A supermoon is an orbital event. The Moon illusion is a perceptual event. They can occur on the same night, which is why the terms get mixed up, but they answer different questions.
The difference at a glance
| Question | Supermoon | Moon illusion |
|---|---|---|
| What is it? | A full Moon near perigee, its closer orbital point. | A visual effect that makes a low Moon seem unusually large. |
| Is the change physical? | Yes: the Moon is genuinely somewhat closer. | No: the perceived enlargement is much larger than the physical change. |
| When can it happen? | Only around a full Moon near perigee. | Whenever the Moon is low on the horizon, at many phases. |
| Does it need buildings or trees? | No. | Not strictly, but horizon context can make the effect much stronger. |
Why do supermoons get so much attention?
“Supermoon” is a pleasingly dramatic label for a real celestial alignment: a full Moon near perigee. It can be a bit larger and brighter than average, and it makes a perfectly good excuse to go outside. But a normal full Moon near the horizon can still look startlingly large because the Moon illusion is so persuasive.
Can both happen at once?
Absolutely. A supermoon rising behind a skyline pairs a small real difference in distance with a large perceptual difference in apparent size. That is prime “why is the Moon so big tonight?” territory. It is also why the best answer is often not either/or.
What is not causing the giant-Moon effect?
The atmosphere can change a horizon Moon’s colour and slightly alter its shape through refraction. It does not magnify the Moon into a billboard. For the perceptual side of the case, read our Moon illusion guide. For an immediate answer on an alarming evening, start with why the Moon looks big tonight.