The heliocentric system of the world: from Aristarchus to Newton

For the first time, the idea that the Earth revolves around the Sun arose in the minds of ancient Greek scientists. The founding father of heliocentrism is considered to be Aristarchus of Samos, who was born around 310 BC. How well he managed to work out his hypothesis is not known for certain, all information available to historians is reduced to only a few references in the works of Archimedes and Plutarch. The only work of Aristarchus that has come down to us concerns measuring the distances to the Moon and the Sun, as well as calculating their sizes.

Heliocentrism eliminated a number of serious problems of the geocentric system of the world, for example, the backward motion of the planets. The further the outer (relative to the earth’s orbit) planet is removed from the Sun, the longer it is period of revolution. Consider, for example, Jupiter with an orbital period of 11.86 years. Due to the higher speed and smaller radius of the orbit, the Earth regularly approaches it and “overtakes”.

If at this time Jupiter is observed against the background of fixed stars, then the impression is created that it is moving in its orbit in the opposite direction, towards the Earth. To explain this observed behaviour of the planets in the geocentric model, a complex system of epicycles was used, requiring a huge amount of non-trivial calculations.

Archimedes spoke rather positively about the heliocentric system of the world, the Babylonian astronomer Seleucus was a well-known supporter of this hypothesis, the ancient Greek philosopher Sextus Empiricus wrote about the followers of Aristarchus. However, the hypothesis turned out to be too bold for its time and was forgotten by the Greeks, the probable reasons are: an increased level of religious dogmatism; astrology, which replaced astronomy; the general crisis of science after the II century AD.

“And yet it turns!”

In the Middle Ages and in the era of the early Renaissance, heliocentrism was practically forgotten, there are only a number of references to similar hypotheses among Indian and Arab astronomers. The year 1543 can be called a landmark for all astronomy, when the monumental work of the Polish astronomer Nicolaus Copernicus “On the Rotations of the Celestial Spheres” was published, in which he outlined his view of the heliocentric system.

The scientist postulated a number of axioms: all six (known at that time) planets revolve around the Sun; The moon revolves around the earth; the distance from our planet to the Sun is much less than the distance to the stars; The earth rotates on its own axis. Copernicus still used epicycles to accurately describe the motion of the planets, but he managed to reduce their number from 77 to 34. Despite a number of serious shortcomings, for example, the preserved special status of the Earth (according to the theory,

One of the great thinkers of the Renaissance who supported Copernicus’ theory was Giordano Bruno. He not only actively defended a model of the world in which the Earth and other planets revolve in a circular orbit around the Sun, but also rejected the concept of celestial spheres as unnecessary, explaining the diurnal trajectories of stars by the rotation of our planet, and also, albeit erroneously, explained the observed effect of precession… Bruno put forward a number of bold assumptions, such as: the infinity of the universe, the existence of other planetary systems and the presence of undiscovered planets in the solar system.

He was one of the first to seriously consider the concept of relativity of motion, and also put forward a hypothesis about the absence of the centre of the universe. Most likely, Giordano Bruno’s “overly progressive” views on cosmology played an important role in the decision of the Inquisition’s court, according to which he was burned alive on February 17, 1600.

The real revolution in the world of heliocentric representations was made by the German astronomer Johannes Kepler, who became a staunch supporter of heliocentrism as a student. Thanks to long-term collaboration with the Danish astronomer Tycho Brahe, he received a huge array of observational data at his disposal. As a result of an intuitive analysis of these data, he formulated three Kepler’s laws, describing the motion of the planets in their orbits with great accuracy.

Thanks to these laws, for the first time, it was possible to calculate with high accuracy the relative distances from the Sun to all planets, as well as to compile tables of their motion, significantly exceeding in accuracy all similar tables that existed before. Astronomers still use Kepler’s laws in their calculations.

The first person to direct a telescope (a telescope, in the terminology of that time) at the night sky was Kepler’s contemporary, the Italian astronomer, physicist and mechanic Galileo Galilei. Thanks to the first threefold, and later 32-fold, increase of the telescopes invented by him, the scientist was able to examine the irregularities of the lunar landscape, open the four satellites of Jupiter, see spots on the Sun and, by the nature of their movement, establish the fact of rotation of the star around its own axis. The phase change of Venus discovered by Galileo indicated the rotation of the planet around the Sun.

This observation can be verified independently using an inexpensive amateur telescope, while it completely destroys the classical geocentric theory. The telescope also made it possible to see stars that were invisible to the naked eye, which corresponded to the position of the Copernican theory of giant distances to stars. Moving along their orbits, the planets either approach each other significantly (opposition or opposition), then, on the contrary, turn out to be on opposite sides of our star (conjunction with the Sun). Following this, and observed by Galileo, changes in the apparent size of the planets also testified in favour of the heliocentric system of the world.

All the discoveries described above, however, did not at all convince the Catholic Church and in 1663, facing the court of the Inquisition, the scientist was forced to renounce his views. It was then that Galileo allegedly uttered the famous phrase: “And yet it turns!” turn out to be on opposite sides of our star (conjunction with the Sun).

Disputes between the followers of various theories continued for some time, until in 1687 Isaac Newton demonstrated that Kepler’s laws can be derived from the law of universal gravitation, thereby putting a bold cross on geocentrism. A century and a half later, in 1835, the Catholic Church finally recognized that the earth revolves around the sun. In Orthodox Russia, active criticism of heliocentrism was carried out until the beginning of the 20th century, quotations from the Bible were used as “arguments”.

The heliocentric system of the world: from Aristarchus to Newton

Empirical evidence for a heliocentric system

It is worth noting that Galileo’s observations are not 100% proof of heliocentrism, and it is quite possible to create a geocentric system of planetary rotation that is consistent with them, albeit completely different from the classical version of Ptolemy. One of these systems was the geo-heliocentric system, popular for some time, which maintained the central position of the Earth, while all other planets in it revolved around the Sun.

This brainchild of the Danish astronomer Tycho Brahe, according to the scientist himself, was created with an eye on the tough position of the Catholic Church and was perceived by many scientists as a veiled version of heliocentrism. After the discovery of the law of universal gravitation in the scientific community, there was no doubt that the Earth revolves around the Sun, but this is a completely different story.

If you just want to make sure with your own eyes that the planets of the solar system revolve around their star, then you just need to repeat the observation of the phases of Venus after Galileo, as well as establish the fact that the apparent diameter of Mars has changed. A telescope in the region of $ 150-200 will be enough for this.

You can start observing Venus almost at any time of the year, its full cycle of phases lasts 584 days – the time during which the planet manages to overtake the Earth by one revolution. Oppositions are important for observing Mars, the next three will occur: July 27, 2018 (great opposition, i.e. maximum approach), October 10, 2020, December 8, 2022. The apparent size of the red planet will gradually increase with the approach of the date of opposition and vice versa.

However, the observational evidence described above does not directly relate to the Earth and is quite consistent with the geo-heliocentric picture. There is, of course, empirical evidence that the Earth also revolves around the Sun, but it is much more difficult to reproduce at home. Moreover, most of them were found after the discovery of the law of universal gravitation, when the scientific community no longer doubted the correctness of the heliocentric system.

The annual parallax of stars

On the motionless Earth, which is located in the centre of the world, the direction of visibility of the stars on the celestial sphere should always be exactly the same. Since our planet still revolves around the Sun, the position of the observer is constantly shifting throughout the year, and, therefore, the angle at which the distant star is visible also slightly changes.

The assumption of the existence of this phenomenon arose back in the time of Aristarchus, however, due to the fact that the distance even to nearby stars is much greater than the diameter of the Earth’s orbit, it was extremely difficult to confirm it with observations (the first successful measurements were carried out only in the 19th century).

For the nearest star, Proxima Centauri, the annual parallax is only 0.77 arc seconds. Launched in 2013, the Gaia spacecraft is capable of measuring angles with an accuracy of 10 microseconds of arc.

The shift of spectral lines

The change in the frequency of sound, depending on whether its source approaches us or recedes, is known to us as a manifestation of the Doppler effect. In everyday life, you may encounter this effect when an ambulance with a siren on passes by your side: first, the sound frequency increases, and then, when the car passes you, it begins to decrease. Such a change in the frequency of radiation, and, consequently, in the wavelength, is also characteristic of light.

As it moves in orbit, the Earth is moving away from the observed star, then approaching it, as shown in the figure. If you constantly analyze the spectrum of a certain star, then cyclical shifts with a period of one year will be visible in it: either towards red (moving away), then towards blue (approaching). Such measurements are greatly complicated by the fact that all-stars

Starlight aberration

Imagine that during the rain you are standing under an umbrella and the drops are falling vertically down. If you run forward, the drops will begin to fall at an angle and you will need to tilt the umbrella at a certain angle in front of you so as not to get wet. This is a rather rough analogy of the aberration of light, when, when passing from one frame of reference to another, the direction of propagation of radiation changes.

Since the Earth changes its direction of motion relative to the star every six months, the latter describes a small ellipse on the celestial sphere in a year. This phenomenon was discovered by the English astronomer James Bradley in 1727 and made it possible to accurately measure the speed of light. The annual aberration angle is approximately 20 arc seconds.

Photos from Mars

From the surface of the red planet, the Sun appears very small. On Earth, the angular diameter of the solar disk is approximately 0.533 degrees, while on Mars we have a value of 0.35 degrees. This means that the distance between our planet and the Sun is less than the distance between Mars and the Sun.

Moreover, if you observe the sunrise from different points of the Martian orbit, then with the naked eye you will not see any changes in the angular diameter (due to the elliptical orbit, there are still minimal oscillations). But that’s not the point. The very fact that humanity has launched its spacecraft to Mars and other distant worlds best confirms the law of universal gravitation and Einstein’s theory of relativity. More convincing evidence is hard to imagine!

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