A so-called supermoon will brighten the night sky tomorrow, though it might not be quite as super as some people believe.

Supermoons occur when the moon's closest approach to the Earth — known as perigee — coincides with a full moon. The moon has an elliptical orbit with one side about 50,000 kilometres closer than the other.

The perigee and the full moon occur within a minute of each other Saturday night (at about 11:35 p.m. ET), and the moon will seem as much as 14 per cent bigger and 30 per cent brighter than other full moons this year, NASA says.

The average distance between the Earth and the moon is about 383,000 kilometres. On May 6 it will be about 356,955 kilometres away.

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While that news will undoubtedly get the curious or romantic gazing skyward, experts say the difference is harder to perceive than it sounds.

"Hanging high overhead with no reference points to provide a sense of scale, one full moon can seem much like any other," NASA Science says on its website.

Eric Briggs, a volunteer with the Royal Astronomical Society of Canada's Toronto Centre, said the moon will be "measurably brighter, but not perceptibly brighter."

Supermoons happen about once a year on average — the March 19, 2011, supermoon drew lots of attention from stargazers and photographers.

Briggs said someone with a high-quality camera might notice the difference, but the average person would have a hard time perceiving a change from month to month.

NASA said the best time to look will be when the moon is near the horizon, to take advantage of the illusion that makes the moon seem bigger. (Many people in northern temperate latitudes take note of the moon illusion in June when it appears near the southern horizon.)

NASA says most major studies show no correlation between the moon and negative human behaviour. Nevertheless, some myths link full moons to increased crime and, in the case of supermoons, to natural disasters.

The supermoon does affect the world's tides, but only by a few centimetres. Briggs said that effect will be more pronounced in areas such as the Bay of Fundy, but not enough for anybody to worry about.

"As far as the end of the world, not much risk of that," he said.

If anything, Briggs said, full moons keep astronomers inside because the dimmest celestial bodies are no longer visible. He's still happy for the attention the supermoon brings, though.

"Any time anyone is interested in looking at the night sky, they're probably going to see something they find fascinating," he said.