John, Fer, and others, John asked : > Let's say you have a spherical room, built like a planetarium, where the > walls curve up into the ceiling and you locate your mirror in > the center. ... Would the sunspot be perfectly round all the time?
I think that the image of the sun coming from any part of the mirror (I'm assuming it is a horizontal mirror for simplicity) is always circular, so that if the mirror is very small compared with the room the sunspot will approximate to a circle. However the circular image of the sun is blurred or enlarged by the apparent size of the mirror (as seen from the sunspot, more or less) and this enlargement will only be equal in all directions when the sun and its reflection are both at the zenith. From the sunspot, the mirror appears as an ellipse and from the zenith it is the special circular case. At sunrise and sunset it appears almost as a line so the enlargement is almost purely horizontal, and at intermediate altitudes the vertical enlargement of the circle is less than the horizontal. (The elliptical appearance of the mirror varies slightly as you move up and down the image of the sun, hence I write "almost".) Regards Andrew James
