Rainbows are not actually circular. They appear as circular arcs because the light that creates them is bent by water droplets in the air in a way that follows a circular pattern. However, the full circle is usually not visible because the ground or other objects block the view.

To understand how rainbows are formed, consider what happens when sunlight passes through a raindrop. Each raindrop acts as a tiny prism, bending the light that passes through it. The different colors of light are bent at different angles, with violet light being bent the most and red light being bent the least. This means that the light from the sun is separated, creating the familiar rainbow colors.

The angle of the sun's rays and the position of the observer also play a role in the appearance of rainbows. When the sun is low in the sky and behind the observer, the rainbow is more likely to be a full circle. As the sun rises higher in the sky, the visible portion of the rainbow gets smaller until it becomes a thin arc.

When the sun is too high in the sky, the rainbow will disappear altogether. This is also why rainbows can't be seen at night. So, the shape and visibility of rainbows depend on a combination of factors, including the position of the sun, the angle of the sunlight, and the location of the observer.

Here are some interesting facts about rainbows:

* Rainbows are always formed in a circle, but we usually only see an arc because of the horizon blocking our view.

* The colors of the rainbow are always in the same order: red, orange, yellow, green, blue, indigo, violet.

* The size of a rainbow depends on the amount of water in the air.

* Double rainbows are caused by sunlight reflecting off the inside of water droplets twice.