The moon often appears much larger when it’s near the horizon than when it’s high in the sky. This phenomenon is known as the Moon Illusion, and it’s a trick our brains play on us, not an actual change in the moon’s size. Want proof? You can conduct a simple experiment to debunk this illusion yourself.
First, photograph the moon when it’s rising near the horizon, preferably around the time of a full moon. Later that same night, using the same camera settings and equipment, take another picture of the moon when it’s high in the sky.
Next, compare the two photographs. You’ll likely notice the moon appears slightly flattened or “squashed” vertically when it’s near the horizon, while it looks circular when higher up. Carefully measure the diameter of the high moon and the longest horizontal “diameter” of the seemingly squashed moon.
You’ll find that the measurements are virtually identical, accounting for the minor difference caused by the Earth’s rotation bringing the camera slightly closer to the moon when it’s higher in the sky. This difference is explained in detail in various resources on DIY Supermoon imaging. This proves the Moon Illusion is just that – an illusion.
The moon near the horizon often appears to have a warmer, reddish hue compared to its higher counterpart. This is because the low moon’s light travels through a thicker layer of Earth’s atmosphere, scattering shorter wavelengths of light (blues and greens) and allowing longer wavelengths (reds and oranges) to reach our eyes. The flattened appearance is due to atmospheric refraction. Earth’s atmosphere acts like a weak lens, bending the moonlight more near the horizon where it travels through more air.
To capture your own comparison photos, use a camera with a long telephoto lens. Mirrorless or DSLR cameras work well with “lenses-only” telephoto lenses. You can also use a teleconverter to increase the effective focal length and magnify the image. Avoid over-enlarging the moon to the point where it exceeds your camera sensor’s field of view. Use a tripod and cable release to minimize vibrations when taking the pictures.
A high ISO setting allows for a faster shutter speed, reducing blur caused by camera shake. A good starting point is a shutter speed equivalent to 1/ISO at f/16. You can also try opening the lens to a lower f-stop and shortening the shutter speed proportionally. Experiment with different shutter speeds (“bracketing”) for both the low and high moon shots to find the best exposure for each.
Saving all bracketed exposures helps you match the low and high moon images more accurately. Atmospheric conditions can significantly affect light absorption, so different exposures might provide the best match for comparing the moon’s apparent size at different altitudes. Avoid using mirror-lens (catadioptric) telephotos, telescopes, or purely reflecting telescopes (except Newtonian telescopes) for this experiment. The changing distance between mirrors and any correcting lenses during focusing alters the effective focal length. Temperature fluctuations can also affect mirror separation and curvature, further impacting focal length and potentially skewing the results. These factors introduce variables that can affect the apparent size of the moon in your images. Refracting (lens) optics are much less susceptible to temperature changes.
