Good question. I Google'ed it and came up with this: "I did not mention refraction in the explanation I just gave. Refraction lifts up the image of a celestial object near the horizon, and the more the closer the object is to the horizon. Refraction can only have a systematic effect in the vertical direction, because the atmosphere is layered only in the vertical direction. It is impossible to make everything appear, for example, twice as large in the horizontal direction, because if that happened everywhere along the horizon, then the horizon would have to be twice as large in circumference, and that doesn't fit. So, the image of the Sun (the solar disk) is equally wide at every height above the horizon.
The effect of refraction in vertical direction can be seen in the Sun or Moon when they are low in the sky, because then the Sun and the Moon appear a little squashed, because the bottom is lifted up more by refraction than the top (because the bottom, as long as it is visible, is closer to the horizon than the top). The Sun appears to be 15% flatter when the bottom of the solar disk touches the horizon. When the bottom of the Sun is still 1 degree (two diameters) above the horizon, then the flattening is 10%. If the Sun is 5 degrees above the horizon, then the flattening is only 2.5%. So, refraction close to the horizon does not make the image of the Sun or Moon larger, but rather smaller, because it is flattened in the vertical direction. The effect is at most 15%, and in the wrong direction, so it cannot explain the "small when high, large when low" effect, which works in the other direction, does not change the shape, and appears much greater than 15%. " Under "Larger near the Horizon, Smaller Overhead": http://www.astro.uu.nl/~strous/AA/en/antwoorden/sterrenbeelden.html Dave On 9/26/05, Rob Studdert <[EMAIL PROTECTED]> wrote: > Does anyone know the technical reason that the sun renders as an ellipse when > shot close to the horizon? > > TIA. > > > Rob Studdert > HURSTVILLE AUSTRALIA > Tel +61-2-9554-4110 > UTC(GMT) +10 Hours > [EMAIL PROTECTED] > http://members.ozemail.com.au/~distudio/publications/ > Pentax user since 1986, PDMLer since 1998 > >
