Technical: Sidereal Time

Scientists and philosophers have long known that time is relative. You might think that a day has twenty-four hours, but that's only because we define a day as the average time it takes the sun to loop from high noon to high noon. But a day can also be measured relative to other objects outside of our solar system, and when we view a day on Earth from the perspective of an extrasolar celestial body, we find something curious: a day lasts only 23 hours, 56 minutes and 4 seconds.

The reason for these missing minutes is due to the relative position of the Earth to the sun in space. Assume we’re able to stand on the sun and watch the Earth, focussing on London, for example. If the Earth stayed at a fixed point in space as it rotated on its axis, it would take that same 23 hours, 56 minutes and 4 seconds for it to complete a rotation and for London to perform a single lap. But Earth does not stay still—it orbits the sun at a colossal 67,000 miles per hour, circling around our vantage point on the sun’s surface. Its constant movement requires our angle of view to change to keep it in view, and that means London has to come further around to face us, which takes longer—3 minutes and 56 seconds longer, totalling twenty-four hours exactly. This is solar time, from which our day-to-day time system is derived.

Now, let’s move ourselves to the next nearest star, Proxima Centuri, over 4.2 lightyears away. From that distance, we always view Earth from the same angle, avoiding the problem we had on the sun as the Earth orbited around it. This new perspective gives us the same view as the hypothetical orbit-less Earth scenario, where a full rotation takes just under four minutes less than solar time. We call this measurement of time relative to faraway objects sidereal time, from the Latin sidus, meaning ‘star’. Over the course of a year, those missing minutes add up to a whole extra sidereal day.

So why is this useful? Well, for navigators and astronomers charting the heavens, it can make all the difference when tracking a celestial object. Quite simply, sidereal time can predict a star’s location accurately; solar time can’t. Sidereal complications have existed for centuries, but, because the engineering involved is so difficult, they are extremely rare. The difference between solar and sidereal time—1:0.99726956633 to be precise—is so tiny that it takes excruciatingly precise calibration to show them both using the same drivetrain.

One particularly impressive proponent of the sidereal complication is the Sidérale Scafusia from IWC's Portuguese collection. With a constant force tourbillon, deadbeat seconds, perpetual calendar and complete star chart to complement the sidereal time indicator, the Sidérale Scafusia is by far the brand's most complicated watch ever. Shown on a twenty-four–hour subdial, the Scafusia's sidereal display is so accurate that it deviates by only 11.5 seconds per year. Another famous example of this achievement is Patek Philippe's legendary Skywatch, which offers even more complications in a highly ornate case.

Other watchmakers have got around the difficulty presented by sidereal time by using separate systems in the same watch. Based on two groundbreaking timepieces by John Arnold at the end of the eighteenth century, Arnold & Son's recent DBS watch uses two separate movements running at different speeds to show solar and sidereal time side by side with maximum accuracy.

To you and I, sidereal time is of little relevance, but as a reminder of the sheer scale of the universe, for the very understanding that what we see of ourselves here on Earth differs to how we’re viewed from across the vast expanse of cosmos, its a complication of truly grandiose implications. Who would’ve thought that those two little hands could hold so much of time and space between them?

Equation of Time

As if one extra form of time, sidereal time, wasn’t enough, how about two more? Apparent solar time, measured from the actual position of the sun in the sky, varies throughout the year due to the tilt of Earth’s spin and its elliptical orbit. Mean solar time, however, averages those variations to give us a consistent unit of time. The difference, as it fluctuates throughout the year by as much as fifteen minutes, is known as the equation of time.