Tarek Mida/Shutterstock
This month’s full moon is a special spectacle: it hangs low on the horizon and seems noticeably larger than usual. The effect is an optical illusion that has intrigued scientists for decades, yet the moon itself does not change in size or distance.
In the Northern Hemisphere, the July full moon is commonly called a “buck moon” because it coincides with the time of year when a male deer’s antlers reach full growth. Buck moons often attract extra attention for two reasons. First, this year’s buck moon is the very first full moon of the summer season, arriving less than a month after the summer solstice. Second, it tends to appear low on the horizon, sometimes taking on a warm orange hue similar to a harvest moon.
During the summer solstice, the sun reaches its highest point in the Northern Hemisphere sky, which means the moon also reaches its lowest annual point on the horizon. As a result, the buck moon – or, in June, the occasional “strawberry moon” – is frequently the lowest full moon of the entire year.
When the moon is very low, it appears much larger to the human eye. However, photographs taken of the buck moon reveal the same angular diameter as a full moon high in the sky, indicating that the illusion does not transfer to images and is therefore a trick of perception.
One of the most widely accepted explanations is that foreground objects such as trees, buildings, or distant mountains frame the moon, making it seem closer and larger by comparison. Yet this theory is challenged by reports from astronauts in low Earth orbit who also experience the illusion, suggesting that the effect may involve complex visual and cognitive processes beyond simple depth cues.
JamesChen/Shutterstock
Despite the prevailing theories, no consensus has emerged on why the moon appears larger when it is low on the horizon. Researchers continue to investigate the interplay of visual cues, eye movement, and brain interpretation that culminate in this enduring mystery.
