The phenomenon of seeing two moons in the sky is known as a "moon illusion" or "double moon illusion." It is an optical illusion that occurs due to a combination of factors, including the Earth's atmosphere, the position of the Moon, and the way our brains perceive depth and distance.
The most common occurrence of the moon illusion happens when the Moon is near the horizon. When the Moon is low in the sky, we tend to perceive it as larger compared to when it is higher overhead. This is known as the "Moon illusion of size." The exact reasons behind this illusion are still debated among scientists, but some explanations include the relative size of familiar objects in the foreground, atmospheric refraction, and the way our brain interprets visual cues.
In addition to the size illusion, atmospheric conditions can contribute to the appearance of a double moon. The Earth's atmosphere can act as a lens and cause the light from the Moon to refract or scatter. This atmospheric effect can create a faint, ghostly image of the Moon that appears alongside the real Moon. This secondary image is often dimmer and less distinct than the primary Moon.
It's important to note that the double moon illusion is purely an optical effect. There is only one Moon in the sky, and what we perceive as a second moon is an artifact of our perception. The illusion tends to occur when the Moon is near the horizon, and as it rises higher in the sky, the illusion typically disappears.
The moon illusion is a fascinating example of how our brains interpret visual information and can sometimes lead us to perceive things differently from reality.