Change into Passive voice

[Shashikant]

Please tell me whether the passive of the following sentence is correct if not kindly suggest me the correct answer.

Active: For a long time the earth was believed to be flat.
passive : It was believed for a long time that the earth was flat.

[Mohammed Abu Risha]

For a long time the earth was believed to be flat.

The above is already in the passive!

When put in the active, the sentence could look like:

For a long time, people believed that Earth was flat.