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Full moon is about to set in the West. Where is visibility greatest?

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Full moon is about to set in the West. Where is visibility greatest?

Old 17th Oct 2018, 16:31
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Full moon is about to set in the West. Where is visibility greatest?

Can anyone explain this met question and answer to me please?
"At night, the full moon is just about to set in the West. In which direction will the visibility be the greatest?"
Correct answer: B - Looking West.
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Old 17th Oct 2018, 16:55
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The full moon won't set in the west at night, it will be early morning as the sun has to be on the opposing hemisphere.

So the correct answer if the statement is true would be C) on another planet ;-)

https://www.timeanddate.com/moon/uk/york
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Old 17th Oct 2018, 18:06
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Where on earth do they come up with these questions? What is the point of them?
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Old 17th Oct 2018, 18:10
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The full moon ALWAYS sets close to (edit for clarity) the time of sunrise (by definition). The rest of the question is therefore ambiguous, and impossible to answer.

Last edited by GordonR_Cape; 17th Oct 2018 at 22:51.
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Old 17th Oct 2018, 21:31
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Well the answers so far have shown that those that replied probably know more about flying than astronomy.
The Sun and Moon are not connected in any way. So their periods are dissimilar. The moon orbits at about 25 hours, so its setting time can be at any time of day or night, and it always rises in the East and sets in the West.


The question asks where the greatest visibility will be.. obviously the area that the Moon is still shining upon, which will be to the West.

One interesting fact is that the Moon creates two High Tides per day, separated by 12 hours 25 minutes.
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Old 17th Oct 2018, 21:48
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Originally Posted by scifi View Post
Well the answers so far have shown that those that replied probably know more about flying than astronomy.
The Sun and Moon are not connected in any way. So their periods are dissimilar. The moon orbits at about 25 hours, so its setting time can be at any time of day or night, and it always rises in the East and sets in the West.


The question asks where the greatest visibility will be.. obviously the area that the Moon is still shining upon, which will be to the West.

One interesting fact is that the Moon creates two High Tides per day, separated by 12 hours 25 minutes.

The answers above show that the previous posters have actually read the question fully. Probably the most important thing to do in any exam.

Note it says FULL MOON
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Old 17th Oct 2018, 22:45
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Originally Posted by scifi View Post
The Sun and Moon are not connected in any way.
The sun, the moon, and all of the major planets are connected in one very specific way: all their orbits are fairly closely aligned in a plane called the ecliptic.

That is why we have lunar and solar eclipses (as well as signs of the zodiac). The positions within the orbits change, but the orbits themselves are very stable and mostly predictable.
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Old 17th Oct 2018, 23:00
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The moon orbits in 25 hours? Try 28 ish days...
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Old 17th Oct 2018, 23:04
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Here is a photo of the moon during a lunar eclipse proving that the earth is flat.



I suspect that SciFi might be a metman.
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Old 17th Oct 2018, 23:11
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Originally Posted by n5296s View Post
Where on earth do they come up with these questions? What is the point of them?
Simple.

Examiners trying to show you how much THEY know instead of finding out how much YOU know.

And itís not limited to ground exams/tests...
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Old 18th Oct 2018, 08:35
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Thanks all.
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Old 18th Oct 2018, 09:23
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The Earth takes 24 hours to complete one rotation. If the Moon was stationary in the heavens (not geostationary), then, to an observer on the Earth, the Moon would be directly overhead at 24 hour intervals.

But the Moon is not stationary. It orbits the Earth in the same direction as the Earthís rotation.

The Moon takes 28 days to complete one rotation (360deg). When our observer on the Earth has completed one rotation (1 day) he will find that the Moon is not overhead but is 12.85deg east of his position (360deg/28 days) and that he will have to travel for longer before the Moon is overhead.

The Earth rotates at 15deg/hour so for any spot on the Earth the Moon will be directly overhead at approx 24 hours 50 min intervals.
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Old 18th Oct 2018, 09:36
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I’m surprised that no one actually answered the question. I thought it was well known!

Visibility range

The contrast of an object to its background has a notable effect on its visibility, if contrast is low (for example a white building against a snowy white mountain) then the range at which objects can be seen is reduced. A clean and dry windscreen also helps increase visibility range (and reduces accidents), as do clean sun glasses and spectacles.

Illumination by the sun or the moon does not alter visibility, it does alter range. The best conditions occur when looking with the sun in your back (down sun) or towards the moon, one can see the silhouette of the objects better that way.

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Old 18th Oct 2018, 10:36
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What they're looking for is, "Down sun, up moon." so the answer is to the west.

Sunlight on hazy air causes flare and scattering of light looking towards the sun so vis is usually better down-sun whare the air/haze is backlit.

Moonlight works the other way around, probably helped by the lower light levels giving better contrast towards the moon rather than away from it.

This is usually only significant when the light source is fairly low in the sky.

I thought every PPL was taught this?

Last edited by meleagertoo; 18th Oct 2018 at 10:48.
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Old 18th Oct 2018, 15:20
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Down sun, up moon.
Supposing this to be true (I have no idea for the moon bit, though I certainly agree for the sun bit), why is it of any practical use to know it when flying? You're not going to choose a destination based on the way the moon is shining. So once you get up there, whatever it is, it is. If you notice that you can make out ground features better on one side of the aircraft than the other, and this is useful, good for you. If you're flying at night you're hopefully not relying on pilotage for navigation anyway. This seems like it could have been a useful thing to know ahead of time in about 1920, before there was anything BUT pilotage.

The FAA written tests generally seem to be stuck in aviation as it was about 30 years ago (complicated questions about NDB navigation, for example). I guess the CAA hasn't quite got that far yet.
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Old 23rd Oct 2018, 23:25
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Originally Posted by n5296s View Post
Supposing this to be true (I have no idea for the moon bit, though I certainly agree for the sun bit), why is it of any practical use to know it when flying? You're not going to choose a destination based on the way the moon is shining. So once you get up there, whatever it is, it is. If you notice that you can make out ground features better on one side of the aircraft than the other, and this is useful, good for you. If you're flying at night you're hopefully not relying on pilotage for navigation anyway. This seems like it could have been a useful thing to know ahead of time in about 1920, before there was anything BUT pilotage.
You've been told it's true so there's no need to "suppose" anything, or don't you believe us?
Practical use? Use a bit of imagination. Planning a flight in hazy conditions you'll know from which direction it's best to approach the field in order to find it. This is particularly important for people flying to farm strips or helo landing sites. It also tells you if your nav is going to be affected by poor forward visibility hindering the "pilotage" (I assume you mean conventional navigation) that you seem to be so dismissive of - despite an awful lot of people still doing it and not just following a magenta line. It'll also suggest that landmarks upsun or downmoon of your track may not be as easy to spot as those in the opposite direction or that a runway oriented in the "wrong" direction might turn out to have unacceptably poor vis for a safe landing.
Why would one "hopefully not" be relying on conventional nav at night? Is there something wrong or risky about it? Or are you just a child of the magenta line and can't actually navigate?

fyi conventional navigation (pilotage?) didn't end "in about 1920", there was nothing else until satnavs became available about 1990. Even if you were clumsily flying VFR via VORs and NDBs you still need to see the destination...

Thus I submit this is a pretty useful bit of knowlege.
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Old 24th Oct 2018, 10:54
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Thus I submit this is a pretty useful bit of knowledge.
Agreed, even if one is using a magic navigation box finding a destination or maybe something to avoid like a mast would be affected by this. Not everyone flies between well lit runways that are at a known position.

Also there is the engine failure situation, useful to know to glide up moon to find somewhere to land.

Last edited by Romeo Tango; 24th Oct 2018 at 11:07.
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Old 24th Oct 2018, 18:50
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Will it not be obvious to the pilot in which direction visibility is best at that time? Atmospheric variations will affect it.
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Old 25th Oct 2018, 05:11
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Maoraigh1, be careful, that's just too simple and logical !
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