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hirov
2009-Dec-30, 04:37 PM
I live in Costa Rica about 10 degrees North. A week ago we saw the moon crescent here in the western sky and it's delimiter angle was almost horizontal, angling a few degrees up and to the right.
My question is....What determines the angle to the horizon of the crescent delimiter? We note that at times it is almost vertical and at other times it is nearly horizontal.
Would the angle be different here near the equator than in the Northern hemisphere say 40 or 50 degrees North?

hhEb09'1
2009-Dec-30, 05:01 PM
I live in Costa Rica about 10 degrees North. A week ago we saw the moon crescent here in the western sky and it's delimiter angle was almost horizontal, angling a few degrees up and to the right.
My question is....What determines the angle to the horizon of the crescent delimiter? We note that at times it is almost vertical and at other times it is nearly horizontal.
Would the angle be different here near the equator than in the Northern hemisphere say 40 or 50 degrees North?Welcome to BAUT, hirov! Interesting question, which can get contentious.

What you call the delimiter is mainly determined by the position of the sun--it is more or less perpendicular to the line between the moon and sun. Consequently, it's approx. perpendicular to the ecliptic, which was tilted to the south on those evenings, making that delimiter tip up to the north.

ETA: the angle of the ecliptic to an observer does depend upon the latitude. I checked skymap for a week ago, 10N, and it shows the moon angling a few degrees up and to the left! At my latitude it is a few degrees up and to the right. At higher latitudes, even more up and to the right.

grant hutchison
2009-Dec-30, 05:45 PM
Jean Meeus had an interesting chapter on this phenomenon in his fourth Mathematical Astronomy Morsels book. Chapter 4, "The Moon as a Boat": that is, the crescent moon lying on its back on the horizon with the horns pointing straight up.
As the new moon passes the sun, its horns can be at any angle at all, but it's generally invisible at that time, both because of glare and because of its extremely narrow phase angle. So Meeus made some reasonable assumptions about the necessary distance between sun and moon before the crescent could be visible.
The effect is strongly dependent on latitude: it's very common to have the crescent lying on its back (or front) at low latitudes, because the moon rises and sets in these latitudes with its north-south axis roughly aligned with the horizon. The higher the latitude, the more strongly the latitude affects the perceived tilt of the lunar crescent, and so the more extreme the illumination angle has to be in order to "undo" the latitude effect.
Meeus found that his "Moon as Boat" phenomenon was impossible to view at latitudes higher than 50 degrees.
He then did a search for the phenomenon for various locations in the next decade. He found an example in Paris (2010 Mar 16) and Toronto (2012 Feb 22), and many examples scattered across the USA. South of 34 degrees, he got very long lists of "hits": the phenomenon is sufficiently common to be unremarkable in those latitudes.

Grant Hutchison

RussT
2009-Dec-31, 11:33 AM
This seems like a good place for this question. I have done several searches on this and found little to nothing.

I have 'heard/seen' that there are 'reflecting angles' where we are seeing 'assumed' reflection of light rays from the moon that are 'supposedly' impossible angles for photons to be refecting to earth. Anyone have anything on this?

Hornblower
2009-Dec-31, 12:36 PM
This seems like a good place for this question. I have done several searches on this and found little to nothing.

I have 'heard/seen' that there are 'reflecting angles' where we are seeing 'assumed' reflection of light rays from the moon that are 'supposedly' impossible angles for photons to be refecting to earth. Anyone have anything on this?

I cannot tell from your words what you are trying to describe, or whether or not it is relevant to the OP topic. Can you give us something geometrically specific about the supposedly impossible angles?

grant hutchison
2009-Dec-31, 02:14 PM
Sounds like this might be something to do with the old optical illusion in which the lunar terminator appears not to be orthogonal to the sun-moon line. (It can be quite a compelling illusion when the moon is gibbous.)

Despite endless demonstrations that this arises from our faulty judgement about how to draw a mental line in the sky connecting the sun and the moon, there's a steady trickle of folk out there who claim they've noticed something truly strange and new about the behaviour of photons.

Grant Hutchison

hirov
2010-Jan-01, 12:04 AM
Welcome to BAUT, hirov! Interesting question, which can get contentious.

What you call the delimiter is mainly determined by the position of the sun--it is more or less perpendicular to the line between the moon and sun. Consequently, it's approx. perpendicular to the ecliptic, which was tilted to the south on those evenings, making that delimiter tip up to the north.

ETA: the angle of the ecliptic to an observer does depend upon the latitude. I checked skymap for a week ago, 10N, and it shows the moon angling a few degrees up and to the left! At my latitude it is a few degrees up and to the right. At higher latitudes, even more up and to the right.

Thanks for the helpful reply
Would you send a link or reference for Skymap? I looked around but could not find a useful moon map. It would be very interesting for me to look at moon images for different months and latitudes. Thanks