Comparing day length on different planets
Physics · Key Stage 2 · The School
A day is a spin, not an orbit
A DAY is the time a planet takes to spin once on its own axis. A YEAR is the time it takes to orbit the Sun once. These are two completely different journeys, and a planet's day and year do not have to be linked at all — some planets have a day far shorter than Earth's, and some far longer.
Some planets spin fast
Jupiter is the largest planet, yet it spins the FASTEST of all — a Jupiter day is under ten hours, less than half of Earth's. Despite its huge size, it completes a full spin before a school day would even finish, which feels backwards until you remember that spin speed and size are simply not linked.
Some planets spin painfully slowly
Venus spins so slowly that one Venus day is longer than one whole Venus YEAR — it finishes orbiting the Sun faster than it finishes spinning once. Venus also spins backwards compared to most other planets. Day length varies enormously across the solar system, for reasons still studied by astronomers today.
Mercury's day lasts about 59 Earth days. Earth's day lasts 24 hours, by definition. Jupiter's day lasts under 10 hours, despite Jupiter being by far the largest planet listed. Venus's day lasts about 243 Earth days — longer than its own 225-day year. Lined up in order of day length, the four planets do not follow their order of size or distance from the Sun at all, which is exactly the point: day length is decided by how a planet actually formed and spins, not by simple rules like "bigger means slower".