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![]() ![]() GPS satellites have their own atomic clocks, which run around 38 microseconds faster per day than if they were on earth. And the faster you go the slower time flies. The less gravity there is, the faster time flies. The same is true for seconds.And then there’s general relativityAnd then, as Einstein predicted, there’s gravity and velocity. The difference between them is that the length of the former fluctuates while the latter remains fixed. ![]() There are two ways to define a day: astronomically by measuring the time between midday to midday and with a clock. UTC (International Coordinated Time) is calculated from an average of over 400 atomic clocks around the world.įar side of the moon as it crosses between the DSCOVR spacecraft and the earth 1,600 millions kilometers away (image courtesy NASA) That was in 1957.Atomic clocks turned observatories on their heads: instead of astronomers using the positions of stars to calculate the precise time, the incredibly precise time of atomic clocks enabled observatories to more precisely plot the positions of the stars. In the case of cesium 133, the frequency is nearly 9,192,631,770 times per second (nearly 9.2 billion Hz).The length of the second measured in the experiments was based on the average length of the day (determined by observing the stars) when the experiments were taking place. They then experimented with the wavelength (frequency) of the microwaves to discover which got them most excited and that determined the natural resonance frequency of the atom. To make an atomic clock, scientists excited cesium atoms with microwaves in a vacuum. National Institute of Standards and Technology physicists Steve Jefferts (foreground) and Tom Heavner with the NIST-F2 cesium fountain atomic clock, which in 2014 set the civilian time standard for the United States (image courtesy NIST) And as telescopes became more powerful, we could measure the positions of the stars to ever greater precision, which enabled us to measure the time with ever greater precision. In the last few hundred years, the exact time on earth and the length of our days were determined by the positions of stars using telescopes. That means that billions of years in the future, the length of a day on earth and the length of a month will be the same: around 47 of our present days. The inverse is that the same effect slows the earth down. ![]() As the earth is rotating relatively fast – in 24 hours compared with the moon’s rotation of 27.3 days – that fast-rotating bulge of water continually moves slightly ahead of the moon’s orbit, pulling the moon to move slightly faster. The moon’s gravity pulls a large body of water toward it, causing a huge bulge in the ocean on the side facing the moon (and on the other side as well but we can discount that here). Relative rotation velocities of the earth and the moon, not to scale (image courtesy Science ABC) And to that end he studied tides and the impact of tidal friction on the earth and the moon. He became interested in determining the age of the earth and the formation of the moon. George Darwin (1845-1912), son of the famous naturalist Charles Darwin, was a lawyer, physicist, and astronomer. One hour = 0.00069444 or 1/1,440th of an average solar day Then the invention of telescopes allowed us to get even more precise time from the stars, and ever more precise atomic clocks allowed us to create our own time on earth, independent of the heavens.One year = 365.25 days Obelisks and sundials then enabled us to break down the day further into hours and even minutes. The seasons repeated yearly as our planet rotated around the sun the moon gave us monthly intervals as it rotated around the earth and our days passed regularly with the earth’s rotation on its own axis. So why are they still measuring the length of a second as though it’s 1957? It’s because of the moon.Until relatively recently in the history of our species, we have divined and calculated the time from astronomy. The latest generation of optical atomic clocks are accurate to less than one-tenth of a second over the age of the universe (13.8 billion years). ![]()
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