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  #1  
Old 27th January 2016, 04:35 PM
tmorg tmorg is offline
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Default The moon illusion

I have been wondering for a number of years why the moon appears bigger near the horizon. Last night I was going for a walk along the beach on "Straya Day" looking at the moon's huge disc on the horizon. Why?
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Old 27th January 2016, 04:50 PM
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Default Re: The moon illusion

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tmorg said View Post
I have been wondering for a number of years why the moon appears bigger near the horizon. Last night I was going for a walk along the beach on "Straya Day" looking at the moon's huge disc on the horizon. Why?
It only appears bigger because you have a reference point. If you actually measure how bit it is, you'll find that it is the same size no matter where it is in the sky.

The simplest way to measure it is to compare it to a coin at arms length, or hold a ruler or similar at arms length.
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Old 27th January 2016, 05:01 PM
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Default Re: The moon illusion

Quote:
The simplest way to measure it is to compare it to a coin at arms length, or hold a ruler or similar at arms length.
Cannot wait to try it! Thank you, Cyclist.
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Old 27th January 2016, 05:04 PM
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Default Re: The moon illusion

Yeah, Dr Karl did this as well.

https://twitter.com/doctorkarl/statu...06464203624448
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Old 27th January 2016, 05:07 PM
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Default Re: The moon illusion

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cyclist said View Post
...
If you actually measure how big it is, you'll find that it is the same size no matter where it is in the sky.
...
Unrelated to the original question, but the apparent size of the moon does vary, due to its elliptical orbit.

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Old 27th January 2016, 05:42 PM
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Default Re: The moon illusion

I've read the same is true of the sun - the size of the image of either the sun or the moon as either rises just above the horizon is no different to when they're directly overhead.
But I recall seeing sunrise at Birdsville when the rising sun's image was so huge I didn't immediately recognise it as the sun. This magnified image lasted only until just a bit more than half the sun was visible at the horizon. After that the image shrank to 'normal' size.
It was an unnerving experience and a sight I took to be a massive fire not far from the town when I first spotted it.
??????
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Old 1st February 2016, 10:47 AM
tmorg tmorg is offline
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Default Re: The moon illusion

The next full moon is February 23. I am going to measure this thing with a coin if I can get organised enough haha.

Quote:
Unrelated to the original question, but the apparent size of the moon does vary, due to its elliptical orbit.
I read that the moon's orbit is nearly circular.
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Old 1st February 2016, 11:20 AM
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Default Re: The moon illusion

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tmorg said View Post
The next full moon is February 23. I am going to measure this thing with a coin if I can get organised enough haha.

I read that the moon's orbit is nearly circular.
Extremely uncircular, just like the earths, it's usually only theists who argue this due to the fine tuning claim. It goes that if the earth was a few kilometres closer to the sun or a few kilometres further away we would either freeze or roast, how wonderful is god! When it's pointed out that the earth orbit varies by about 5 million kilometres between apogee and perigee they suddenly shut up, or tell you to shut up, I have seen that just as often. The orbits only look round because they are so large.

The difference between apogee and perigee for the moon is about 43,000 klms, or nearly 4 times the diameter of the earth. The eccentricity of the moons orbit is much greater in proportion than the suns orbit and because of their close relative sizes compared to the sun it is much more noticeable to the naked eye. Of course you can't look directly at the sun during apogee and perigee to compare sizes but I think you wouldn't notice the difference without proper equipment anyway.
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Old 1st February 2016, 11:35 AM
tmorg tmorg is offline
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Default Re: The moon illusion

Quote:
The orbits only look round because they are so large.
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Old 1st February 2016, 12:08 PM
stevebrooks stevebrooks is offline
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Default Re: The moon illusion

No problem. I used to be a member of the Australian branch of the L5 Society many years ago so I am well versed in orbital mechanics, basically a circular orbit with more than one body in a solar system is impossible, and even with one body there are going to be perturbations due to the fact that nothing actually orbits around anything else, that's just an easy way of describing something that's good enough to use (Newtonian mechanics) but is in fact completely wrong in every regard

The earth and the moon have a common point of rotation called the Barycentre, this orbit around the barycentre is why the orbit of the moon is not absolutely fixed but varies slightly, resulting in perigees and apogees of different distances on different orbits. In fact the earth and sun also have a barycentre which causes perturbations in the earth/sun orbit although in this case the earth/sun barycentre is actually below the surface of the sun and not between them.

Basically if anyone tells you orbital mechanics are simple or mention circular orbits tell them they are dreaming, it just doesn't happen.
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