or less straight line through space from the sun to the moon, though. The only side that will be illuminated is the one facing the sun. That's irrespective of perspective, too.

When you talk about the path that the moon traces, are you talking about its daily path, or its monthly path? It would seem you meant the latter, right?

I just looked at Jeff Root's diagram too, and I don't like it either. The top half says it shows what your eyes see--but the text seems to indicate that the observer is under the moon, rather than away from it. I don't think you can explain this one with diagrams--it's just an illusion, it's not a real effect, just like the moon illusion.

Hmm. I totally disagree. There is no doubt that, if you draw the perpendicular to the terminator and extend on the bright side of the waxing gibbous moon shortly after sunset, it is pointing upward into the sunless sky, as diagrammed by Jason. Of course if you allow yourself to follow a curved line, as Jason and Tony do, you can convince yourself that there is no discrepancy, but it isn't sufficient in itself, particular as the curved line is almost defined by being at right angles to the terminator.

The issue may be a misinterpretation of relative distance to sun and moon as Jeff suggests but even that doesn't seem sufficient to me. I think it's more likely a misinterpretation of the absolute distance to both sun and moon. If they were fixed in a sphere about 5km (3 miles) away, ie. horizon distance, rotating about me personally, you might expect the moon's terminator to lie perpendicular to the line to the sun. Being off equator on a round planet that rarely obstructs the moon's illumination, the geometry is more than we can instinctively cope with. (Intellectually - that's different, of course).

I'm sure these shadow lines will not be angled in the same direction. Can anyone manage a photo of a rising moon 4-5 days before full (next couple of days), plus a tennis ball, just before sunset?

But they will! That's what's so cool about this trick. Hold the ball anywhere in the sky and it will show you what phase the Moon would have if it were hidden behind the ball.

I'll try to take the picture and post it here if these pesky clouds will ever leave.

It is an illusion, but I'm confident that a diagram can show
how the illusion is caused. My diagram is lacking in that it
is only two-dimensional, rather than three-dimensional. I'm
pretty sure that if you consider the geometry in 3-D, it will
fully explain the illusion.

I don't understand what you meant by my text indicating that
"the observer is under the moon, rather than away from it."
What does that mean?

The geometry I depicted is with the Sun and Moon 90 degrees
apart in the sky. I did that simply because a right angle
is familiar to everyone, and easy to visualize.

The observer is at the top of the Earth. That is where all
observers always are! The Moon is shown directly above the
Earth and the Sun is to your left. Observers in the northern
hemisphere can interpret that to mean that the Moon is on your
meridian directly south of you, shortly after sunrise. Those
in the southern hemisphere can interpret it to mean the Moon
is directly north of you, shortly before sunset. The Moon
could be low in the sky, or high in the sky.

None of that is important.

What is important is that there is a definite angle from the
Sun to you to the Moon (90 degrees in the diagram). Given that
the Sun and Moon appear to be roughly the same distance away,
your visual system interprets the angle of the path of light
from Sun to Moon as about 45 degrees (for the case diagrammed),
though it is actually well under one degree.

I think the diagram needs to be 3-D to be complete.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves