The time it takes our planet to do a full rotation has increased every century due to friction caused by tides. The work of ancient astronomers reveals that the Earth's spin is slowing down — though not as much as scientists believed. Each century, the length of the solar day, or the time it takes the planet to do a full rotation, grows by 1.8 milliseconds, according to a new study using astronomical observations going back to 750 B.C. Researchers have known that the planet's rotation is slowing because of friction caused by the tides, as water that's being tugged on by the moon's gravity sloshes against the solid Earth. However, measurements of this tidal effect suggest that the planet should be slowing in its rotation by 2.3 milliseconds per century, slightly more than the new research finds. The difference between 2.3 milliseconds and 1.8 milliseconds over a century may seem trivial, said study researcher Leslie Morrison, who worked at the Royal Greenwich Observatory for nearly 40