How big is the moon?

[Geoff45]

If you hold each of the following objects at arms length, which best represents the apparent size of the moon?
1. A dinner plate
2. A saucer
3. An orange
4. A cherry
5 A pea
6 A pinhead.

Of course, you can check this by going outside with the six objects in hand, but I'm asking you to go by gut feeling as you sit in front of the computer.
Geoff

[Geoff-]

I'm going for "cherry", after an initial first instinct of "orange".

Mr Tickle could probably hold a dinner plate at arm's length, though.

[Karlsson]

Whoa... I voted first and then read the posts. I'd say 'dinner plate' but I seem way out... 'barlow eyes'?:eyepop: Or simply a narrow FoV?

[mrsnipey]

Depends what length your arms are.

[leon]

I reckon about the size of a cherry, never really tried that experiment before.

Leon

[vash]

I find it's close to the size of the top of my thumb at arms length, so I'd go with a cherry.

[iceman]

I'm lead to believe it's the size of a thumbnail, so I went with pea.

[matt]

I'll also say cherry.

[33South]

Remember something about the size of a thumbnail, so Cherry.

Or maybe a large pea ;)

[Omaroo]

You must have child-like thumbs then Mike! :P

[iceman]

little finger nail, i'll say then.

I've seen big cherries!

[Geoff45]

Let's go with 60 cm

[goober]

I went with pea. The moon does change in size by around 15% too.

[Matty P]

I went with a cherry, but i'm not sure.

[Omaroo]

I'm taking cherries to be the typical Woolies supermarket type - about 15-20mm in diameter.

[iceman]

I'm taking pea to be your typical garden variety - not a shrivelled up dried pea :) 7mm or so.

[Geoff45]

Yep.

[wavelandscott]

I voted Pea...but like others I was thinking thumbnail...

[Geoff-]

Really! We might want to get some astrophysists onto that, ASAP :)

(I know what you meant though.. :rofl:)

[tbentley]

I went with an orange. I'm almost 2m tall and have very long arms, even for my height. Besides, isn't it a male thing to overestimate size?:rofl:
Trav

[spearo]

Well,
I immediately thought "cherry", then read the posts and voted cherry.
Now I just need to find a cherry that matches the size of the Moon at arm's length.
frank
:lol:

[[1ponders]]

Definately the pea. You can cover the full moon with the tip of your little finger held at arms length. Mind you you have to close one eye ;)

[sheeny]

I voted for a pea.

Al.

[astroron]

The Moon takes up half of a degree of the sky, the same as the Sun and can be covered by your little finger held at a normal persons arms length,:P so you don't have to wait fo a Full Moon to check.;)
But don't stare at the Sun with unaided eyes for to long:thumbsup:
Ron

[leon]

Although I did vote about cherry size, it is still to bloody big when you want to image. :lol: :lol:

Leon :thumbsup:

[iceman]

LOL too right! :)

[spearo]

Bugga! NOW you tell me ...

:doh:
frank

[Sharnbrook]

Well, no one has come up with any maths to justify their thoughts/guesses/beliefs, so;

The distance from my eye to my fingers "holding a pea" is 710mm
The pea is 8mm, so the angle subtended is 0.65 degree

The cherry is 18mm, so the angle subtended is 1.45 degrees

The moon tonight is 399715 km away, and its diameter is 3474km, so the angle is 0.5 degree. (Starry Night)

So if you are holding a small pea, that's the answer, approximately.

I guessed at the cherry, so I was wrong, as are 52% of the guesses, but a cherry stone would be nearer the mark.

[erick]

I did a kneejerk "cherry" and immediately knew I was wrong. :sadeyes: Since I started this hobby, I've always held up my little finger at arms length to the full moon and can always more than block it out. And the tip of my little finger is no cherry:whistle:

The brain is a strange thing. Hold your little finger up now at arms length and think whether the tip of that finger would cover the full moon. "No way", your brain says, "the Moon is much larger than that!" Well, we can all try it in three weeks time. :)

[RAJAH235]

I voted 'Pea'.
Ya'sawll knows thats the moons izonly 1/2 degree/30 AM. :P

[Blue Fire]

I voted cherry because I like red more than orange, green, silver, or undefined. :D

[astroron]

There are a lot of people who like their cherries on this forum:rofl::rofl::rofl:
Ron

[Night Owl]

I have read plenty of times how 'big' the moon appears in the sky, and reasons why.

One I have read, is that a full moon rising just above the horizon looks 'so big' because it is a psychological trick of the mind. That being because the moon is closer to objects on the horizon, the mind has some terrestrial objects to 'compare' it size too. And that is what makes the rising full moon 'look so big'. Personally I disagree with this psychological explanation, like I disagree with many things psychologists say are psychological 'facts'.

It was when I was a child that I became aware of a phenomenon of how light is transmitted through liquids. For example, using a ruler go and try and poke a fish in bowl of water, or a fish tank and you will see what I mean. Even better try going spear fishing flounder. The water, at an angle 'bends' the light, so if you are pointing your spear straight at the target, you are going hungry mate!

The next time I became aware of the transmission of light through water was also when I was a kid, when we used to go to a take away restaurant. They had a massive fish tank full of tropical fish. I noticed that if you looked through the end walls of that tank objects outside the other end of the tank appear much bigger / closer than if you put your head around the corner of that tank and looked at them through just air.

I also noticed this when I used to got snorkeling / spear fishing. Underwater objects appear 'much bigger' and closer than they actually are, and many a time I was disappointed to find that my spear never had the range, and when I did get a hit what I thought would feed a family of 10 underwater wasn't so great when landed back on dry land. Also when underwater, looking up through the surface, objects above the surface appear magnified.

Now, the atmosphere at mean sea level is 40 times denser than water. But the atmosphere acts like a liquid.

I have a hunch, based on my previous visual experiments with water / liquid, that a rising full moon 'appears' much bigger as it rises as the greater thickness of the atmosphere you are looking through as the moon rises above the horizon, is acting on the transmission of the light from the moon, just like is does when viewing objects through water in a fish tank, and underwater.

I have tested the 'psychological' explanation, and it just doesn't cut it.

I have made a point of being at the bottom of an extremely steep granite cliff, that was close to 2500 feet, and I have observed the moon 'rise' above the top of the cliff, through the trees, and a fire watch tower, and at that elevation it would be above the horizon, roughly 30 degress, it does not 'appear' to look the same 'big' size it does as it 'rises' above the horizon as if you were out on the plains watching it rise.

Well, that's what I think is the reason why.

In any event watching a full moon on the rise usually makes me stop for a few minutes to enjoy it.

[circumpolar]

Pea.
Or about the size of your smallest fingernail.

[circumpolar]

[quote=Night Owl;301931]
I have a hunch, based on my previous visual experiments with water / liquid, that a rising full moon 'appears' much bigger as it rises as the greater thickness of the atmosphere you are looking through as the moon rises above the horizon, is acting on the transmission of the light from the moon, just like is does when viewing objects through water in a fish tank, and underwater.

I have tested the 'psychological' explanation, and it just doesn't cut it.

I have made a point of being at the bottom of an extremely steep granite cliff, that was close to 2500 feet, and I have observed the moon 'rise' above the top of the cliff, through the trees, and a fire watch tower, and at that elevation it would be above the horizon, roughly 30 degress, it does not 'appear' to look the same 'big' size it does as it 'rises' above the horizon as if you were out on the plains watching it rise.

quote]

I have no way to measure wether there is an actual size difference due to atmospheric distortion close to the horizon. It would be great if someone out there could do the experiment.

I Disagree with the lack of size illusion at higher elevations though. I find that there is little difference compared to looking at the moon rising over a mountain peek or distant tree line. I have done the "hold little fingertip out at arms length" experiment at different moon elevations (very scientific :lol:) and found the same effect: it does look bigger.
Well, thats just my thoughts.

[circumpolar]

OMG.
Somebody said Dinner Plate.
Now thats psychology. :P

[gaa_ian]

1/2 a Degree (if I remember correctly !) so I went with a Pea ;)

[Geoff45]

Yep, that's right. I was thinking pea at 6mm, held 60 cm away. The angle is 0.573 degrees, or 34'. Since the average size of the moon is 31', the pea is about right. Really unbelievable!
Geoff

[johnno]

Absolutely Agree.

I can Block a Full Moon with the tip,of my little finger ,held at arms length.

I voted "Pea" also.

John

[Sharnbrook]

Now, the atmosphere at mean sea level is 40 times denser than water
The density of water is 1 gram per cc, or 1 kg per litre, or 1 tonne per cubic meter. The weight of 1 cubic meter of dry air at sea level, at 15 deg C, is approximately 1.226 kg, or less than 1/800th the density of water.

The atmospheric pressure at sea level is approximately 101 kPa, but pressure is not the same as density. The weight of a column of "atmosphere" with a base area of 1 square centimeter is approximately 1 kg, but I'm not sure what you mean by atmosphere is 40 times denser than water?

I can recall my physics master at school, sometime in the late 1950s, explaining that the moon looked bigger at moonrise because of the refraction of the denser air at just a few degrees of elevation, whereas when the moon is overhead, the light is striking the atmosphere at 90 degrees, and therefore there is no (or minimal) diffraction. However, I have heard that if you photograph the moon at moonrsie, or at its zenith, the size of the image is identical. That's another project to be undertaken sometime...........

[Suzy_A]

As well as many people not knowing the angular size of the moon, most people have even less of an idea how 'bright' the moon is.

Best done later in the afternoon with the sun behind you down fairly low (20 deg or so above the horizon) and the moon 20 deg above the opposite horizon, get a sheet of white paper and hold it up so that the moon is just off to the side of it and the sun is illuminating them both. (Not that you have much control over the moon!)

Which is brighter? The moon or the paper? Is the moon white, light grey, medium grey or dark grey?

The moon is actually very, very dark grey. It's albedo is 8.6% as compared to paper at about 95%.

Here's a 8.6% circle on a white background...


Also I'm surprised that people would discuss whether the moon's angular size is bigger or not when it's rising or setting than at other times - get outside with your camera and 'scope and take a few photos and you will soon learn if its true or not! I thought you people were astronomers!