Old days were shorter

'Comets are hammering through the inner solar system at a constant rate. This proves that previous suppositions were incorrect'

Modern astronomers are using ancient and mediaeval records to map changes in the number of comets passing through the solar system and the length of the day.

Although ancient observations are less precise than modern methods, they are useful because they cover long periods of time. Long-term trends can therefore be identified by combining all the information.

One surprise result to emerge is that the number of comets passing through the inner solar system has not changed in 2,000 years. Astronomers had thought that there were more bright comets in the past than now.

"Comets are hammering through the inner solar system at a constant rate," said David Hughes of the University of Sheffield. "This proves that previous suppositions were incorrect." He intends to write up the work in the next few weeks. Dr Hughes has also demonstrated that comets are evenly spread throughout the inner solar system.

In a separate study, Leslie Morrison, of the Royal Greenwich Observatory in Cambridge, and Richard Stephenson, of the University of Durham, have used Babylonian records to measure the change in the rate of the Earth's rotation. "Ancient eclipses give a marker of the exact position of the Earth, the Moon and the Sun," said Dr Morrison. "By identifying many of these, we can identify changes in the length of the day."

Between 700bc and ad1000, Babylonian astronomers recorded eclipses on clay tablets. Virtually all of the known records are now in the British Museum. By working with experts on the script and experts on the calendar, Dr Morrison and Professor Stephenson demonstrated that the length of the day has increased by 42.5 milliseconds over the past 2,500 years, or 1.7 milliseconds per century.

According to theory, the tides should increase the length of the day by 2.3 milliseconds each century. This means that something else is acting against the tides. Dr Morrison believes that this component is probably due to the Earth being more flattened at the poles during the last ice age; it is now speeding up as it becomes more spherical. His study is published in this month's Astronomy and Geophysics.