A supermoon is an orbital event. The Moon illusion is a report about appearance. They can happen on the same night, which is why the words get mixed up, but they answer different questions and use different rulers.
The difference at a glance
| Question | Supermoon | Moon illusion |
|---|---|---|
| What is it? | An informal name for a full Moon near the closest point of its orbit. | The widely reported appearance of a low Moon looking larger than a high one. |
| Is the change physical? | Yes. The Moon is genuinely somewhat closer, so its real apparent diameter is a little larger. | No. A matched photograph shows the same width low and high. |
| When can it happen? | Only around a full Moon that falls near the closest point. | Whenever the Moon is low in the sky, at many phases. |
| How big is the difference? | Up to about 14 percent in diameter between the closest and farthest full Moons of the year. | Reported strength varies by observer and by how detailed the horizon is. |
| Is it explained? | Yes. It is orbital geometry. | Not completely. No single mechanism is uncontested. |
The 14 percent figure is the extreme across the comparison NASA uses. It is not a same-evening horizon change.
Why do supermoons get so much attention?
Supermoon is a pleasingly dramatic label for a real alignment: a full Moon near its closest point. It can be a little larger and brighter than an average full Moon, and it makes a perfectly good excuse to go outside. But a supermoon does not explain why a low Moon looks enormous. That is the horizon effect, and it happens on ordinary nights too.
Can both happen at once?
Yes. A supermoon rising behind a skyline pairs a small real difference in distance with a widely reported difference in appearance. How strong that appearance is varies from person to person and with the scene. That is prime “why is the Moon so big tonight?” territory.
What is not causing the giant-Moon effect?
The atmosphere can change the colour of a horizon Moon and compress its image vertically through refraction. It does not magnify the Moon into a billboard. For the unsettled side of the case, read the longer Moon illusion guide. For an immediate answer on an alarming evening, start with why the Moon looks big tonight.