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I think that it's all to do with the space allocated to images in our brain things around the horizon are important to us and get more attention than things right over our heads and so the brain allocates the more space in our model of the world about us and so things near the horizon look bigger.
Dear AllI received this email (below) recently, and I'd be grateful for an evidence-based argument for why the lensing effect of the atmosphere is wrong.Chris
Musings on the Sun Concerning the Apparent Difference Between the Size of the Sun at Sunset and NoonBy Capt. Raleigh C. Willems, USAF Training Analysis and Development Division, Mather Air Force Base Blast From the PastThe following article originally appeared in the ION Journal, NAVIGATION in June 1952 (Vol. 3, No 4). It appears that some navigators do not understand why the sun appears larger at sunset and sunrise than it does at noon. One of the more popular misconceptions is that it is caused by refraction. Actually, refraction makes the sun appear smaller at rising or setting, because there is more refraction acting on the lower limb of the sun (because of its lower altitude) than on the upper limb. You can check this. Measure the horizontal and vertical diameter of the sun at noon with a transit. You will find it approximately 32 minutes of arc each way. Repeat this measure- ment at sunset. The horizontal diameter will still be 32 minutes of arc. The vertical diameter will be approximately 27 minutes. Our eyes give our brains a false impression of apparent size for the following reason: To the observer on the surface of the earth, the sky looks like a large, flat dome instead of a hemisphere. Points on this flat dome which are at the observerís zenith appear to be nearer to him than points on the rim of the dome (the observerís horizon). Haze near the horizon aids the illusion of greater distance. The human eye automatically makes allowance for distant objects appearing smaller than closer objects of the same size. The image of the sun received by the eye is the same size (discounting the refraction effect) at both the horizon and the zenith. However, because of the illusion of greater distance at the horizon, the eye makes an adjustment in the message transmitted to the brain and the sun appears larger. You can check this illusion in an effective, if undignified, manner. Face away from the setting sun, bend down, and look at it from between your legs. The sun will appear smaller than it did at noon.
You can also use the "looking between your legs" method to spot several planets. Uranus is particularly visible.Chris