Earth rotated once around its axis on Wednesday, June 29, in 1.59 milliseconds less than 24 hours, faster than scientists had ever previously recorded, according to TimeAndDate.com.
It came against a long-term background of a gradual slowing of Earth’s rotation, caused by the the ebb and flow of tides, the flow of magma, the melting of ice caps and other factors that introduce friction.
The observed rotational speed of Earth is measured against what an atomic clock says is the length of the day. That produces a figure called Delta T, the difference between what we think the slowing of the Earth is going to be, and what we actually measure it to be.
However, atomic clocks can’t help scientists measure the rotational speed of Earth in ancient times. For that you need historical records of solar eclipses.
A new paper published in the Publications of the Astronomical Society of the Pacific studied documents from the Byzantine Empire. They were looking for official records of solar eclipses around the Eastern Mediterranean between the 4th and 7th centuries, a period of history for which very few details on solar eclipses have been found.
“Although original eyewitness accounts from this period have mostly been lost, quotations, translations, etc. recorded by later generations provide valuable information,” said Koji Murata, co-author and Assistant Professor of the University of Tsukuba in Japan. “In addition to reliable location and timing information, we needed confirmation of eclipse totality.”
Totality is the period during a total solar eclipse when the Moon completely blocked the Sun. Although it doesn’t always bring darkness in the day, it always results in a deep twilight during which bright stars and planets can be seen as well as a view of the solar corona, the white-hot atmosphere of the Sun.
The next total solar eclipse will occur in Western Australia, Timor Leste and West Papua on April 20, 2023.
The researchers discovered in the records details of the probable times and locations of five total solar eclipses in the years 346, 418, 484, 601 and 693.
The key information they needed to establish as Delta-T, which is critical in calculating where on Earth the path of totality will fall. If the figure is wrong then the path will be too far east or west. Variations in Delta-T reveal the actual length of a day on Earth.
One of the new total solar eclipse is discovered by researchers occurred on July 19, 418. An ancient text reported that stars were seen in the sky during the day as seen from Constantinople, present-day Istanbul. That proved to be critical information because the accepted Delta-T figure placed the city outside the path of totality for that eclipse. The 5th century thus has a new figure for Delta-T, which should help scientists better document changes in the speed of Earth’s rotation through history.
“Our new Delta-T data fill a considerable gap and indicate that the Delta-T margin for the 5th century should be revised upward, whereas those for the 6th and 7th centuries should be revised downward,” says Murata.
Wishing you clear skies and wide eyes.