STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

Dr. N. Subramanian

Slowing the spin of the Earth By Patterson Clark, Published: Sept. 23, 2013

How much does the Earth slow down every year?
AN ANALOGY TO PUT THINGS INTO PERSPECTIVE:
• Imagine that the 2,445-mile distance between Washington and San Francisco represents today's day length of 24 hours.
• 200 million years ago, an analagous distance for day length would stretch only from San Francisco to near the West Virginia/Virginia border.
• Every 100 years, the length of a day increases by 0.002 seconds — or 3.23 inches farther down the road to Washington. The annual gain is an infinitesimal 0.00002 seconds, which on our analogous cross-country trip would amount to 0.82 millimeters, a little more than the thickness of a thumbnail.
Rotating Earth
Moon's gravity pulls back on bulge displaced by the rotating Earth
Axis of bulge
Gravitational friction
Axis of moon's gravity
Tidal bulge (greatly exaggerated)
Moon
Moon's gravity causes tides
Distances not to scale
Baker, W.V.
D.C.
D.C.
102 miles
S.F.
S.F.
24 hours
23 hours
DAY LENGTH
200 million
years ago
Today

SOURCE: NASA.

Why, in the Jurassic era, an Earth day may have been only 23 hours long.                By  Ivan Amato,                                                              Published: September 23      E-mail the writer (health-science@washpost.com?subject=Reader%20feedback%20for%20%27Why,%20in%20the%20Jurassic%20era,%20an%20Earth%20day%20may%20have%20been%20only%2023%20hours%20long.%27)
                                                                                                                                                         Don’t forget to set your clocks ahead two thousandths of second before you go to sleep tonight. Same thing goes for bedtime tomorrow. And every day after that, because that is how much slower the Earth turns on its axis each day now than it did a century ago.
                              All of those sub-eyeblink slowdowns each century have been adding up, too. For Jurassic-era stegosauruses 200 million years ago, the day was perhaps 23 hours long and each year had about 385 days. Two hundred million years from now, the daily dramas for whatever we evolve into will unfold during 25-hour days and 335-day years.

                                                                              Graphic


                              Slowing the spin of the Earth

                                                                              More health and science news
As Earth’s rotation slows, days grow shorter all the time                                                                                                              Ivan Amato SEP 23

               Imperceptible changes in length of day raise issues for smartphones, cellphones and other devices.






                                                        “We naively think there always has been 24 hours per day,” says Thomas O’Brian, chief of the Time and Frequency Division of the National Institute of Standards and Technology (NIST). “But that is not the case.”

For all but the past 60 to 70 years, those extra milliseconds adding to each day did not matter one whit. The boss still can’t tell if you arrive at work two milliseconds after 9 a.m. And twice a year, those accumulating micromoments essentially vanish when most of us adjust our clocks with the start or end of daylight saving time.

Except for one thing: Those micromoments don’t actually vanish, and in an era of intense technology, they now matter a whole lot.
“We have become critically dependent on incredibly precise timekeeping,” O’Brian says. Technologies such as smartphones, GPS devices and the power grid rely on thousands of separated elements — such as satellites, cell towers, generating stations, computers, electrical switches and countless computers — that cannot get more than a millionth of a second out of sync with one another before bad stuff happens.
Consider GPS signals between satellites and receivers on the ground. Those are radio signals that move at the speed of light, which means they travel about one foot every billionth of a second (which is a nanosecond). So if the clocks in GPS satellites and your GPS receiver drift just one millionth of a second — a thousand nanoseconds — out of sync with each other, the system will not pinpoint your location more precisely than within about two-fifths of a mile. If the synchronization drifts off by one thousandth of a second, the system couldn’t tell you for sure if you were in Washington or Boston.
The moment-to-moment monitoring and management by which electrical engineers maintain the flows of current in power grids, whose interconnected components can span thousands of miles, are possible only because of precisely synchronized clocks and high-speed communication by which even the most distant parts of the grid can keep track of each other’s status. And forget about talking and texting on cellphones or Googling on your computer without superlatively timed handoffs of billions of signals between cellphone towers and perfectly timed transmissions of data packets crisscrossing the planet at lightning speed only to miraculously reassemble everywhere into coherent Web pages.
“If we relied on the Earth’s length of day, we could not have any of this,” says O’Brian, whose group at NIST develops, maintains and improves the supremely regular atomic clocks on which all other timekeeping ultimately is based.

Read more at: http://apps.washingtonpost.com/g/page/national/slowing-the-spin-of-the-earth/471/

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