This poll is now closed. The most popular question was: "How long is a day?" We will publish the answer in an article on Plus shortly. Thank you for taking part!

This is the last online poll in our series to celebrate the International Year of Astronomy 2009. Choose your favourite question from the list on the right, and we'll put the one that proves most popular to world-leading astronomers and cosmologists. The poll will remain open for a month and the answer will be published in a Plus article and possibly a podcast soon after.

The winning questions in our previous polls have been

• What happened before the Big Bang?
• Are the constants of nature really constant?
• What is dark energy?
• What is dark matter?
• How does gravity work?
• And "Is time travel allowed?"

The answer to the last question will be published on Plus shortly.

Labels: IYA2009