The full Moon arrives this month on May 4, 2015. This full Moon is often called the “Full Flower Moon” since the flowers are near full bloom in many parts of the northern hemisphere. It’s also called the “Corn Planting Moon” since the agricultural cycle begins this month for many farmers. And it’s called the “Milk Moon” because of the increased productivity of cows grazing on the rapidly greening pastures.
The warmer weather in May also brings stargazers out to see the night sky. So this is a good time to investigate the “Moon Illusion”, the effect which makes it appear the Moon is larger when over the horizon than when higher up. Although the cause of the illusion is still debated, many scientists agree the Moon over the horizon is perceived by our brains to be at the end of a stretch of terrain that recedes into the distance along with distant trees, buildings, and other features of the landscape, all of which trick our brains into perceiving the Moon must be a long way away and therefore quite large. When it’s overhead, the Moon appears closer and is therefore perceived as smaller. The full Moon’s true apparent size, of course, is the same wherever it lies in the sky during the night. You can notice the same illusion when constellations rise. To many observers, for example, Orion looks like it fills nearly half the sky when it rises above the horizon, then appears to slowly shrink as it rises.