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Besides the smog and light polution...

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bentley
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Besides the smog and light polution...

Unread postby bentley » Mon Mar 09, 2009 4:09 pm

Something that has always puzzled me...
Why is it that when looking at the night skies from Kruger park the moon is so much bigger than in Johannesburg ?
When at Kruger you don't need a scope to look at the craters etc. :roll:
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Re: Besides the smog and light polution...

Unread postby onewithnature » Tue Mar 10, 2009 6:47 am

Not true, Bentley!
The moon remains the same size, regardless of where it is in the sky!
However, in darker areas - like Kruger - the clarity of the light reaching our eyes from the moon (and other heavenly bodies) is greater, so the definition of what we see is also clearer.
You can see the same craters from Jo'burg, but the smog and dust particles in the air diffracts the light, making them more unclear.
If you think the moon is bigger in Kruger, test this theory with a coin.
Compare "Kruger's moon" to a "city moon" using a coin (or any other reliably-sized object) and you will see - to your great surprise and chagrin - that the moon remains the same size!
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Re: Besides the smog and light polution...

Unread postby bentley » Wed Mar 11, 2009 4:16 pm

Thank you for clearing my little myth up... :doh: . Would this apply to the full moon aswell ? :hmz: :hmz:
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Re: Besides the smog and light polution...

Unread postby Imberbe » Wed Mar 11, 2009 4:40 pm

Yes! :thumbs_up:

It is also about perception. That is why the moon looks bigger when it rises, as opposed to when it is high in the sky. The moon does not change size, it is your eyes that are playing games with you. :wink:
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Re: Besides the smog and light polution...

Unread postby onewithnature » Thu Mar 12, 2009 4:43 am

:lol:

Actually, it's the brain that misinterprets the information coming from the eyes!
Even though our brain is so amazing, it goofs up on this one.
It will even do it with a crescent moon, but it is most obvious - and impressive - with a full moon.
Take a photograph of the full moon near the horizon and, later, higher up in the sky, and compare: you will be disappointed that the size is identical!
When I was much younger, I thought the enlarged moon was to do with diffraction of light through more air when the moon is rising, but this is also not a factor. (Although it does play a role with colour of the moon.)
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Re: Besides the smog and light polution...

Unread postby NightOwl » Fri Mar 13, 2009 10:32 pm

The above is all true ONLY on the same night. (Or few nights close together, depending how relative you wanna make it).
The moon is in an Elliptical orbit around the earth, which means that it's distance from the earth does vary from relatively close, to relatively far.
Some nights the moon is closer than others.
All depends how far apart the nights were, when you looked at the moon in the city and when you looked at it in Kruger.

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Re: Besides the smog and light polution...

Unread postby onewithnature » Fri Mar 13, 2009 11:09 pm

I can't see, though, that the illusion of the enlarged moon near the horizon has anything to do with the slight variance of moon size based on orbital distances? :huh:
Same with the colour of the moon; the variances are too small (a few percent) to make significant differences here.
If you know something that I do not, please educate me - especially as you are a night owl, and so am I. 8)
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Re: Besides the smog and light polution...

Unread postby NightOwl » Sat Mar 14, 2009 4:39 am

If you look at what the original poster (bentley) posted.
Difference in moon size "from Kruger park the moon is so much bigger than in Johannesburg".
He made no mention from the horizon illusion you're talking about.
He also did not mention how many days difference there was between his observations.
That's why I mentioned the elliptical orbit and how it affects visual moon size observation.
The clearer craters is obvious and as you pointed out, the lack of smog in Kruger.

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Re: Besides the smog and light polution...

Unread postby onewithnature » Sat Mar 14, 2009 4:49 am

I understand what you are saying, but I cannot see that the moon is in fact noticeably larger at any time compared to another (notwithstanding the illusory element as discussed).
I appreciate your elliptical input, but does this really have a bearing on how we view the moon?
I don't claim to know everything, so was looking for more input to justify the claims you made.
Could you perhaps explain it to me again - maybe I'm a little slow?
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Re: Besides the smog and light polution...

Unread postby NightOwl » Sat Mar 14, 2009 4:53 am

When the Moon is at apogee, it is 11% farther from Earth than it is at perigee.
Perigee: 363,300 km
Mean: 384,400 km
Apogee: 405,500 km
As you can see, we're not talking half a percent or one percent here.
11% difference in distance is quite significant and can visually be observed. But as I mentioned it's all relative to each individual's vision and perception.
Some ppl notice things easier than others

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Re: Besides the smog and light polution...

Unread postby onewithnature » Sat Mar 14, 2009 5:21 am

Oh, okay NightOwl, now I see where you're going with this.
I agree with you that the moon does appear 11% larger at perigee than apogee, but the practical aspects of this negate this enlargement.
The problem with the human brain, especially because there is no clear reference point in the night sky with which to compare size differences (everything else is insignificant in size compared to the moon), is that we don't notice these differences.
Even if we know about it, the relatively long time between apogee and perigee means that we have forgotten what we saw two weeks ago (a fortnight exists between apogee and perigee).
So the differences in disc size go virtually unnoticed by almost everyone.
However, at perigee, the moon is about 30% brighter than at apogee and an astute observer may notice this.
However, the brightness does not increase disc width, although a more-brightly defined disc perimeter may allow the keenest and most knowledgeable of us to realise that the moon has grown larger.
Still, I maintain that the difference is virtually unnoticed by people.
So I believe that bentley was referring to the horizon illusion more than anything else?
Perhaps you can confirm this, bentley?
But thanks for the excellent angle on things, NightOwl, something I didn't take into account because I naturally assumed (you know what they say about assumptions, yes?) that he was describing the horizon illusion.
And so we continue to learn from each other ... :D :thumbs_up:
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Re: Besides the smog and light polution...

Unread postby NightOwl » Sat Mar 14, 2009 12:51 pm

Onewithnature: thats why I used the word 'relative' and also the phrase 'some ppl notice things easier than others'.
It really depends from person to person. Some ppl have better memmories than others and 2 weeks is a drop in the bucket for them, whereas for some others 5 minutes is an eternity to remember something.

But the fact is that the moon is in an eliptical orbit, whether we notice it or not.

B.T.W. 11% closer/farther does not always translate into 11% larger/smaller.

I learnt something new as well, the illusion theory. Very interesting. will have to google and read up even more on it.

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Re: Besides the smog and light polution...

Unread postby onewithnature » Sun Mar 15, 2009 12:38 am

Thanks NightOwl for your excellent input. Actually, the moon's disc does vary around 11-12% in disc size between apogee and perigee, if you compare photographs (but not in surface area of disc)! I knew this existed, but being aware of it means I will keep a deliberate eye out for it this month. Next apogee on March 19, and perigee April 2.
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Re: Besides the smog and light polution...

Unread postby Imberbe » Sun Mar 15, 2009 9:24 pm

:clap: Excellent info. :dance:

And the best ... at perigee I will be in the bush! :dance:
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