I hope this isn’t a silly or obvious question ... the way that the moon looks much larger on the horizon than it does when high in the sky? I previously believed that this was an optical illusion, either because when the moon is closer to the horizon the observer’s subconsciously comparing it with whatever’s on the horizon - mountains, trees? Or, an alternative theory I’ve heard of, that it’s an optical illusion caused by seeing the moon through the atmosphere’s thickness ... looks bigger because the atmosphere’s thicker when it’s closer to the horizon than higher in the sky? Are these hypotheses wrong? Because I’ve lately read that there actually IS a measurable difference in the moon, at different times, of up to 10%? Does anyone know if this is true, and whether it’s related to the moon appearing larger when closer to the horizon?