# Where does my computer get the time from? Source: https://dotat.at/@/2023-05-26-whence-time.html Kiwi News: https://news.kiwistand.com/stories/Where-does-my-computer-get-the-time-from?index=0x651ee49c85c5b4c4adfef4419c4160bb451c21457895b97aded9d59b8f29add5e8560eed Submitted by chaomons.eth on 2023-10-05 UTC, 3 upvotes, 0 comments ## Summary A talk on where computers get their time traces the chain from NTP servers to GPS receivers, then to the US Naval Observatory (USNO) and its atomic clocks. The USNO receives Earth-rotation information from the IERS in Paris, and the BIPM in Paris maintains UTC from national timing laboratories' measurements. The article also covers how the caesium second's definition was calibrated in 1955–1958 by Louis Essen and Jack Parry with help from William Markowitz of the USNO, who measured the "ephemeris second." ## Article This week I was in Rotterdam for a RIPE meeting. On Friday morning I gave a lightning talk called where does my computer get the time from? The RIPE meeting website has a copy of my slides and a video of the talk; this is a blogified low-res version of the slides with a rough and inexact transcript. I wrote a follow-up note, “Where does ‘where does my computer get the time from?’ come from?” about some things I left out of the talk. Where does my computer get the time from? from NTP - here’s a picture of an NTP packet and here’s a picture of David Mills who invented NTP simple question, easy answer, end of talk? No! let’s peel off some layers… stratum 3 NTP servers get the time from stratum 2 NTP servers, stratum 2 NTP servers get the time from stratum 1 NTP servers, stratum 1 NTP servers get the time from some reference clock maybe a radio signal such as MSF in Britain or DCF77 in Germany but in most cases the reference clock is probably a GPS receiver here’s a GPS timing receiver and here’s a GPS satellite where does GPS get the time from? Schriever Space Force Base in Colorado they look after a lot of different top secret satellites and other stuff at Schriever, as you can see from all the mission logos so you can’t get close enough to take a nice photo Where does Schriever SFB get the time from? the US Naval Observatory Alternate Master Clock is on site at Schriever in Colorado the US Naval Observatory Alternate Master Clock gets the time from the US Naval Observatory in Washington DC there are three answers the first answer is atomic clocks, lots of atomic clocks in the background there are dozens of rack mounted caesium beam clocks in the foreground the black boxes house hydrogen masers these shiny cylinders are rubidium fountains the USNO has so many atomic clocks they have entire buildings dedicated to them When I was preparing this talk I noticed on Apple Maps that there’s a huge building site in the middle of the USNO campus. It turns out they are building a fancy new clock house; the main limit on the accuracy of their clocks is environmental stability: temperature, humidity, etc. so the new building will have serious air handling. the second answer is that UTC is a horrible compromise between time from atomic clocks and time from earth rotation so the USNO gets the time from the international earth rotation service, which is based at the Paris Observatory twice a year the IERS sends out Bulletin C, which says whether or not there will be a leap second in six months time; leap seconds are added (or maybe removed) from UTC to keep it in sync with earth rotation the IERS is spread across several organizations which contribute to its scientific work for example, you can subscribe to IERS Bulletin A, which is a weekly notice with precise details of the earth orientation parameters Bulletin A is sent out by the US Naval observatory they need to know the exact orientation of the earth under the GPS satellites, so they can provide precise positioning the third answer is, how does the USNO know its atomic clocks are working well? that information comes from the international bureau of weights and measures in Paris, who maintain the global standard UTC how does the BIPM determine what UTC is? the BIPM collects time measurements from national timing laboratories around the world, and uses those measurements to determine official UTC periodically they send out Circular T which has information about the discrepencies between official UTC and UTC from the various national time labs the BIPM is responsible for maintaining the international system of units, which is defined by the general conference on weights and measures the CGPM is an international treaty organization established by the metre convention of 1875 UTC is an implementation of the SI unit of time, based on quantum measurements of caesium atoms where did this magic number, about 9.2 GHz, come from? in 1955, Louis Essen (on the right) and Jack Parry (left) built the first caesium atomic clock the current definition of the second came from the calibration of this clock before atomic clocks, the definition of the second was based on astronomy, so Essen and Parry needed help from astronomers to find out how fast their clock ticks according to the existing standard of time they got help from the astronomers at the US Naval Observatory the way it worked was William Markowitz measured time by looking at the skies, and Louis Essen measured time by looking at his atomic clock, and to correlate their measurements, they both listened to the WWV radio time signal broadcast by the national bureau of standards in Washington DC this project took 3 years, 1955 - 1958 Markowitz was measuring the “ephemeris second” in 1952 the international astronomical union changed the definition of time so that instead of being based on the rotation of the earth about its axis, it was based on the orbit of the earth around the sun in the 1930s they had discovered that the earth’s rotation is not perfectly even: it slows down and speeds up slightly clocks were now more precise than the rotation of the earth, so the ephemeris second was a new more precise standard of time the ephemeris second is based on an astronomical ephemeris, which is a mathematical model of the solar system the standard ephemeris was produced by Simon Newcomb in the late 1800s he collected a vast amount of historical astronomical data to create his mathematical model it remained the standard until the mid 1980s here’s a picture of Simon Newcomb he is a fine-looking Victorian gentleman where did he work? at the US naval observatory! (and the US nautical almanac office) I have now run out of layers: before this point, clocks were set more straightforwardly by watching stars cross the sky so, to summarise my talk, where does my computer get the time from? it does not get it from the Royal Greenwich Observatory!