The Handy Astronomy Answer Book

ASTRONOMY

FUNDAMENTALS

IMPORTANT DISCIPLINES

IN ASTRONOMY

What is astronomy?

Astronomy is the scientific study of the universe and everything in it. This includes, but is not limited to, the study of motion, matter, and energy; the study of planets, moons, asteroids, comets, stars, galaxies, and all the gas and dust between them; and even the study of the universe itself, including its origin, aging processes, and final fate.

What is astrophysics?

Astrophysics is the application of the science of physics to the universe and everything in it. The most important way astronomers gain information about the universe is by gathering and interpreting light energy from other parts of the universe (and even the universe itself). Since physics is the most relevant science in the study of space, time, light, and objects that produce or interact with light, the majority of astronomy today is conducted using physics.

What is astrochemistry?

Astrochemistry is the application of the science of chemistry to the universe and everything in it. Modern chemistry—the study of complex molecules and their interactions—has developed almost exclusively at or near Earth’s surface, with its temperature, gravity, and pressure conditions. Its application to the rest of the universe, then, is not quite as direct or ubiquitous as is the application of astrophysics. Even so, astrochemistry is extremely important to cosmic studies: The interactions of chemicals in planetary atmospheres and surfaces is vital to understanding the planets and other bodies in the solar system. Many chemicals have been detected in interstellar gas clouds throughout the Milky Way and other galaxies, including water, carbon monoxide, methane, ammonia, formaldehyde, acetone (which we use in nail polish remover), ethylene glycol (which we use in antifreeze), and even 1,3-dihydroxyacetone (which is found in sunless tanning lotion).

What is mechanics?

Mechanics is the branch of physics that describes the motions of objects in a system. Systems of moving bodies can be very simple, such as Earth and the Moon, or they can be very complicated, such as the Sun, planets, and all the other objects in the solar system put together. Advanced studies of mechanics require complex and detailed mathematical techniques.

What is astrobiology?

Astrobiology is the application of the science of biology to the universe and everything in it. This branch of astronomy is very new. The serious use of biology to study the cosmos has blossomed in recent years, however, and has become very important in the field as a whole. With modern astronomical methods and technology, it has become scientifically feasible to search for extraterrestrial life, look for environments where such life could exist, and study how such life could develop.

What is cosmology?

Cosmology is the part of astronomy that specifically examines the origin of the universe. Until the advent of modern astronomy, cosmology was relegated to the domain of religion or abstract philosophy. Today, cosmology is a vibrant part of science and is not limited to gazing out into the cosmos. Current scientific theories have shown that the universe was once far smaller than an atomic nucleus. This means that modern particle physics and high-energy physics, which can be studied on Earth, are absolutely necessary to decipher the mysteries of the very early universe and, ultimately, the very beginning of everything.

Why is astronomy, the scientific study of the universe, important?

Science is a process of learning about how things work by following a series of steps: the gathering of factual knowledge, the asking of questions, the posing of falsifiable hypotheses, and the testing of those hypotheses with experiments and observations. Following all these steps is essential to building scientific knowledge. If these steps are not followed, the result of learning is non-scientific knowledge. Whereas non-scientific knowledge can be very valuable and very important, it is also very different from scientific knowledge, and cannot be used for many purposes (such as developing technology) that scientific knowledge enables. Astronomy, as the scientific study of the universe, thus serves a very important role in humanity’s quest to understand everything around us and to benefit from that understanding.

Which of the many related scientific disciplines is most important to astronomy?

Physics is by far the most important and relevant scientific discipline to the study of the universe and everything in it. In fact, in modern times the terms astronomy and astrophysics are often used interchangeably. That said, all sciences are important to astronomy, and some disciplines that are not very relevant now may someday be extremely vital. For example, if astronomers eventually find extraterrestrial intelligent life, psychology and sociology could become important to the study of the universe as a whole.

HISTORY OF ASTRONOMY

When did people first begin to study what is now called astronomy?

Astronomy is probably the oldest of the natural sciences. Since prehistoric times, humans have looked at the sky and observed the motions of the Sun, Moon, planets, and stars. As humans began to develop the first applied sciences, such as agriculture and architecture, they were already well aware of the celestial objects above them. Astronomy was used by ancient humans to help them keep time and to maximize agricultural production; it probably played an important role in the development of mythology and religion, too.

What did early astronomers use to measure the universe before telescopes were invented?

Ancient astronomers, such as Hipparchus (in the second century B.C.E.) and Ptolemy (in the second century C.E.), used instruments such as a sundial, a triquetrum (a sort of triangular ruler), and a plinth (a stone block with an engraved arc) to chart the positions and motions of planets and celestial objects.

By the sixteenth century C.E., complex observational tools had been invented. The famous Danish astronomer Tycho Brahe (1546–1601), for example, crafted many of his own instruments, including a sextant, a quadrant with a radius of six feet (almost two meters), a two-piece arc, an astrolabe, and various armillary spheres.

What is an astrolabe, and how does it work?

An astrolabe is an instrument that can be used by astronomers to observe the relative positions of the stars. It can also be used for timekeeping, navigation, and surveying. The most common type of astronomical astrolabe, called the planispheric astrolabe, was a star map engraved on a round sheet of metal. Around the circumference were markings for hours and minutes. Attached to the metal sheet was an inner ring that moved across the map, representing the horizon, and an outer ring that could be adjusted to account for the apparent rotation of the sky.

To use an astrolabe, observers would hang it from a metal ring attached to the top of the round star map. They could then aim it toward a specific star through a sighting device on the back of the astrolabe, called an adilade. By moving the adilade in the direction of the star, the outer ring would pivot along the circumference of the ring to indicate the time of day or night. The adilade could also be adjusted to measure the observer’s latitude and elevation on Earth.

The astrolabe helped mariners navigate the seas for hundreds of years by measuring the positions of the stars.

Who is thought to have invented the astrolabe?

The ancient Greek mathematician Hypatia of Alexandria (370–415 C.E.) is thought to be the first woman in western civilization to teach and study highly advanced mathematics. During her lifetime, the Museum of Alexandria was a great learning institution with a number of schools, public auditoriums, and what was then the world’s greatest library. Hypatia’s father, Theon of Alexandria, was the last recorded member of the Museum.

Hypatia was a teacher at one of the Museum’s schools, called the Neoplatonic School of Philosophy, and became the school’s director in 400 C.E. She was famous for her lively lectures and her many books and articles on mathematics, philosophy, and other subjects. Although very few written records remain, and much information is missing about her life overall, the records suggest that Hypatia invented or helped to invent the astrolabe.

What is the art of astrology?

Astrology is the ancient precursor of the science of astronomy. Ancient people understood that the Sun, Moon, planets, and stars were important parts of the universe, but they could only guess what significance they had or what effects they might cause on human life. Their guesses became a practice in fortune-telling. Astrology was an important part of ancient cultures around the world, but it is not science.

What did ancient Middle Eastern cultures know about astronomy?

The Mesopotamian cultures (Sumerians, Babylonians, Assyrians, and Chaldeans) were very knowledgeable about the motions of the Sun, Moon, planets, and stars. They mapped the twelve constellations of the zodiac. Their towering temples, called ziggurats, may have been used as astronomical observatories. Arab astronomers built great observatories throughout the Islamic empires of a thousand years ago, and we still use Arabic names for many of the best-known stars in the sky.

Are astrolabes used for astronomy today?

Prismatic astrolabes are sometimes still used to determine the time and positions of stars, and for precision surveying. However, newer technology, such as sextants, satellite-aided global positioning systems, and interferometric astrometry are far more common.

What did ancient American cultures know about astronomy?

Ancient American cultures were very knowledgeable about astronomy, including lunar phases, eclipses, and planetary motions. Almost all of the many temples and pyramids of the Inca, Mayan, and other Meso-American cultures are aligned and decorated with the motions of planets and celestial objects.

For example, at Chichen Itza in southern Mexico, on the days of the vernal equinox (March 21) and autumnal equinox (September 21), shadows cast by the Sun create the vision of a huge snake-god slithering up the sides of the Pyramid of Kukulcan, which was built more than one thousand years ago. Farther north, among the Anasazi ruins of Chaco Canyon, New Mexico, the work of ancient Native American astronomers survives in the famous sun dagger petroglyphs, which appear to mark the solstices, equinoxes, and even the 18.6-year lunar cycle.

What is the Dresden Codex, and what does it say about Mayan astronomy?

There are three well-known records from what is believed to have been an extensive Mayan library, dating back perhaps one thousand years to the height of the Mayan civilization. One of these books is called the Dresden Codex because it was discovered in the late 1800s in the archives of a library in Dresden, Germany. It includes observations of the motions of the Moon and Venus, and predictions of the times at which lunar eclipses would occur.

Perhaps the most remarkable section of the Dresden Codex is a complete record of the orbit of Venus around the Sun. Mayan astronomers had correctly calculated that it takes Venus 584 days to complete its orbit. They arrived at this figure by counting the number of days that Venus first appeared in the sky in the morning, the days when it first appeared in the evening, and the days that it was blocked from view because it was on the opposite side of the Sun. The Mayans then marked the beginning and ending of the cycle with the heliacal rising, the day on which Venus rises at the same time as the Sun.

Why did some people think ancient Mayans predicted that the world would end on December 21, 2012?

A few years ago, archaeologists studying Mayan calendar records noticed that one particular calendar cycle would reset on December 21, 2012, similar in a way to how the modern calendar resets each year on January 1. Other people who heard about this began to speculate, without any scientific evidence, that the Mayans who had put together that calendar thought the world would cease to exist on that date. The rumor spread, and a number of wild and fanciful hypotheses about how and why the world would be destroyed on that day were created and circulated. Of course, none of these hypotheses had any scientific validity, and none of them came true—it is now well past December 21, 2012, and we are still here.

Why do people make doomsday predictions based on astronomical phenomena?

Doomsday predictions, in which the world ends in some sort of giant catastrophe, have been made periodically since ancient times. Since the universe encompasses Earth, it’s easier to imagine that such a destructive event would come from the cosmos rather than from something purely human in origin. So many such predictions have come and gone over the years that imaginative, unscientific scenarios—many of which have inspired books and movies as well as scientific-sounding rumors—have no real credibility in the modern world. Astronomers, by scientifically studying the universe and how it works, can provide testable, legitimate ideas of any real cosmic threats to Earth.

An ancient astronomical dial is on display in China. Chinese astronomers were studying the night skies as early as 1500 B.C.E.

What did ancient East Asian cultures know about astronomy?

Some of the world’s earliest astronomical observations were made by the ancient Chinese. Perhaps as early as 1500 B.C.E., Chinese astronomers created the first rough charts of space. In 613 B.C.E., they described the sighting of a comet. Within a few centuries after that, Chinese astronomers were keeping track of all the eclipses, sunspots, novae, meteors, and celestial and sky phenomena they observed.

Chinese astronomers made numerous contributions to the field of astronomy. They studied, for instance, the question of Earth’s motion and created one of the earliest known calendars. By the fourth century B.C.E., Chinese astronomers had produced a number of star charts, which depicted the sky as a hemisphere—a perfectly logical strategy, since we can only see half the sky at any one time. Three centuries after that, Chinese astronomers began to regard space as an entire sphere, showing they were aware of Earth’s spherical shape, as well as of Earth’s rotation around its polar axis. They created an early map of the celestial sphere on which they placed stars in relation to the Sun and to the North Star.

Chinese astronomers were the first to observe the Sun; they protected their eyes by looking through tinted crystal or jade. The Sung Dynasty, which began in 960 C.E., was a period of great astronomical study and discovery in China. Around this time, the first astronomical clock was built and mathematics was introduced into Chinese astronomy.

What did ancient African cultures know about astronomy?

The ancient Egyptians built their pyramids and other great monuments with a clear understanding of the rhythms of rising and setting celestial objects. The Egyptians established the 365-day solar year calendar as early as 3000 B.C.E. They established the twenty-four-hour day, based on nightly observations of a series of thirty-six stars (called decan stars). At midsummer, when twelve decans were visible, the night sky was divided into twelve equal parts—the equivalent to hours on modern clocks. The brightest star in the night sky, Sirius the Dog Star, rose at the same time as the Sun during the Egyptian midsummer; this is the origin of the term dog days of summer.

What did other ancient cultures around the world know about astronomy?

A knowledge of the night sky seems to be a common thread among all the major cultures and societies of the ancient world. Polynesian cultures, for instance, used the Pleiades (the cluster of stars also known as The Seven Sisters) to navigate around the Pacific Ocean. Australian aboriginal cultures, south Asian cultures, Inuit cultures, and northern European cultures all had their own sets of myths and legends about the motions of the Sun and the Moon, as well as their own maps of the stars and of constellations.

What contributions did ancient Greek astronomers make to the science of astronomy?

The contributions of ancient Greek astronomers are numerous. Many of them were also pioneers in mathematics and the origins of scientific inquiry. Some notable examples include Eratosthenes (c. 275–195 B.C.E.), who first made a mathematical measurement of the size of Earth; Aristarchus (c. 310–230 B.C.E.), who first hypothesized that Earth moved around the Sun; Hipparchus (c. 190–120 B.C.E.), who made accurate star charts and calculated the geometry of the sky; and Ptolemy (c. 85–165 C.E.), whose model of the solar system dominated the thinking of Western civilization for more than a thousand years.

What is Stonehenge?

Stonehenge is one of the world’s most famous ancient astronomical sites. This assembly of boulders, pits, and ditches is located in southwestern England, about eight miles (thirteen kilometers) from the town of Salisbury. Stonehenge was built and rebuilt during a period from about 3100 B.C.E. to 1100 B.C.E. by ancient Welsh and British nature-worshipping priests called druids.

Archaeologists think Stonehenge had astronomical significance. It was certainly built with astronomical phenomena in mind. One pillar, called the Heel Stone, appears to be near the spot where sunlight first strikes on the summer solstice. Thus, Stonehenge may have served as a sort of calendar. Other evidence suggests that Stonehenge may have been used as a predictor of lunar eclipses.

What is the Ptolemaic model of the solar system?

About 140 C.E. the ancient Greek astronomer Claudius Ptolemy, who lived and worked in Alexandria, Egypt, published a thirteen-volume treatise on mathematics and astronomy called Megale mathmatike systaxis (The Great Mathematical Compilation), which is better known today as The Almagest. In this work, Ptolemy built upon—and in some cases, probably reprised—the work of many predecessors, such as Euclid, Aristotle, and Hipparchus. He described a model of the cosmos, including the solar system, that became the astronomical dogma in Western civilization for more than one thousand years.

According to the Ptolemaic model, Earth stands at the center of the universe, and is orbited by the Moon, the Sun, Mercury, Venus, Mars, Jupiter, and Saturn. The stars in the sky are all positioned on a celestial sphere surrounding these other objects at a fixed distance from Earth. The planets follow circular orbits, with extra additions on their orbital paths known as epicycles, which explain their occasional retrograde motion through the sky. Ptolemy also cataloged more than one thousand stars in the night sky. Although the Ptolemaic model of the solar system was proven wrong by Galileo, Kepler, Newton, and other great scientists starting in the seventeenth century, it was very important for the development of astronomy as a modern science.

What happened to astronomy after the fall of the Roman Empire?

During the Middle Ages in Europe, the study of astronomy continued to progress, though slowly. The Arabic cultures of western Asia, on the other hand, made many advances in both astronomy and mathematics for many centuries. This remained the case until the European Renaissance. Meanwhile, astronomers in China and Japan continued their work completely unaffected by events in the Roman world.

England’s ancient Stonehenge may have served as a type of astronomical calendar used by the druids.

MEDIEVAL AND

RENAISSANCE ASTRONOMY

What influence did the Catholic Church have on astronomy in medieval Europe?

Most historians agree that the immense power of the Catholic Church during the Middle Ages stifled astronomical study in Europe during that time. One tenet of Catholic dogma stated that space was eternal and unchanging; so, for example, when people observed a supernova in 1054 C.E., its occurrence was recorded in other parts of the world but not in Europe. Another part of Church dogma erroneously declared that the Sun, Moon, and planets moved around Earth. By the 1500s, a thousand years after the fall of Rome, the Catholic Church finally began to contribute again to the science of astronomy, such as with the development of an accurate calendar.

Who first began the challenge to the geocentric model of the solar system?

Polish mathematician and astronomer Nicholas Copernicus (1473–1543; in Polish, Mikolaj Kopernik) suggested in 1507 that the Sun was at the center of the solar system, not Earth. His heliocentric model had been proposed by the ancient Greek astronomer Aristarchus around 260 B.C.E., but this theory did not survive past ancient times. Copernicus, therefore, was the first European after Roman times to challenge the geocentric model.

How did Copernicus present the heliocentric model of the solar system?

Copernicus wrote his ideas in De Revolutionibus Orbium Coelestium, which was published just before his death in 1543. In this work, Copernicus presented a heliocentric model of the solar system in which Mercury, Venus, Earth, Mars, Jupiter, and Saturn moved around the Sun in concentric circles.

Nicholas Copernicus

How did the heliocentric model of the solar system advance after the death of Copernicus?

Regrettably, De Revolutionibus Orbium Coelestium was placed on the Catholic Church’s list of banned books in 1616, where it remained until 1835. Before it was banned, word of the heliocentric model nonetheless spread among astronomers and scholars. Eventually, Galileo Galilei (1564–1642) used astronomical observations to prove that the heliocentric model was the correct model of the solar system; Johannes Kepler (1571–1630) formulated the laws of planetary motion that described the behavior of planets in the heliocentric model; and Isaac Newton (1642–1727) formulated the Laws of Motion and the Law of Universal Gravitation, which explained why the heliocentric model works.

Who was Galileo Galilei?

Italian scholar Galileo Galilei (1564–1642) is considered by many historians to be the first modern scientist. One of the last great Italian Renaissance men, Galileo was born in Florence and spent a good deal of his professional life there and in nearby Padua. He explored the natural world through observations and experiments, wrote eloquently about science and numerous other philosophical topics, and rebelled against an established authority structure that did not wish to acknowledge the implications of his discoveries. Galileo’s work paved the way for the study and discovery of the laws of nature and theories of science.

What happened between Galileo and the Catholic Church?

Galileo’s support of the heliocentric model was considered a heretical viewpoint in Italy at the time. The Catholic Church, through its Inquisition, threatened to torture or even kill him if he did not recant his writings. Ultimately, Galileo did recant his discoveries and lived under house arrest for the last decade of his life. It is said that, in a private moment after his public recantation, he stamped his foot on the ground and said, Eppe si muove (Nevertheless, it moves).

Galileo Galilei

How did Galileo contribute to our understanding of the universe?

Galileo was the first person to use a telescope to study space. Even though his telescope was weak by modern standards, he was able to observe amazing cosmic sights, including the phases of Venus, mountains on the Moon, stars in the Milky Way, and four moons orbiting Jupiter. In 1609 he published his discoveries in The Starry Messenger, which created a tremendous stir of excitement and controversy.

Galileo’s observations and experiments of terrestrial phenomena were equally important in changing human understanding of the physical laws of the cosmos. According to one famous story, he dropped metal balls of two different masses from the top of the Leaning Tower of Pisa. They landed on the ground at the same time, showing that an object’s mass has no effect on its speed as it falls to Earth. Through his works A Dialogue Concerning the Two Chief World Systems and Discourse on Two New Sciences, Galileo described the basics of how objects move both on Earth and in the heavens. These works led to the origins of physics, as articulated by Isaac Newton and others who followed him.

Who was Tycho Brahe?

Tycho Brahe (1546–1601), despite being a Danish nobleman, turned to astronomy rather than politics. Granted the island of Hven in 1576 by King Frederick II, he established Uraniborg, an observatory containing large, accurate instruments. At the time, Uraniborg was the most technologically advanced facility of its type ever built. Brahe’s measurements of planetary motions, therefore, were more precise than any that had been previously obtained. This facility and these measurements helped Brahe’s protégé, Johannes Kepler, determine the elliptical nature of the motion of planets around the Sun.

Who was Johannes Kepler?

German astronomer Johannes Kepler (1571–1630) was very interested in the mathematical and mystical relationships between objects in the solar system and geometric forms such as spheres and cubes. In 1596, before working as an astronomer, Kepler published Mysterium Cosmographicum, which explored some of these ideas. Later, working with Danish astronomer Tycho Brahe and his data, Kepler helped establish the basic rules describing the motions of objects moving around the Sun.

A diagram by Johannes Kepler from his 1609 work Astronomia nova, depicting Mars orbiting the sun, to illustrate two of his laws of planetary motion.

How did Johannes Kepler contribute to our understanding of the universe?

Kepler worked with Tycho Brahe until Brahe’s death in 1601. He succeeded Brahe as the official imperial mathematician to the Holy Roman Emperor. This position gave him access to all of Brahe’s data, including his detailed observations of Mars. He used that data to fit the orbital path of Mars using an ellipse rather than a circle. In 1604, he observed and studied a supernova, which he thought was a new star. At its peak, the supernova was nearly as bright as the planet Venus; today, it is known as Kepler’s supernova. Using a telescope he constructed, he verified Galileo’s discovery of Jupiter’s moons, calling them satellites. Later in his career, Kepler published a book on comets and a catalog of the motions of the planets, called The Rudolphine Tables, that was used by astronomers throughout the following century. Kepler is perhaps most famous for developing his three laws of planetary motion.

What is Kepler’s First Law of planetary motion?

According to Kepler’s First Law, planets, comets, and other solar system objects travel on an elliptical path with the Sun at one focus point. The effect can be subtle or profound; Earth’s orbit, for example, is very nearly circular, whereas the orbit of Pluto is noticeably oblong, and the orbits of most comets are highly elongated.

What is Kepler’s Second Law of planetary motion?

According to Kepler’s Second Law, planetary orbits sweep out equal times in equal areas. This means that a planet will move faster when it is closer to the Sun and slower when it is farther away. Future scientists, such as Isaac Newton, showed that the Second Law is true because of an important property of moving systems called the conservation of angular momentum.

What is Kepler’s Third Law of planetary motion?

According to Kepler’s Third Law, the cube of the orbital distance between a planet and the Sun is directly proportional to the square of the planet’s orbital period. Kepler discovered this law in 1619, ten years after the publication of his first two laws of planetary motion. It is possible to use this third law to calculate the distance between the Sun and any planet, comet, or asteroid in the solar system, just by measuring the object’s orbital period.

Christiaan Huygens

Who was Christiaan Huygens?

Dutch astronomer, physicist, and mathematician Christiaan Huygens (1629–1695) is one of the most important figures in the history of science. He was a key transitional scientist between Galileo and Newton. His work was crucial to the development of the modern sciences of mechanics, physics, and astronomy. Huygens helped develop the Law of Conservation of Momentum, invented the pendulum clock, and was the first person to describe a wave theory of light. He designed and built the clearest lenses and most powerful telescopes of his time. Using these tools, he was the first person to identify Saturn’s ring system, and he discovered Saturn’s largest moon, Titan.

Who was Isaac Newton?

English mathematician, physicist, and astronomer Isaac Newton (1642–1727) is considered one of the greatest geniuses who ever lived. He had to leave Cambridge University in 1665 and work on his family farm when the university was closed owing to an outbreak of bubonic plague. During the next two years, he made a series of remarkable advances in mathematics and science, including the calculus and his laws of motion and universal gravitation. Newton returned to Cambridge University in 1667, and eventually assumed the position of the Lucasian Professorship of Mathematics. While there, he made fundamental discoveries about optics, invented a new kind of telescope, and published his greatest work, Principia, in 1687, with the encouragement and financial backing of his acquaintance, the astronomer Edmund Halley.

In his later career, Newton won a seat in the British Parliament and was appointed Master of the Royal Mint. He invented the idea of putting ridges around the edges of coins so people could not shave the coins and keep the precious metal shavings for themselves. The Queen of England knighted him in 1705, the first scientist to be given such an honor. He was also elected head of the Royal Society, the most significant academic body in the world at that time. Sir Isaac Newton died on March 31, 1727, in London, England.

How did Newton contribute to our understanding of the universe?

In his work Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), or just Principia, Newton articulated the law of universal gravitation and his three laws of motion. He also described, in other works, major advances in many areas of knowledge. In optics, he showed that sunlight is really a combination of many colors; in mathematics, he developed new methods that form much of the modern foundation of mathematics, including the calculus, which was also developed by German philosopher and mathematician Gottfried Wilhelm von Leibniz. In cosmology, he supplied a theoretical framework that modern astronomers used to calculate the density of an expanding universe; while in astronomy, he invented a kind of telescope that uses mirrors rather than lenses. It is the basis of all major astronomical research telescopes built today.

Isaac Newton (inset) and an illustration he drew of the telescope he invented.

What is Newton’s First Law of Motion?

According to Newton’s First Law, Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it. This is also known as the law of inertia: it simply means that an object tends to stay still, or stay in motion in a straight line, unless it is pushed or pulled. This law is an expression in words of a fundamental property of motion called the conservation of linear momentum. Mathematically, the momentum of an object is its mass multiplied by its velocity.

Why are Newton’s Laws of Motion important?

With Principia, and the theories he described in it, Newton radically changed our understanding of the universe and the interconnectedness of its components. After Newton’s Laws of Motion were accepted, it was clear that the motion of objects in space followed the same natural rules as the motion of objects on Earth. This realization altered the fundamental relationship that humans felt with the sky and space. Things in space could now be studied and interpreted as objects, rather than as unknowable gods or supernatural entities. This helped lead to the entire enterprise of scientific research today.

What is Newton’s Second Law of Motion?

According to Newton’s Second Law, The change of motion is proportional to the motive force impressed and is made in the direction of the right line in which that force is impressed. This is also known as the law of force, and it defines force as the change in the amount of motion, or momentum, of an object. Mathematically, the force of an object is its mass multiplied by its acceleration.

What is Newton’s Third Law of Motion?

According to Newton’s Third Law, To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal and directed to contrary parts. That means that to exert a force on an object the thing doing the exerting must experience a force of equal strength in exactly the opposite direction. This law explains, for example, why an ice skater goes backward when she pushes another skater forward.

What is Newton’s Law of Universal Gravitation?

According to Newton’s Law of Universal Gravitation, every object in the universe exerts a pulling force on every other object; this force between any two objects is directly proportional to the product of their masses, and it is inversely proportional to the square of the distance between the two objects. In other words, gravity follows what is known as the inverse square law: a mathematical relationship that governs both the strength of gravity and the propagation of light in space.

What was the importance of Newton’s Law of Universal Gravitation to astronomy?

Newton’s Law of Universal Gravitation shows that the objects in the solar system move according to a mathematically predictable set of rules. It shows scientifically why Kepler’s three laws of orbital motion are true, and it allows astronomers to predict the locations and motions of celestial objects. When Edmund Halley, for example, used the law to predict the seventy-six-year orbital period of a well-known comet—a prediction confirmed after Halley’s death—it marked a milestone in astronomy: the final transformation from superstition and ignorance to science and knowledge.

EIGHTEENTH-AND

NINETEENTH-CENTURY ADVANCES

What significant scientific advances occurred in the 1700s that most advanced astronomy?

In the 1700s, the study of mathematics beyond the calculus first established by Leibniz and Newton led to the development of the branch of physics called mechanics. Scientists began to understand the nature of electricity through experiments in laboratories and with lightning. Opticians began to develop telescopes that could let astronomers observe objects invisible to the unaided eye. And using those telescopes, astronomers began to take systematic surveys of the sky, making detailed sky catalogs.

Who was Pierre-Simon de Laplace and what did he contribute to mechanics?

French mathematician and astronomer Pierre-Simon de Laplace (1749–1827) made a number of key contributions to mathematics, astronomy, and other sciences. Together with chemist Antoine-Laurent Lavoisier, Laplace helped develop our understanding of the interrelationship of chemical reactions and heat. In physics, Laplace applied the calculus, recently invented by Isaac Newton and Gottfried Wilhelm von Leibniz, to calculate the forces acting between particles of matter, light, heat, and electricity. Laplace and his colleagues created systems of equations that explained the refraction of light, the conduction of heat, the flexibility of solid objects, and the distribution of electricity on conductors.

In astronomy, Laplace was primarily interested in the movements of the objects in the solar system and their complex gravitational interactions. He published his results over many years in a multi-volume book called Traite de Mechanique Celeste (Celestial Mechanics). The first volume of Celestial Mechanics was published in 1799. Laplace also developed a nebular theory of the formation of the Sun and our solar system, and, along with his colleague John Michel, he introduced the idea of a dark star, which later came to be called a black hole. Because of his brilliance, and because his work expanded on the gravitational theories of Isaac Newton, Laplace earned the nickname The French Newton.

Pierre-Simon de Laplace

Who was Joseph-Louis Lagrange and what did he contribute to mechanics?

Joseph-Louis Lagrange (1736–1813) was an Italian mathematician who developed some of the most important theories of mechanics, regarding both Earth and the universe. Generally remembered as a French scientist because he spent the last part of his career in Paris, his analysis of the wobble of the Moon about its axis of rotation won him an award from the Paris Academy of Sciences in 1764. Lagrange also worked on an overall description of the way that forces act on groups of moving and stationary objects, a project that Galileo Galilei and Isaac Newton had begun years before. He eventually succeeded in devising several key general mathematical tools to analyze such forces. These were published in a 1788 work called Mechanique Analytique (Analytical Mechanics). Lagrange went on to explore the interaction between objects in the solar system as a complex system of objects. He discovered what are called Lagrange points: places around and between two gravitationally bound bodies where a third object could stay stationary relative to the other two. This proves useful today for placing satellites in space.

In 1793, Lagrange was appointed to a commission on weights and measures, and helped create the modern metric system. He spent his final working years trying to develop new mathematical systems of calculus.

Who was Leonhard Euler and what did he contribute to mechanics?

The Swiss mathematician Leonhard Euler (1707–1783) was probably the most prolific mathematician in recorded history. He helped unify the systems of calculus first created independently by Leibniz and Newton. He made key contributions to geometry, number theory, real and complex analysis, and many other areas of mathematics. In 1736, Euler published a major work in mechanics, appropriately called Mechanica, which introduced methods of mathematical analysis to solve complex problems. Later, he published another work on hydrostatics and rigid bodies, and he did tremendous work on celestial mechanics and the mechanics of fluids. He even published a 775-page work just on the motion of the Moon.

Who was Adrien-Marie Legendre and what did he contribute to mechanics?

The French mathematician Adrien-Marie Legendre (1752–1833) taught at the French military academy with Pierre-Simon de Laplace, starting in 1775. In 1782 he won a prize for the best research project on the speed, path, and flight dynamics of cannon-balls moving through the air. Elected to the French Academy of Sciences the next year, he combined his research on abstract mathematics with important work on celestial mechanics. In 1794, Legendre wrote a geometry textbook that was the definitive work in the field for nearly a century. In 1806, he published Nouvelles methods pour la determination des orbits des cometes (New Methods for the Determination of the Orbits of Comets). Here he introduced a technique for finding the equation of a mathematical curve using imperfect data. Legendre is best known today for his work on elliptical functions and for inventing a class of functions called Legendre polynomials, which are valuable tools for studying harmonic vibrations and for finding mathematical curves that fit large series of data points.

Who created the Messier Catalog?

French astronomer Charles Messier (1730–1817) was a famed discoverer of comets. Discovering comets with a telescope was a very difficult task at the time, and successes