When we note our departure time from a clock and then record the clock time at a subsequent fix, we are using the third definition of time as a point in duration. The difference between these two times is the interval between them, which is an example of the second definition. As we learned in basic DR, this time interval and the measured distance traveled is used to calculate our speed expressed in miles per hour, feet per second, or knots. Knots as you will recall, means nautical miles per hour.
Time as a point in duration can use many different reference points. When we talk about local time, noon for my location is quite different from noon to someone in Europe. If we want to reference time to some common point so that everyone in the world knows what that time is, then we must all agree on the point. Greenwich Mean Time (GMT) is the primary time used for official events and is measured at the zero meridian in Greenwich, England, which has been agreed upon by all nations of the world. Time and longitude are so intertwined in navigation that it is difficult to speak of one without understanding the other. But before we get into this relationship, let me first cover some history in the search for time, its measurement, and the inventions used to measure it.
Though their accuracy was not great, these devices provided a way to measure time without the need for the sun to be visible. Each of these time-measuring devices carried markings designed to give sundial time. In addition to a lack of accuracy, sandglasses, waterclocks, and candles had to be reset frequently.
After the discovery of the laws of the pendulum, a more accurate clock was invented that could not only count the hours, but eventually minutes and seconds. The idea of measuring time by splitting it into equal, discrete intervals and counting them was at odds with the concept of time as something that flows. The division of a day into 24 equal hours, of each hour into 60 minutes, and of each minute into 60 seconds are all human inventions required by the need for a more accurate measurement of time.
Despite the various improvements, most early clocks were highly unreliable. However, this was of little consequence, since they could be checked and adjusted regularly by reference to the sun. Thus, despite the technology and the mechanical nature of the time clocks produced, time was still ultimately dependent on the sun.
By the middle of the seventeenth century, pendulum clocks that were accurate to within 10 seconds per day were being manufactured. This was far more precise than the sundial. But, for the vast majority of the world's population, the sun would continue to provide the principal means of telling the local time. However, the definitive time was provided by a clock. From then on, clocks were used to set and calibrate sundials, rather than the other way around as previously had been the case.
Of course, any system that used the sundial as a primary reference point was using local time. As we all know today, local time on the US East Coast is quite a few hours different from the local time on the West Coast. After the great westward migration in the early 1800s, the railroads followed. As the US railway system grew between 1840 and 1850, most railroad companies operated according to the time of their home city. The result was many different railway timetables being used around the country.
In 1918 the four-zone time system was legalized. After 2,000 years, a completely abstract, man-made, uniform, mathematical notion of "time" was starting to work its way into our view of the world. But there were still some additional developments to take place. By the late nineteenth century, many countries had adopted uniform time systems within their borders but there was hardly any coordination between nations. In particular, there was the fundamental issue of where to locate the base line for measuring longitude.
The establishment of a worldwide system to measure longitude brought with it a notion of worldwide time. Since there are 24 hours in a day and 360 degrees in a circle, each 15 degrees of longitude represented one hour. Thus, by wrapping a 360-degree longitudinal grid around the earth, the planet was divided into 24 time zones, each one hour different from its neighbors.
Just as the adoption of uniform time zones in the US came about in response to the growth of rail travel, so too the main impetus for having a uniform worldwide system of measuring time was a global worldwide maritime industry and Marconi's invention of the radio. With instantaneous communications between countries around the world, and between land and ships at sea, it became imperative to have a uniform system of world time.
Today, we live much of our lives by the clock. Indeed, not only are most of our daily activities regulated by the clock, they are often ruled to the precise minute. To live according to the regular beat of man-made time, we have to carry time around with us. The accuracy and inexpensive aspect of today's watches comes from an observation made by Pierre Curie in 1880. He noticed that when pressure is applied to quartz crystals they vibrate at a highly constant frequency. Subsequent investigations showed that subjecting crystals to an electric current also caused them to vibrate. The first use of this phenomenon was in the design of radios to provide a broadcast wave of constant frequency. Then in 1928, Bell Laboratories built the first quartz-crystal clock that eventually led to the replacement of the pendulum with the constant vibrations of the quartz crystal. The quartz clock was so accurate and reliable, that by 1939 it had replaced the mechanically regulated clocks at the Observatory in Greenwich.
But even quartz-crystal clocks were not sufficiently accurate to provide the precision of measurement required for modern technologies. Far greater accuracy is provided by the atomic clock. It makes use of the fact that when suitably energized, the outer electron of a cesium atom flips its magnetic direction relative to the nucleus. In this process the electron emits or absorbs a quantum of energy in the form of radiation with a constant frequency of 9,192,631,770 cycles per second. The idea behind the atomic clock is to bombard cesium with microwaves at close to 9,192,631,770 cycles per second. The microwaves cause an energy oscillation of exactly 9,192,631,770 cycles per second in the cesium atoms, and that in turn regulates the microwaves, holding them to exactly that frequency, hence the ultimate, perfect timekeeper. The official definition, adopted in 1967, for the second is "9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom."
Although units of time less than about a 10th of a second are not perceptible to us, present-day life depends heavily on the extremely accurate measurement of time provided by quartz and atomic clocks. A prime example is desktop computers, which derive their speed from a highly accurate internal clock capable of measuring extremely short periods of time, currently approaching the 1000 MHz range, or 1 GHz.
Another good example of the use of atomic clocks is provided by the Global Positioning System. GPS depends on a network of 24 satellites orbiting the earth. Each satellite continually beams down a signal giving its position and the mean-time determined by the four atomic clocks it carries on board. By picking up and comparing the time signals from three satellites, a ground receiver can compute its latitude and longitude with an accuracy of 30 feet with selective availability turned off. The clocks on the satellites have to be extremely accurate. The determination of the position of the ground receiver depends on the tiny intervals of time it takes an electromagnetic signal to travel from each of the satellites to the receiver. Since the signal travels at 186,000 miles per second, a timing error of one-billionth of a second will produce a position error of about one foot. The onboard clocks are accurate to one second in 30,000 years.
The US Naval Observatory (USNO) in Washington, DC is charged with the responsibility for measuring and disseminating time. The USNO Master Clock is based on a system of many independently operating cesium atomic clocks and a dozen hydrogen maser (a devise emitting microwave radiation produced by the natural oscillations of atoms or molecules between energy levels) clocks. Their website at http://tycho.usno.navy.mil/ provides a rich source on information on modern timekeeping.
Though largely hidden from our view, the fine-grained notion of time in use today, based on the movement of pulsars and measured by the tiny quantum energy states of the atom, quite literally affects the very fabric of our daily lives and the way we view ourselves and the world we live in. We live by the clock, and in many ways we are slaves to the clock. The use of Greenwich Mean Time for celestial navigation is required since all the Nautical Almanac tables are referenced to GMT and it is the official time for all maritime navigation. Accurate time is very important to celestial because any clock errors will throw the fix off by many miles. I'll cover this concept in more detail in my next article.
When dealing with navigation, you're apt to come upon a number of ways in which time is referenced. Here are the most common time references and their definitions:
Local Apparent Time - Sun Dial Time or hour angle of the real sun.
Local Mean Solar Time - The hour angle of a fictitious, mean sun; used to determine Standard Time.
Standard Time - The Local Mean Solar Time at the standard meridian of your time zone. Standard meridians are spaced approximately 15 degrees apart in longitude around the earth.
Equation of Time - The difference between the apparent time and the mean time. The Equation of Time value for the date is used to convert Sun Dial Time to Local Mean Time.
Local Sidereal Time - Often known as "star time," this is the hour angle of the vernal equinox; or in more useful terms, the right ascension of the stars on your local celestial meridian.
Julian Day - A linear count of days starting on January 1, 4713 BC and is commonly used for astronomical calculations. The Julian Day starts at Noon Universal Time (UT), hence at 0h UT the Julian day is some whole number + 0.5.