One of a small number of planets that have been used for SRV circumnavigation of a planetary body, the first planet of this star system is a small, hot, rocky world with large amounts of open terrain. Completed in 3304, before the invention of the FSS or various other planetary travel enhancements, the historical record for this planet looks a bit different than it would when traveling there in the modern day.

The original forum thread details the circumnavigation, as well as the history pertaining to the importance of Mercury in our understanding of space and physics. These historical essays are included here.

The Celestial Sphere

The term celestial sphere refers to what you see in the sky, and the relative apparent positions of astronomical objects. All objects in the sky can be imagined as though they are projected on the inside of a giant sphere. In gaming terms, you might think of this as the "sky box", but it's a useful tool in astronomy.

Since astronomical objects are extremely distant, for practical purposes (such as constructing star maps of the sky) they can be thought of as infinitely distant, with parallel sight lines from the ground. This imaginary, projected sphere would be infinitely large, with Earth at its center. While this isn't physically real, astronomers still use a set of two-dimensional coordinates to describe an object's position in the sky, namely Right Ascension and declination, and still use the term "celestial sphere" to describe this coordinate system. For a full three-dimensional description of an object's position in space relative to our solar system, a distance is also needed, but historically this has been the most difficult coordinate to measure. The 2D location within the sky, however, can be measured with great precision, and historically this has been possible for centuries.

In ancient times, there was a widespread belief that these astronomical objects were literally placed on a sphere that surrounded the Earth. There was also a belief that the planets, whose motion in the sky didn't match the more distant objects, might be objects on large, concentric, nested crystal spheres that rotate around the Earth. Planetary motion defied understanding for quite some time, as their movement in the sky often contained strange loops and squiggles, with "retrograde" movement.

As we continue, we'll look at the history of orbital mechanics, and the advances made by various astronomers and philosophers, from geocentric to heliocentric models of the solar system, leading to the present day.

Aristotle & Ptolemy

For most of human history, it was believed that Earth was the center of the universe. Everything in the sky seemed to rotate around us, in a giant celestial sphere. This view of the cosmos continued well after it was understood that the Earth is spherical, rather than a flat surface.

For instance, Aristotle, who is often considered the "father of science", believed in a geocentric system. His view of a stationary Earth at the center of a revolving universe persisted for well over a thousand years.

Several centuries later, Ptolemy created one of the first models of the solar system that seemed to explain the strange retrograde motion that planets would occasionally have in the sky. He claimed to have based his geometric model on writings from other astronomers spanning over 800 years before him, such as the writings of Hipparchus, who described the motion of the moon using the concept of an Epicycle to explain its variations in speed during its orbit. While epicycles were first proposed by Apollonius of Perga, Ptolemy formalized it and extended the use of epicycles to explain planetary motion, both in terms of speed variations, and the anomolous retrograde loops and squiggles that the planets would draw out against the background stars.

The epicycle (literally "circle moving on another circle") explained these motions by having the planets revolve on circles (or crystal spheres), that in turn would orbit the Earth in circular paths. While this model was able to predict the motion of the planets, it still lacked precision. Nevertheless, this system persisted as the dominant model for centuries, well into the medieval period. It had been a long held belief, that was further perpetuated by Christian Europe, that everything in the heavens should be perfect, and there was nothing more perfect than a true circle, and therefore the orbits must have been perfect circles.

(As an interesting aside-- Ptolemy attempted to calculate the size of the universe, using his epicycle planetary model. He estimated that the Sun was at an average distance of 1,210 Earth radii, and the radius of the celestial sphere was 20,000 times the Earth's radius).

Aristarchus of Samos

Aristarchus was considerably ahead of his time. While he preceded Ptolemy, he is among the earliest to propose a heliocentric view, in which the Earth orbits the Sun. His ideas were rejected in favor of the incorrect geocentric models of Aristotle and Ptolemy, for centuries.

Unfortunately much of his writings were lost. However Archimmedes references his works and describes his heliocentric ideas. Nicolaus Copernicus, who is credited with formalizing a heliocentric view of the solar system, also credits Aristarchus as the originator of the heliocentric theory.

In addition to this, Aristarchus also correctly suspected that the stars were other suns, and that they were far enough away to have no observable parallax. This was unprovable at the time, since Stellar Parallax is only detectable with telescopes.

Ironically, the only surviving work attributed to Aristarchus uses geometry to calculate the size and distance to the sun and the moon, in a geocentric view. His calculations were incorrect due to the lack of precision in the measurements he had access to, but his geometry was sound. The descriptions from Archimedes of his other works show that he had better numbers and calculations at other times, and his surviving work isn't the best example.

Copernicus

Nicolaus Copernicus was a mathematician and astronomer, in the Renaissance era. His book De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres) brought about a revolution in science, with his heliocentric theory of the solar system. Interestingly, he was reluctant to publish it, despite the urging of his closest friends, because he feared the scorn that it would bring him. The dominant theory until then, was still Ptolemy's geocentric system of epicycles.

Among the revelations presented in his book were Earth's rotation on its axis, the fact that Earth is a planet like the others in the solar system, and the correct order of orbits of the planets, from the sun outward. These ideas were not widely popular when he first published his theory, but it gradually took hold in secret.

His system of circular orbits around the sun still had problems with accuracy, as the elliptical nature of orbits was yet to be discovered. He made corrections by retaining a set of small epicycles, which he called "epicyclets." However the correct ordering of the planets naturally fell into place, as he scaled the orbits of each planet relative to the Earth's motion. While he was mostly correct, and his theory could explain the retrograde motion of the planets in the sky, it couldn't fully prove the heliocentric model until Kepler later cracked the math behind elliptical orbits, eliminating the need for epicycles altogether. In the meantime, his model was comparable in accuracy to Ptolemy's model that was still in widespread use after more than a thousand years. Copernicus offered a much simpler system that achieved comparable results.

Kepler & Galileo

Galileo, born approximately 100 years after Copernicus, revolutionized astronomy with the use of the telescope. He was able to observe the phases of Venus, which is an inner planet that will never show us its fully illuminated side, which proved the heliocentric model. He also observed moons circling Jupiter.

Around the same time, Johannes Kepler was working out a series of laws to describe the orbits of the planets around the sun. It took him about 10 years to complete his book, Astronomia Nova (A New Astronomy), before publishing it. In his book, he outlined equations and terminology to fully describe elliptical orbits in three dimensions, all of which are still in use today.

Prior to working out his theory, Kepler had worked as an assistant to the astronomer Tycho Brahe, who had collected a lifetime of astronomical observations, but kept it mostly hidden from Kepler for fear that he would use it to try to prove a Copernican heliocentric theory. When Tycho Brahe died, the collection of data passed on to Kepler, who was able to use it for completing his theory.

Like those who came before him, Kepler believed that orbits should be perfect circles, but struggled to make that work with the recorded observations of Mars that Brahe had collected. Eventually he stumbled upon the fact that an imaginary line drawn from the Sun to a planet's position will sweep out an equal area of space in an equal time, regardless of which side of the orbit the planet is in, and how fast it is moving there.

This discovery, which became his second law of orbital motion, led to what became his first law: that the planets move in elliptical orbits, with the sun at one focus of the ellipse. His third law shows that there is a precise relationship between the planet's distance from the sun, and the period of the orbit (the length of time it takes to complete one orbit).

Kepler had tried various other orbital shapes, including an egg-shaped ovoid, to explain the equal area over equal time phenomenon. This was unsuccessful until after approximately 40 attempts, that he tried fitting it to an ellipse. He had previously dismissed it as too simple of a solution to have been overlooked by his predecessors. Once he demonstrated that this perfectly fit the Mars data, he concluded that it must be true for all of the planets.

What Kepler lacked though, was an understanding of why. He proposed that there were two forces acting upon the planets: one to propel them forward through their paths, and one to attract them toward the sun. What he didn't understand was that these were simply inertia and gravity, respectively. He did however correctly deduce that whatever solar force was acting on the planets, it must lose strength over distance, since the planets move more slowly when they're further away.

While very precise, there are still orbital effects that can't be accounted for using his perfect elliptical equations. His system assumes a 2-body system, with planets represented as point-masses orbiting a stationary sun. In reality, the sun orbits a barycenter that is contained within itself, and the planets interact with each other in subtle ways. This is referred to as the N-body problem, and it results in the orbits changing over time, in many cases oscillating in eccentricity, and perhaps also a precession of the orbit, meaning the direction of the ellipse's long axis can slowly rotate. Newtons laws of motion and universal gravitation were still decades away.

(As an aside, my ship is named Astronomia Nova, after Kepler's book)

EDIT: Aside #2: Elite uses perfect Kepler orbits in the game.

Newton

Sir Isaac Newton brought further understanding to the orbital motion of the planets, beyond the pure elliptical mathematics that Kepler described. As previously noted, Kepler didn't fully understand why the planets moved as they did, and only postulated the existence of forces acting upon them.

Newton came to the conclusion that all objects in motion, whether it was the orbit of a moon or planet, or an apple falling from a tree, must follow the same principles. Previous Aristotelian lines of thought had assigned different rules and types of motion to different things. Newton shifted the overall scientific perspective to unified patterns in nature.

In his book, Philosophiæ Naturalis Principia Mathematica ("Mathematical Principles of Natural Philosophy", often simply referred to as Principia, pronounced with a hard-C, and published in 1687), Newton laid the foundations of classical mechanics, and proposed that all matter exerts a gravitational force that attracts matter toward its center. The strength of this force depends on the mass of the object, and weakens with distance.

In a famous example, Newton created a thought experiment with the idea of climbing a tall mountain, above the atmosphere, and firing a cannon parallel to the ground. He proposed that as the power of the shot increased, the cannonball would travel further with each shot, and that there would be a speed at which the trajectory of the cannonball would match the curvature of the planet, returning the shot to the point of origin. This would effectively create an orbit (diagram below).

Newton's laws and equations worked so well, that they are still used today as classical mechanics, and the foundation of modern classical physics.

Newton's laws of motion and universal gravitation seemed to complete the entire picture of how objects move through space, and how gravity works. But this too was incomplete. Astronomers began to notice something odd about the planet Mercury. Even after taking into account the influences of all of the other planets in the solar system, the orbit of Mercury seemed to precess (rotate) more than it should. This anomalous effect was measurable, and could not be explained away until a new, additional theory came along.

Precession of Perihelion of Mercury, and Einstein

Newton's laws of motion and universal gravitation stood relatively unchallenged as the best theory of gravity for over two centuries. However, during the latter part of this period, a problem was noticed.

Due to interactions between the planets, their orbits can precess over time (rotation of orientation of the orbit). Newton's laws described this very well, for the most part. However, in 1859 Urbain Le Verrier discovered that the rate of precession for the planet Mercury disagreed with the predictions from Newton's theory. This anomalous precession lacked a good explanation for decades. Though many ideas were suggested, they all failed to hold up to scrutiny.

Part of the problem is that Newton's theory depended on the assumption that mass, distance, and time are constant regardless of where you observe them.

Albert Einstein published his theories of Special Relativity in 1905 and General Relativity in 1915, which proposed a deeper underlying reality in which time, space, and mass are much more fluid, and depend on the frame of reference in which you measure them. Newton's theory is still mostly true within a single frame of reference, and on small scales and at low speeds where the differences from Einstein's system are negligible. However objects in orbit have independent reference frames, and Mercury is in a particularly fast orbit, deep within the sun's gravity well.

This new theory described a system in which space and time (together referred to as spacetime) can be distorted by the presence of matter and energy. Time is flexible, and runs slower in places that are deeper within gravitational fields. With the curvature of spacetime mediating the gravitation between the sun and Mercury, whose orbit is very close, the remaining anomalous precession could now be easily explained.

Einstein was aware of this problem in astronomy, and in his paper he proposed three separate tests that could be performed to prove his theory, and included Mercury's precession as one of them. In fact, it was soon demonstrated that his theory's predictions matched very closely with observed measurements, which cemented General Relativity as something to be taken seriously, and it remains the currently accepted theory of gravity.

It should be noted that only at extreme speeds or scales will Einstein's relativistic effects be noticed. Even with Mercury's orbit, the effects were subtle, and required precise measurements to be detectable. Newton's and Kepler's mathematical models work so well within the scale of the solar system that they are still used to launch satellites and other spacecraft to destinations throughout, and predict their motion. For this reason Einstein's theories often are not seen as a replacement, but rather a more complete realization of the same physical laws.

And this brings us full circle, to why I wanted to use a Mercury-like planet in a tight, fast orbit for my circumnavigation! ;) :D

This of course was a very superficial historical look at the development of an understanding of gravity and planetary orbits. If you're interested in more detail, there's certainly a wealth of information online about these subjects. We've only just scratched the surface, of course.

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