Elon Musk mistook the moon for Mars in a tweet — and the internet is calling him out.

The SpaceX CEO tweeted a picture of the moon with the caption "Occupy Mars" and other Twitter users were quick to correct him.

Musk's tweet came within hours of him saying that he is accelerating the development of Starship, a rocket that is designed to go to Mars.

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The internet is mocking Elon Musk after he mixed up the moon and Mars.

Musk tweeted out a photograph on Sunday with the caption "Occupy Mars" but Twitter users were quick to point out that the image was not of the red planet, but of the moon during the July 2018 lunar eclipse — a phenomenon known as the blood moon.

"That's the moon," said Twitter user Viv, to which Musk responded with two laughing face emojis. He later said "moon too," adding: "Moon 1st, as it's only 3 days away & u don't need interplanetary orbital synchronization."

It's not clear whether Musk tweeted the meme in error, or whether it was a deliberate effort to troll some of his followers. According to a reverse image search, Musk's space firm SpaceX first started using the "Occupy Mars" meme in 2014 — with an actual picture of Mars. It even sells it on merchandise likes T-shirts.

Musk is currently working on a multibillion-dollar rocket called the "Starship" through SpaceX. He plans to use the rocket to take people to the moon and Mars.

Read more: Elon Musk says people can buy seats to Mars after the first orbital Starship prototype completes its test flight

On Sunday, Musk said he is accelerating the development of Starship and that people could start reserving their places to Mars after it returns from its first orbit around earth. Musk previously said he wants to launch a cargo mission to Mars by 2022 and a crewed mission by 2024.