Time, at least on the larger scale, is typically “measured” in days and years, using the rotation of the Earth and the orbit of the Earth around the Sun respectively as reference. When the Earth completes “one full rotation,” a day is said to have occurred. Similarly, when the Earth completes “one full orbit” around the Sun, a year is said to have occurred. However, on the smaller scale, a different reference is typically used.
The “most accurate measure” of time is kept by atomic clocks. An atomic clock is a device that counts very brief changes in atomic and subatomic particles. That is, in modern atomic clocks, when an electron changes energy levels it emits an electromagnetic pulse that the clock can detect; the atomic clock counts these pulses and determines that a specific amount of time has passed upon detecting the specified number of pulses. Putting this another way, through scientific inquiry it has been determined that electrons will change energy levels in certain elements (at very specific temperatures and other conditions) at certain specific intervals, thus the atomic clock, upon counting an appropriate number of these changes, can report that a specific amount of time has passed.
I admit that even this brief description of how an atomic clock works is over-simplified, but the basic structure of counting events is still present in the process of “measuring” time. If I assume that the event in question is sufficiently regular and reliable, then I can simply count the events to determine how much time has passed in a particular situation. Atomic clocks are considered to be very, very accurate—given that the conditions under which the atomic clock operates are kept as constant and unchanging as possible—when compared to other sorts of time keeping devices, such as wristwatches which often use a quartz crystal and count the oscillations of the crystal’s natural vibrations.
It is not strictly important to understand the very particular nature of physics and the universe to follow what I am suggesting here. Each situation of time measurement is essentially the same: find a naturally occurring event, one that is considered to be reliable and regular, and then count those events in order to determine an accurate accounting of time. Whether one uses the rotation of the Earth, or the vibration of atoms, in theory the end result should be the same. Variance should generally only occur if the reference I select is less reliable or less regular for some reason. It is generally considered the case that the vibration of atoms is much more reliable and regular than the rotation of the Earth, and thus those time measuring devices that use a reference of the vibration of atoms are considered more accurate and precise than those that rely upon the rotation of the Earth. And thus, if there is a difference between the time measured on different devices, the one that is considered more accurate should be used to correct the one that is less accurate. And this is where the idea of leap years and leap seconds enter into the discussion.
As I had suggested in my previous post, the rotation of the Earth about its axis, and the orbital period of the Earth about the Sun, are not consistent. How do I know this? Because when I use other methods of measuring time, I find that the time it took for the Earth to rotate, or the Earth to complete an orbit around the Sun, is different from one count to the next. If the last orbit of the Earth around the Sun occurred within 365.24221 days (according to a particular atomic clock), and the current orbit occurred within 365.24220 days (according to the same particular atomic clock), and then another measure from another previous orbit occurred within 365.24219 days, then I would suggest that either the orbital period is changing with each orbit, or the event that the atomic clock is using to measure time with is not as regular or reliable as I might think. Or it is also possible that both methods have error to them, and that neither event I am using as a reference is entirely reliable and regular. The general consensus by the international community of humans on this planet suggests I take the atomic clock as most accurate. If you want to learn more about this, I’d suggest starting here.
At this point, many of my friends and family will suggest I am being far too pedantic. That it is not so important to worry about such minor differences between everything. It doesn’t affect most people’s day-to-day lives whether one time measuring device is more or less accurate than another, so long as we can all agree to one standard. And, for the most part, they are correct. The standard that virtually all human beings agree to is that from the point in time the Sun is at its highest point in the sky, to the point in time when the Sun is again at the highest point in the sky is exactly one day. Most people are not interested in the measure provided by the atomic clock, even if science will tell us that its information is far more accurate. Furthermore, when the time comes to synchronize our clocks, it is the atomic clock that will be adjusted to conform its measure of time to the rotation of the Earth by adding or subtracting seconds to its value. These are leap seconds.
In a similar move, the orbit of the Earth around the Sun does not occur in a time frame that coincides nicely with the rotation of the Earth. That is, in the time it takes for one complete orbit of the Earth around the Sun, the Earth will rotate approximately 365 1/4 times. And again, when it is time to synchronize our devices, it is the year that is adjusted to conform with the day, hence why I get one extra day added to the year every four years, except every hundred years. These are leap days.
Thus, the agreed upon measure of time is actually the day, which in turn is measured by observing the rotation of the Earth about its axis. Furthermore, the day is not a complete rotation of the Earth about its axis, but something slightly more than a complete rotation, as the measure is made by observing when the Sun is at the highest point in the sky until it is again at the highest point in the sky. All other devices and measures are adjusted in accordance to this standard. Why this may seem unimportant to most people is that most people exist and spend the entirety of their lives on the Earth, where making such observations are so easy as to be unremarkable. However, if any humans were to leave the Earth, to perhaps colonize Mars, or to explore beyond our solar system, it becomes a great challenge to figure out what time it is.
Having now established how time is measured and maintained for humans upon the Earth, it is time now to take this discussion in another direction. I have some idea of how time is accounted for. I can say that it is Sunday, May 31, 2020 at about 2:41 pm, and I can feel quite confident that if you are a human being living upon the Earth, you will understand what I mean and when I mean. But this still doesn’t really answer any questions regarding what time might be in itself. In fact, the best I can say at this moment is that time, for humans on the Earth, is simply a count of various agreed upon reliable, regular events. So what is time in itself?