The Sidereal Day – Little Known & Little Understood, Here’s a Quick Guide

When Daniel and I started designing our own watch, we determined that it had to be different.  We would not be content with merely different colours or shapes – it had to be fundamentally different.  This goal led us down many avenues of exploration.  Initially, our focus was driven by our love of the intricacies and masterful engineering in each aspect of a watch, but we soon discovered there was little value we could add.

After a while, we decided to sit back and have a more fundamental look at the purpose of a watch.  That seems an easy question – to measure time.  But thinking about the measurement of time led us in many different directions, mainly catching my interest in the development of the definition of a day.  Again, something that, at first look, seems blindingly obvious, but once the question is researched a little bit, the complexity of a simple concept becomes abundantly clear. Through this exploration of “time,” I came across the concept of the “sidereal day” and how it was observed and used historically and still used to this day in certain professions. 

A Brief History of the Day


A day was simple to observe for the ancient people of the world. It started at a given point, let’s say sunset, and continued until the next sunset. This was good enough for all purposes as, in general, the farmers were most intent on following the seasons. However, the drive for a higher degree of resolution on time measurement began when guards were deployed on “watches.” The “Guard Watch” was typically set up to last a period of time measured by reference to the sun during the day, using a sundial or other similar instrument, and with reference to the stars during the night. This measurement by reference to the stars defined the concept of a day for many centuries.

The Big Leap Forward

In 1543, Nicolas Copernicus published his radical new theory that the earth orbited the sun (as opposed to the opposite). Copernicus’s theory was so controversial at the time that his theory that the earth orbited the sun became widely accepted a century later. By then, the best minds in the world, generally all astronomers and astrologers, were focused on the movement of the earth with reference to celestial bodies and heralded the advent of the “sidereal.” The word is derived from the Latin word “sidus,” meaning star, and sidereal is defined as “with respect to the distant stars, i.e., the constellations or fixed stars, not the sun or the planets.” These days, it is unusual to hear the term sidereal day being used – unless you are an astronomer – and the reason will soon become evident. Here is an elementary guide to the sidereal day and why it remains relevant today.

The Sidereal Day

Just as the Sun and Moon appear to rise in the east and set in the west due to Earth’s rotation, so do the stars. The sidereal day uses the regularity of Earth’s rotation about its axis with reference to the stars. A day is defined as the time it takes the earth to rotate once on its axis. Two reference points must be defined to measure the earth’s rotation (or any rotating body, for that matter). One reference point needs to be stationary (or at least as stationary as possible), and the other reference point should be on the rotating body so that when the reference point on the rotating body returns to the same position in relation to the stationary reference point a full rotation has been completed.

It is All Relative

The choice of these reference points differentiates different measurements of a day, for example, the difference between a solar day and a sidereal day. For the sidereal day, the stationary point is chosen as a distant star (hence the name sidereal day). Ideally, the chosen star would be stationary, but unfortunately, there is nothing stationary in the universe, so the farthest star that can be reliably observed is chosen. In this case, the star can be considered stationary in relation to the earth’s rotation as the star’s movement over the period that the earth’s rotation is being observed (a day) will be so infinitesimally small that it can be considered stationary for all intents and purposes.

Selecting the reference point on Earth is a bit more complicated, mainly because it does not rotate around its vertical axis. This reference point is the “equinoctial point” on Earth, defined as “either of the two points at which the ecliptic cuts the celestial equator.” As you can see, this becomes quite technical, but for our purposes, suffice it to say there are two specific points on Earth where the entire rotation is best observed. It is most important to know that the stationary reference point is a distant star for our purposes.

A sidereal day is now simple to observe. For example, suppose you point a telescope at your chosen reference point star while sitting at the equinoctial point on Earth. In that case, a sidereal day will have elapsed when the reference star has returned to the same position in the telescope.

Why is The Sidereal Day Useful?

The sidereal day is a very convenient timekeeping system for astronomers and astrologers when observing the sky. Reversing the definition of the sidereal day, its utility becomes evident. For example, if a telescope points to specific coordinates in the sky on any given day and the observer returns one sidereal day later, the observer would see the same celestial bodies as the previous day. This allowed astronomers to study specific areas of the sky, knowing exactly when to return to see the same object.

The other utility of the sidereal day in the 17th century was that it was the most accurate measurement of a day. Point a telescope east to west so that it could only move north-south. This simple configuration is called a transit instrument and was the primary instrument for measuring days (sidereal), while clocks were used to measure the subdivisions of days. You may note that the transit instrument did not have to be placed at an equinoctial point. This was addressed by adjustment tables published at the time for various locations on the earth. These tables also converted sidereal time into solar time.

The big problem with the sidereal day (and why it is not in everyday use today) is that it does not coincide with the earth’s rotation around the sun. The day that we refer to in everyday use today is the “Solar Day,” which is one rotation of the earth with reference to the sun. The solar day is slightly longer than the sidereal day because it necessarily needs to account for both the earth’s axial rotation on its axis and the earth’s orbit around the sun.

diagram that shows the difference between a sidereal day and a solar day in a schematicThis is most easily explained by reference to the diagram above. Position 1 is the starting position for both the sidereal and solar day. Position 2 is the end of the sidereal day, and position 3 is the end of the solar day. This simple diagram shows that the sidereal day is slightly shorter than the solar day. Another way to look at it is for the solar day. The earth must rotate past an absolute single rotation due to its orbit around the sun, making a solar day longer. This results in one more sidereal day in a year than there are solar days, or a sidereal day is one 366th, or nearly 3 minutes and 56 seconds shorter than a solar day.

The Drawback of the Sidereal Day 

It may not be immediately apparent, but if there is one more sidereal day in a year, then solar time and sidereal time can only be identical at one instance in any given year. Furthermore, and this is the inconvenient point of sidereal time, as the year progresses, 7am in sidereal will start at being close to sunrise and advance over the following six months to be closer to sunset and then continue to advance over the next six months to be back to closer to sunrise. This material shifting of the sidereal time with regard to sunrise and sunset makes it almost impossible to use for daily interactions, which are generally governed by the rhythm of sunrise and sunset. For this reason, although the sidereal day is a technically accurate definition of a day – an absolute single rotation of the earth around its axis – the sidereal day has remained in use only for those who study the stars, and the practical solar day has been adopted universally.

A Contemplation of Time

The sidereal day is the purists’ day; it represents the ideal of a single rotation of the earth on its axis, but when we consider it in relation to our daily lives, it is of little use. So, instead, we need to consider the complexity of our reality. We are sitting on a celestial body spinning on its axis at 1,000 miles per hour while orbiting the sun in an ellipse at 67,000 miles per hour.

Our construct of time is one of pure convenience throughout history. It has evolved as the human race has been able to, or desired, more resolution on measuring the periods being experienced. Of course, there are many other areas in time measurement where the utility of convenience has been selected over the absolute answer – but we take these for granted today

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Comparison Ohi2 vs. Ohi4

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Finally, depending on the movement you select the watch case will have a different profile as the OHI-4 movement is thinner than the OHI-2 movement. The key differences are that the case for the OHI-4 movement has a double domed crystal and a flat caseback. The OHI-2 case has a flat crystal and a curved caseback. All the details are in the product page.