Perception vs Reality: Why Does the Moon's Shadow Look Wrong?

[Farsight]

I was out back in the garden earlier, just before sunset. The three-quarter moon was bright yellow in the clear blue sky, and as I drew on my cigarette, something struck me as odd. I mentally drew a perpendicular line from the shadow on the moon and tried to trace it over the shed, skimming the apple trees, over the hydrangea, to the sun. But it didn't seem to point at the sun. It looked as if it was maybe ten degrees out. Huh? I said to myself, and got my rake. I held it up as best I could between the moon and the still-bright sun. That shadow on the moon still looked wrong.

So, guys. Was I kidding myself? Is there something odd about arcs and great circles and horizons that deceives the human eye? Do I just need a pair of sunglasses and a longer rake? Or (ho ho!) do I get a Nobel Prize?

Your information will be gratefully received.

 

[russ_watters]

No, you can't see the warping with the naked eye unless the sun is occulting a star while in a total eclipse. That's the extent of the curvature around here.

I'm not sure what you witnessed, but it may have been refraction caused by the atmosphere. When objects are close to the horizon, the atmosphere causes them to look like they are higher up - so the day is actually always longer than advertised (well, depends on who you ask - some sources will account for that). I'm not sure exactly how much refraction you get, but it isn't more than a couple of degrees.

 

[pervect]

Sounds like atomspheric refraction to me, too. The same phenomenon causes mirages.

see for example
http://www.islandnet.com/~see/weather/elements/mirage1.htm

 

[Farsight]

I don't think it was any kind of mirage. I know I said just before sunset, but the sun was still above the houses opposite. Anyhow, after fiddling around with the rake, I held up an apple and tried to compare the shadow angle on it with the shadow angle on the moon. It didn't look the same. But the apple wasn't perfect so that doesn't really count.

I expect it's just some optical illusion related to how we see straight lines in the sky. But: I'm going to buy a billiard ball and try again.

 

[Garth]

Remember the "straight lines in the sky" are actually great circles across the celestial sphere.

As the Moon was 'three-quarters' it would have been about 135⁰ from the Sun and you should have traced an obvious arc across the sky.

Garth

 

[LURCH]

I also think it was refraction. This is a mirage, but not of the same sort as the kind in the dessert. according to my astronomy prof, when you first see the sun on the horizon, it isn't actually lined up to where you could see it; it is still a full two minutes from coming over the horizon. But the image of the sun appears this early because the atmoshphere bends the light around the curvature of the Earth (much the same way a stick appears to bend when you put it through the surface of some water). Likewise, by the time you see the sun (or the moon) set, it's already been gone for two minutes.

Two minutes is quite a difference between perceived position and actual; certainly much bigger than I would have geussed. It may eb the amount of discrepency you observed.

 

[Farsight]

It's not a refraction lurch. It's more likely to do with great circles like Garth said, but try it yourself and see if you can see what I mean.

 

[hover]

not tring to get in the way but some of it could be refraction which has to do with the Earth's atmosphere not being stable. Depending where the moon was from "your view" it would varry. If the moon is closer to the horrizon then there will be more refraction because your looking though more atmosphere (which astronomers know about since you can't see much with a telescope near the horrizon). If the moon is more in the middle of the sky than there will be less refraction but some because your still looking through the atmosphere.

 

[DaveC426913]

Farsight, I know what you're talking about, I've experienced the same thing. The arc of the Moon's shadow doesn't seem to point directly at the Sun.

I am completely convinced it has to do with Great Circles and one's perception of straight lines across the sky. It has nothing to do with refraction or other phenomena.

The same effect occurs when trying to star-hop across large distances of the night sky - which I do when stargazing. You mentally draw a straight line, but that is incorrect, you have to draw a *curved* line, which is very difficult to do.

See attached diagram.