We decided to study motion from the point of view of an inertial frame of reference. Won’t we then have to reject the services of the terrestrial observer? As a matter of fact, the Earth rotates about its axis and revolves around the Sun, as was proved by Nicolaus Copernicus (1473–1543). It may be difficult for the reader to feel now how revolutionary Copernicus’ discovery was to realize that Giordano Bruno was burned at the stake, and Galileo suffered humiliation and exile for championing the truth of Copernicus’ ideas.

What was it that Copernicus’ genius accomplished? Why may we place the discovery of the Earth’s rotation and revolution on one plane with the ideas of human justice for which progressive-minded people have been willing to give up their lives?

In his Dialogue on the Two Chief Systems of the World (the Ptolemaic and the Copernican), for whose writing he was persecuted by the Inquisition, Galileo gave the opponent of the Copernican system the name Simplicio, which means “simpleton”.

In fact, from the point of view of a simple direct observer of the world, that which is not very aptly called “common sense”, the Copernican system seems mad. How can the Earth rotate? As a matter of fact, I see it and it is stationary, but the Sun and the stars are really moving.

The attitude of theologians to Copernicus’ discovery is shown by the following conclusion of the Assembly of Theologians (1616):

The doctrine that the Sun is located at the centre of the world and is immovable is false and absurd, formally heretical and contrary to the Bible. More than that, the doctrine that the Earth does not lie at the centre of the world and moves, possessing in addition a daily rotation, is false and absurd from the philosophical point of view and at least erroneous from the theological one.

This conclusion, in which a lack of understanding of the laws of nature and a belief in the infallibility of religious dogmas are mixed up with a false “common sense”, testifies better than anything else to the strength of Copernicus’ spirit and mind, and those of his disciples having so resolutely broken with the “truths” of the 17th century.

But let us return to the question posed above.

If the velocity of an observer’s motion changes or if he rotates, he must be deleted from the list of “correct” observers. But it is precisely under these conditions that an observer on the Earth is found. However, if the change in velocity or the observer’s rotation during the time he is investigating a motion is small, such an observer may be conditionally regarded as “correct”. Will this pertain to an observer on the Earth?

During a second the Earth will turn 1/240 of a degree, i.e. about 0.00007 radian. This isn’t very much. The Earth is therefore quite inertial with respect to a great many phenomena.

Nevertheless, one can no longer forget about the Earth’s rotation when dealing with prolonged phenomena.

Under the dome of St. Isaac Cathedral in Leningrad hangs an enormous pendulum. If we start oscillating this pendulum, within a short time it will be possible to notice that the plane of its oscillation is slowly turning. After several hours, the plane of oscillation will turn through, a noticeable angle. Such an experiment with this kind of pendulum was first performed by the French scientist Leon Foucault (1819-1868), and has born his name ever since. The Foucault experiment yields a visual demonstration of the Earth’s rotation (Figure 1).

Thus, if the observed motion continues for a long time, we shall be forced to reject the services of the terrestrial observer and take a frame of reference associated with the Sun and the stars as our ba sis. Such a frame of reference was used by Copernicus assuming the Sun and the surrounding stars to be fixed. However, in reality Copernicus’ frame of reference is not completely inertial.

The Universe consists of a great number of star-clusters—islands of the Universe, which are called galaxies. In the galaxy to which our solar system belongs, there, are approximately one-hundred billion stars. The Sun is revolving around the centre of this galaxy with a period of about 180 million years and a speed of 250 km/s. What error will be made by assuming a solar observer to be inertial?

For a comparison of the merits of terrestrial and solar observers, let us compute the angle through which the solar frame of reference turns during a second. If a complete revolution takes place every \(180\times 10^6\) years (\(16\times 10^{15}\) s), then in one second the solar frame of reference will turn through an angle of \(6\times 10^{-14}\) degree or \(10^{-15}\) radian. We may say that the solar observer is 100 billion times “better” than the terrestrial one.

Desiring an even closer approximation to an inertial frame of reference, astronomers take a frame of reference associated with several galaxies as a basis. Such a frame of reference is the most inertial of all possible kinds. It is impossible to find a better frame of reference.

Astronomers may be called star gazers in two senses: they observe stars and describe the motions of heavenly bodies from the point of view of the stars.