My Science School

Watching the full Moon rise over the horizon can be a spectacular experience. The Moon often looks humongous during this time, making people pull out their smartphones and capture the scene. As the night passes, the same Moon begins to look smaller higher up in the sky. But is there actually a difference in the size?

It's all in our head

The Moon never really changes in size. It remains the same whether you see it at the horizon or higher up in the sky. However, to our eyes the Moon looks different in size due to illusion. This illusion is popularly termed Moon Illusion.

Why do we see it?

Like several other mysteries that science is yet to find an answer for, there is no proven scientific explanation for Moon Illusion. However, many theories have been proposed.

Most theories on Moon Illusion revolve around how we visually perceive the world. Our brain perceives the size of objects nearer and farther away differently. Due to this, it is thought that the brain doesn't realise that the Moon's size doesn't change no matter at what point we see it in the night sky on a particular day.

Another theory is that the position and size of the objects in the foreground also plays a role. Trees, mountains, buildings or other objects in the foreground can trick the brain into thinking the Moon is closer and bigger than it actually is. Ponzo illusion is the term used to describe this effect. However, this is also not considered a perfect explanation since astronauts in orbit also see the Moon Illusion and they have no objects to act as distance cues.

Argue it out with the brain

There are a couple of ways to prove that what we are seeing is an illusion One of the simple ways is to hold up your outstretched index finger next to the Moon. You will notice that your fingernail and the size of the Moon is the same, no matter where you spot it.

Another way to check the size is through photographs. Take a photo of the Moon when it is near the horizon and another when it is higher up. Keep the camera zoom settings the same while taking both the photos. You will notice that the Moon's width is the same side-to-side. Note that the Moon might appear a little squashed in the vertical direction when it is near the horizon because the atmosphere acts like a weak lens.

 

Picture Credit : Google

 

We know that the planets in our solar system orbit the Sun and rotate around their own axis in a counterclockwise (west to east) direction. But while all the planets orbit the Sun in this direction, not all of them rotate the same way around their own axis. Venus and Uranus rotate clockwise (east to west), also called retrograde rotation. During the formation of our solar system these two planets spun the same way as others, but there is no definite answer as to when and why they started spinning backwards. However, there are several theories.

The backward-spinning Venus

There are two main theories that try to explain Venus backward / retrograde rotation. The first theory suggests that at some point in time, post the formation of the solar system, Venus flipped its axis 180 degrees. This means, Venus continues to spin in the same direction as it always has, except it is now upside down. So, when looked at from other planets, Venus looks like it is spinning in the opposite direction.

Scientists believe Venus axis might have flipped due to the Sun's strong gravitational pull on the planet's dense atmosphere, which could have caused strong atmospheric tides. These tides, combined with the friction between Venus' mantle and core might have caused the flip.

The second theory states that Venus might not have flipped at all. Scientists propose that the planet's rotation slowed to a standstill and reversed direction. Scientists suggest this theory taking into account the factors mentioned in the first theory coupled with the tidal effects from other planets. They believe this might have caused Venus to eventually spin in a more stable retrograde state.

The side-spinning Uranus

Uranus, like Venus, rotates in the clockwise direction but its axis is tilted at 97.77 degrees, making the planet appear as though it is spinning sideways and orbiting the Sun like a rolling ball. The most popular theory suggested by scientists for Uranus tilt is the collision of the planet with an Earth size object. A more recent theory suggests that Uranus wasn't hit once by a giant object, but collided multiple times with objects of a smaller size.

A collision-free theory suggests that during the initial days of planetary migration, Uranus had a large Moon whose gravitational pull caused the planet to fall on its side. During the same planetary migration, this moon is believed to have been knocked out of orbit by another planet.

 

Picture Credit : Google