Mechanical watches are full of features with practical origins but with virtually no use to most people who wear them. Will you ever use a chronograph's tachymeter or a dive watch's helium escape valve? Highly doubtful. But these kinds of features can make watches so much more cool and interesting, right? We love watches for these reasons, rather than in spite of them.
Most such features are relatively straightforward once you understand them, but the measurement of time can get a lot deeper than just hour, minute and seconds hands spinning around a dial. As timekeeping gets more esoteric it also gets fascinating as hell — especially when you discover that a watchmaker has been able to express a complicated concept in a form that fits on your wrist.
Some of the most exotic features, or complications, have a history that goes back centuries and were found in clocks or pocket watches that measured all manner of celestial movement. Today, there are still some watchmakers that recreate them purely for the sake of their historical interest, the horological challenge — and the fact that they're just astounding and kind of poetic to contemplate.
The rare complications below can give you a sense of watchmaking's astronomical origins and maybe even a grander perspective of time itself — not to mention an appreciation of some of the most complicated, impressive and expensive watches ever created.
What it is: Sidereal time is an alternative way of timekeeping based on the earth's position relative to the stars rather than to our sun.
The way we measure days is all about our relationship to the sun. It's practical and most relevant to us, but it's only one way of looking at the earth's movement. While you might reasonably think that the Earth turns 360 degrees from noon one day to noon the next, for example, it's also moving on its path around the sun and needs to turn just a little bit further to reach noon again — because it's not in the same position relative to the sun that it was at noon yesterday. Get it?
Measuring time in terms of the 360-degree turn is called sidereal time, whereas measuring the practical length of day from noon to noon is simply called solar time (more on this below). A sidereal day is only 23 hours, 56 minutes and 4.0905 seconds, rather than 24 hours — but this difference can add up significantly over time.
How it's used: Sidereal time is mostly useful to astronomers. It's often represented on watches as a traditional-looking secondary dial.
Luckily, we don't need to calculate the difference between sidereal and solar time in order to be punctual for a meeting. Tracking sidereal time was important to the historical development of science, but why would watchmakers include it on wristwatches today?
Frankly, because it's cool, but astronomers can also use sidereal time to know exactly when a star will be in the same position in the sky as it was the night before. Because the difference between sidereal and solar time is so small, watches that track both timekeeping systems simultaneously need to be extremely accurate and, thus, this type of complication also functions to show off a watchmaker's skill.
Equation of Time
What it is: The equation of time is a way of representing the difference between the way clocks measure time and the earth's actual movement in relation to the sun.
Our standardized 24-hour days and 60-minute hours are useful, but the earth's movement around the sun isn't nearly so regular. Compare your watch to a sundial, and it'll only agree exactly four times per year. This is due to several factors: not only is the earth's path around the sun is elliptical rather perfectly circular, but it speeds up as it's closer to the sun and slows down when it's further away. In addition to this, the earth tilts on its axis as it turns.
All this leads to different lengths of days and different hours of sunlight throughout the year, and this is called apparent solar time. Compared to our artificially averaged system of timekeeping, called mean solar time, apparent solar time can be as much as around 15 minutes fast or slow at certain times of the year.
How it's used: Equation of time on watches mostly serves as a curiosity. It's usually expressed in a gauge-like display that indicates actual solar time's deviation as plus or minus 15 minutes.
Most people don't have a lot of practical use for the equation of time, but there may be some circumstances in which it would actually come in handy — if you're an astronomer. However, it's more likely that watchmaking fans will find it poetic and fascinating the same way they're enchanted by watches with moonphase indicators. It's also another way for watchmakers to stand out and offer something different, complicated and interesting.