Mars will not be the same size as the moon on Friday — it's a heavenly hoax.

An email is making the rounds describing a once-in-a-lifetime event in the night sky on Friday. It says Mars will be 56 million kilometres away, making it seem like the Earth has two moons.

But don't be fooled, astronomers say.

"It's totally made up," said David Lane, with the astronomy department at Saint Mary's University in Halifax.

The hoax has been around since 2003 when Mars was 56 million kilometres away — the closest it has ever been to Earth. But it wasn't anywhere near as close as the moon and looked like a star.

If the red planet really was close enough to appear the same size as the moon, the tides would be affected and large tracts of land would be submerged, experts say.

Lane said stargazers will be able to see Mars on Friday night. However, at 320 million kilometres away, it will be no brighter than the average star.

"It's actually a lousy time to look at Mars," he said.