We know, instinctively, that when we see something that appears small in our field of vision, there are multiple possibilities. It could be an intrinsically small object that’s close by, a medium-sized object that’s an intermediate distance away, or a very large object that’s a large distance away. It’s why a bird, a plane, and the Moon can all appear to be the same size in our field of vision, taking up the same angle on the sky — what astronomers refer to as angular diameter — despite their vastly different intrinsic sizes. It’s simple geometry: an object twice as far away appears to have half the size, and apparent size decreases as distance increases.
But that’s assuming that the geometry of the Universe is fixed, grid-like, and Euclidean. In our actual, expanding Universe, things aren’t so simple, and that’s why Doug Plata writes in to ask about how Andromeda, or an Andromeda-sized galaxy, would appear to us if we viewed it at different epochs throughout cosmic history: