# Moon's/Orion's Apparent Sizes **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [December 29, 1999, 8:22pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426 "1999-12-29T20:22:00Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [December 29, 1999, 8:22pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/1 "1999-12-29T20:22:00Z")
This refers back to the question on the moon’s apparent large size near the horizon. First, Uncle Cecil states the moon actually appears smaller on the horizon because it is 4000 miles further away. Huh? As an amateur astronomer, I fail to understand the logic here. The moon’s actual distance from Earth is not dependent on whether it is observed near the horizon or not! If that were true, the moon’s distance from Earth would be relative to the observer’s location! Seeing the moon on the horizon is a local event! How can it be 4000 miles further for observere “A” than observer “B”? Can someone please explain what Uncle Cecil might have meant? This issue has another twist, too. If you are a stargazer, you might notice that the constellations will appear distorted as they are near the horizon. For example, many can identify Orion in the Winter sky. Watch it rise in the East and then note its relative size later that same evening. Why should this be? A few text books addressing this topic suggest that the curvature of the sky is simply an illusion and does not cause the apparent distortion. Yet, no other explanation has been given! Any enlightenment would be appreciated. * * * yyz2112 ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [December 29, 1999, 8:31pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/2 "1999-12-29T20:31:00Z")
M MMM M - | | | | | 2 3 0 K ``` M i . | | | | - M E |--------230K Mi.------------|MMM EEEEE M EEEEEEE EEEEE E |4K| ``` Even though you’re only about 4 miles from the horizon, you’re 4000 more miles from the moon. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [December 29, 1999, 8:32pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/3 "1999-12-29T20:32:00Z")
That didn’t come out right. ---
**Author:** ![Boris\_B](https://avatars.discourse-cdn.com/v4/letter/b/fbc32d/32.png) [@Boris\_B](https://boards.straightdope.com/u/Boris_B)\ **Post date:** [December 29, 1999, 8:41pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/4 "1999-12-29T20:41:00Z")
The moon’s distance from the center of the earth is (roughly) constant. When the earth is straight over head, we’re located between the earth and the moon (closer to the earth, of course). When the moon is low on the horizon, we’re (roughly) as far away as the center of the earth is from the moon. So the difference between distance at moonrise and moon-zenith should be about half the diameter of the earth. ---
**Author:** ![Boris\_B](https://avatars.discourse-cdn.com/v4/letter/b/fbc32d/32.png) [@Boris\_B](https://boards.straightdope.com/u/Boris_B)\ **Post date:** [December 29, 1999, 8:44pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/5 "1999-12-29T20:44:00Z")
To my surprise, I was able to find the parent column: [http://www.straightdope.com/classics/a2\_110.html](http://www.straightdope.com/classics/a2_110.html) * * * - Boris B, Hellacious Ornithologist ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [December 29, 1999, 8:56pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/6 "1999-12-29T20:56:00Z")
Thanks, Boris B for your suggestion to see the link to Cecil’s answer, but… my question stems from this initially. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [December 29, 1999, 9:26pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/7 "1999-12-29T21:26:00Z")
Jinx revision - Yes, now I understand the 4000 miles now. I wasn’t picturing the astro-geometry quite correctly. Still, any thoughts on the second half of the question? * * * I’d rather have a bottle in front of me than a frontal lobotomy - Hawkeye 4077th ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [December 29, 1999, 10:51pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/8 "1999-12-29T22:51:00Z")
Read this link on the Moon Illusion. [http://www.GriffithObs.org/IPSMoonIllus.html](http://www.GriffithObs.org/IPSMoonIllus.html) ---
**Author:** ![Boris\_B](https://avatars.discourse-cdn.com/v4/letter/b/fbc32d/32.png) [@Boris\_B](https://boards.straightdope.com/u/Boris_B)\ **Post date:** [December 29, 1999, 11:03pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/9 "1999-12-29T23:03:00Z")
Well, I’m not sure if I follow the second part of the question. Is this a distortion of shape, or size? That is, is Orion being thinned or fattened, or expanded or contracted? I think an (apparent) distortion in size could be caused by the same optical illusion which causes the moon to apparently contract as it climbs. If what you mean is a distortion in _relative_ dimensions, you’ve got me. I haven’t ever noticed such a thing, but it seems like I only see Orion when pretty high in the sky. (I told him the habit was killing him, but he doesn’t listen…) ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [January 4, 2000, 9:20pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/10 "2000-01-04T21:20:00Z")
Boris - The link provided by “The Irishman” gives much food for thought, and it does address the same phonomena occurring with the constellations. When I say distorted, I mean Orion et al. appears proportionally stretched when low to the horizon than at ohter times. If you know the night sky well, you can observe this with all the constellations when nearing the horizon. Overall, I’m still thinking and re-thinking the proposed answers to this question. Keep looking up! * * * I’d rather have a bottle in front of me than a frontal lobotomy - Hawkeye 4077th ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [January 5, 2000, 2:27pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/11 "2000-01-05T14:27:00Z")
Jinx, the reason for posting the link to Cecil’s column is not for YOU, but for the sake of readers/posters who may want to respond to you. You get more effective responses if the people posting have read the column, too; otherwise, you have to read through dribble (statements that Cecil may have debunked in his column) or repetition (someone brilliantly says the same thing that Cecil said, because he didn’t read the column.) That was kinda long-winded, I hope you follow me. Having the OP posting the link to the column is a courtesy for others. ---
**Author:** ![knappy](https://avatars.discourse-cdn.com/v4/letter/k/b3f665/32.png) [@knappy](https://boards.straightdope.com/u/knappy)\ **Post date:** [January 5, 2000, 11:36pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/12 "2000-01-05T23:36:00Z")
Here’s another link: [http://unisci.com/stories/20001/0104001.htm](http://unisci.com/stories/20001/0104001.htm) ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [January 6, 2000, 2:40am UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/13 "2000-01-06T02:40:00Z")
About constellations distorting near the horizon, I must say that though I’ve spent a lot of time stargazing, I haven’t really noticed it. Then again, I rarely get to stargaze with a view clear down to the horizon. Still, there are a couple of possible answers. One is that they are not distorted, but merely tilted. Constellations are merely patterns of dots, and if they are tilted, as they would be when nearing the horizon, they can look somewhat different. Another possibility is the explanation given by Cecil about the moon - i.e. we perceive the sky to be not a hemisphere but a flattened spheroid (bowl). Therefore we expect some distortion - shrinking at the bottom, I’d guess. If you’re expecting distortion, a ‘correctly’ distorted image looks right, and a non-distorted image looks wrong. Finally, there really is a refraction of light by the earth’s atmosphere. This tends to squash the patterns in the vertical direction. I didn’t think this was large enough to cause visible distortion in large constellations though - it’s less than one degree shift, if I remember correctly. ---
**Author:** ![jeremybb](https://avatars.discourse-cdn.com/v4/letter/j/76d3ee/32.png) [@jeremybb](https://boards.straightdope.com/u/jeremybb)\ **Post date:** [January 7, 2000, 6:30am UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/14 "2000-01-07T06:30:00Z")
There is an article on the moon illusion in the latest issue of the Proceedings of the National Academy of Sciences: Vol. 97, Issue 1, 500-505, January 4, 2000 Here’s the abstract: An old explanation of the moon illusion holds that various cues place the horizon moon at an effectively greater distance than the elevated moon. Although both moons have the same angular size, the horizon moon must be perceived as larger. More recent explanations hold that differences in accommodation or other factors cause the elevated moon to appear smaller. As a result of this illusory difference in size, the elevated moon appears to be more distant than the horizon moon. These two explanations, both based on the geometry of stereopsis, lead to two diametrically opposed hypotheses. That is, a depth interval at a long distance is associated with a smaller binocular disparity, whereas an equal depth interval at a smaller distance is associated with a larger disparity. We conducted experiments involving artificial moons and confirmed the hypothesis that the horizon moon is at a greater perceptual distance. Moreover, when a moon of constant angular size was moved closer it was also perceived as growing smaller, which is consistent with the older explanation. Although Emmert’s law does not predict the size-distance relationship over long distances, we conclude that the horizon moon is perceived as larger because the perceptual system treats it as though it is much farther away. Finally, we observe that recent explanations substitute perceived size for angular size as a cue to distance. Thus, they imply that perceptions cause perceptions. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [January 7, 2000, 6:52am UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/15 "2000-01-07T06:52:00Z")
scr4 - The constellations wouldn’t look tilted. Alpha Centauri, a nearby star, is about 480,356,352,000,000 miles away. The relative positions of stars in relation to each other, as the Earth spins, isn’t much. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [January 6, 2000, 8:53pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/16 "2000-01-06T20:53:00Z")
> [@](#): > > The constellations wouldn’t look tilted. > Alpha Centauri, a nearby star, is about > 480,356,352,000,000 miles away. > The relative positions of stars in relation > to each other, as the Earth spins, isn’t > much. I don’t think you understood the point. When you look at Orion high in the sky, you’re facing mostly south, so the three stars of Orion’s belt appear mostly horizontal. When it’s nearer the horizon, the same three stars will be almost vertical, because you’re now facing east. I know that if you take a tall object such as a telephone pole and lay it on its side, it appears to be shorter than it was when it was vertical. This strikes me as a plausible explanation. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [January 6, 2000, 9:34pm UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/17 "2000-01-06T21:34:00Z")
Just for the record, I think CurtC is not suggesting that the constellations are foreshortened (like if you looked at a telephone pole from the end, and it might look only two inches tall), but instead is suggesting an optical illusion, like this: - - - * * * The line of stars lying down would look shorter. (I’m not saying that it does–and this example sucks because of ascii art. ---
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [January 12, 2000, 6:02am UTC](https://boards.straightdope.com/t/moons-orions-apparent-sizes/426/18 "2000-01-12T06:02:00Z")
here’s a link to the press release for the aforementioned article. A web simulation is also available. [http://www.research.ibm.com/news/detail/moon\_illusion.html](http://www.research.ibm.com/news/detail/moon_illusion.html)