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all about astronomy
Click on any of the terms below to get a full explanation of that topic.
•  What is Astronomy?
•  The Celestial Sphere
•  Right-Hand Rule
•  Other Stuff You Need to Know
•  The History of Astronomy
•  Ptolemy
•  Retrograde Motion
•  Discoveries by other
Greek Astronomers
•  Before Copernicus
•  The Copernican Revolution
•  Tycho Brahe: Astronomical
Jerk
•  Johannes Kepler
•  Kepler's Laws
•  What's a Focus?
•  Huh?
•  What's a Period?
•  Then what's a?
•  Example
•  Galileo
•  Newton
•  Newton's Universal Law of Gravitation
•  Newton's Laws of Motion
•  Spectrum of Visible Light
•  Conclusion

Intro

Hi. I'm the voice in your head you hear when you read. How's everything going?

This is the Standard Deviants' guide to astronomy. It covers everything in the episode of Standard Deviants Television: Astronomy, and a little bit extra.

Here's what we'll cover. We'll focus on the history of astronomy, starting with the ancient Greeks, and get the scoop on the geocentric (Earth-centered) model of the universe.

Then we'll follow the advances in astronomy that led to the heliocentric, or sun-centered, model of the universe, and meet some nifty astronomical thinkers like Copernicus, Galileo, and Sir Isaac Newton.

Along the way, we'll learn about Kepler's laws of planetary motion and Newton's three laws of motion. Ready to learn? Good! Then, let's get started.

What is Astronomy?

Every one of us is an astronomer, in a sense. Every time you look up and see the stars, you're doing astronomy, just like humans have done for thousands of years.

Let's give astronomy an official definition. Astronomy is the study of everything that lies beyond the Earth's atmosphere. That's a lot of stuff.

The Celestial Sphere

Here's a little something to help us put the heavens in perspective—the celestial sphere.

When people first started looking at the sky, they imagined that all those little lights out there were hanging on the inside of a large bowl. Well, not so much a bowl, really, but a sphere that surrounded the Earth. We call that imaginary sphere the celestial sphere.

All the stars are on the surface of the imaginary celestial sphere and the Earth is inside in the center. Now, we know that all the stars and planets and everything are different distances from us, but the idea of the celestial sphere is still convenient for pinpointing things in the sky.

To use the celestial sphere, we'll pretend that everything in the sky is attached to the inside of this big imaginary sky ball that encloses the Earth, even though we know everything isn't attached to it. If we imagine that all the stars and stuff are attached to the celestial sphere, it's easier to visualize their position in the sky when we're looking up at it from the Earth.

Let's run down the different parts of the celestial sphere. Oui?

The celestial sphere has poles in the same places as the Earth. Imagine that we set up big searchlights on the north and south poles of the Earth. The north and south poles of the celestial sphere are where the searchlights hit the celestial sphere.

In between the celestial poles, right on the surface of the celestial sphere, we draw an equator, just like the Earth's equator.

Now, imagine that we draw a line from one pole of the celestial sphere to the other. It goes from the north celestial pole, through the middle of the Earth, and right down through to the south celestial pole.

The line between the north and south poles of the Earth is the axis the Earth revolves around. When we look at the stars on the celestial sphere over a period of hours, they seem to revolve around this axis.

Just a few more terms. You may have heard of these two before: zenith and nadir.

The zenith is the point on the celestial sphere that is exactly above your head. The exact opposite point, the one underneath your feet all the way through the Earth and on the other side of the celestial sphere, is called the nadir. No matter where you are on the Earth, the point above you is the zenith, and the point below you is the nadir. Pretty cool!

Okay, last term. Imagine that we draw a great circle on the celestial sphere from one pole to the other, through the zenith of our position. This line is called the meridian.

Whew. We're done.

Right-Hand Rule

Time for a new topic. Let's say you're looking at some strange planet. How do we know which way is north?

The answer? The right-hand rule.

To use the right-hand rule, point your fingers in the direction the planet rotates. Your fingers should curl around the planet, as if you're grabbing it like a ball and your fingers are pointing the exact same way the planet rotates. Now, stick out your thumb. That way is north.

I know, you're thinking this doesn't seem to be very helpful. Well, if you're lost, it really isn't. But when astronomers look at other planets, this is how they decide which way is north on those strange and different planets.

Okay, so now you know how to deal with the sky and the planets. What else should you know?

Other Stuff You Need to Know

Let's have a look at the first two terms you'll need to know: rotation and revolution.

Rotational period, for a planet or moon, is how long the planet or moon takes to rotate 360 degrees on its axis, once around. The Earth, for example, completes one rotation every 24 hours.

One revolution, for a planet or moon, is one cycle around whatever it orbits. You know, the moon orbits the Earth, the Earth orbits the sun.

One revolution for the Earth is the 365 days it takes to orbit the sun.

A revolution is also sometimes called an "orbital period" or a "revolutional period." So remember, when you hear revolution or orbital period, it means the same as revolutional period.

Quick review!

Rotation = Planet rotates on its axis.
Revolution = Planet revolves around something else.

Sometimes you might hear the term ions. Ions are atoms that have lost or gained electrons. Electrons are negatively charged particles. If an atom loses an electron, it has a positive charge, and is called a positive ion. If an atom gains an electron, it picks up a negative charge, and is called a negative ion.

We should touch on temperature too. Usually, we'll talk about temperature in terms of degrees Fahrenheit since that's what you're probably familiar with.

Sometimes we'll talk about temperatures in degrees centigrade, or Celsius. Oh, stop cheering, you Canadians. To get you to think in Celsius terms, water freezes at zero degrees. A really hot day is about 30 degrees. And water boils at 100 degrees Celsius.

There's one more temperature measure we'll use, which is called Kelvin. The Kelvin scale doesn't have degrees, its units are called Kelvins.

One Kelvin is the same size as one degree Celsius, but the Kelvin scale starts numbering at absolute zero, which is the coldest anything can ever be. So in the Kelvin scale, absolute zero is zero degrees Kelvin, water freezes at 273 degrees Kelvin, and water boils at 373 degrees Kelvin.

Enough terms. Let's move on to the history of Astronomy.

The History of Astronomy

So where did all this thinking about astronomy begin? Let's find out.

In the Western world, the science of astronomy began with the Greeks. Around 600 B.C., guys like Thales and Democritus wondered what made up the world, and how far away the stars were. Other people probably thought about it before them, but these smart-guys were the first to write it all down.

Later, Pythagoras (these guys only have one name) showed up and started thinking about astronomy stuff. He and his followers believed several things, including that the Earth is round.

They decided the Earth is round because the phases of the moon seemed to imply the moon is round. To them, the moon looked like a spherical object that had light shining on it from different angles. Guess what? It pretty much is.

Unfortunately, some of the Pythagoreans also believed that the Earth was the center of everything. Why unfortunately? Because they were wrong! We call this concept of a universe a geocentric universe. Remember: geocentric universe = Earth is the center of everything.

[Actually, some of the Pythagoreans believed that Earth moved in small circles near the center of the universe. This is really splitting hairs, though. They essentially believed the Earth was the center of everything.]

The Pythagoreans also believed that everything in the sky was fixed on big celestial spheres. The celestial spheres for the Pythagoreans were different than the one we use now.

For the Pythagoreans, the celestial spheres were big glassy circles that actually held the stars and planets in the sky. They said there was one celestial sphere for all the stars, one celestial sphere for each planet, and the sun and our moon had their own celestial sphere, too. The Pythagoreans believed that if you listened really closely, you could hear the spheres rubbing together. Silly Pythagoreans.

The next big dude in Greek philosophy was Plato. Plato thought about a lot of stuff and was very influential, so his thought on science and astronomy stuck around for awhile. Plato thought everything in the world was made up of a mix of four elements: earth, air, fire, and water.

Everything but the heavens, that is. The stars and planets were made of a godly stuff called quintessence. Since they were made of god stuff, Plato reasoned, the planets and stars must move in god paths. What shapes are god paths? Circles.

Plato reasoned that the planets, the sun, and the stars moved in circular paths around the Earth. Circular paths seemed to make sense to everyone, and the idea that objects make circles in their movement dominated for quite a while.

After Plato was Aristotle. Aristotle was Plato's student and probably the most influential person in ancient astronomy.

Aristotle believed that astronomical objects moved in circular paths, and that the Earth was round. Aristotle figured the Earth was round because during a lunar eclipse, the Earth cast a round shadow on the moon. Pretty clever, huh?

But Aristotle also believed the Earth was the center of the universe. Aristotle rejected the Pythagorean idea that the Earth moved. Aristotle said the Earth was immobile and the heavens revolved around it. Why would he think that?

Aristotle reasoned that if the Earth did indeed move around the sun, we should be able to notice a stellar parallax. What's stellar parallax?

Stellar parallax has to do with how we see nearby stars as the Earth shifts in position going around the sun. In fact, it is the angle formed by our lines of sight to a star when it is looked at from two different positions of the Earth.

You can see it first hand for yourself. Hold your finger out and look at it from one eye, then the other. See how the image shifts depending on your point of view? That's parallax.

Aristotle figured that if the Earth moved, we should detect a shift in position between nearby stars and far away ones. But when he tried to detect parallax with the stars, he couldn't.

The only reason that there wouldn't be parallax was if the Earth weren't moving or if the stars were so far away you couldn't see their parallax. So, Aristotle reasoned that the Earth didn't move. To Aristotle, it just didn't seem plausible that the stars were so far away that you couldn't see parallax for them.

Aristotle said the Earth was the center of the universe and everyone believed him. After a while, the Catholic Church even decided that it was a sin to believe otherwise.

Ptolemy

After Aristotle, people continued to believe that things moved in circles. Ptolemy tried to incorporate the idea of a geocentric universe with the planets moving in circular paths around the Earth.

Ptolemy's model was really complicated. A rough summary is that the planets would move along circular orbits around the Earth. While traveling on the orbit, they would also spiral along it in smaller circles. This model tried to explain the movement of all the planets, including (vocabulary term alert!) retrograde motion.

Retrograde Motion

What's retrograde motion? If we look at where a planet is in the sky every night, it tracks a slow steady line across the sky. But not always! Sometimes, when we're looking at a planet that is farther away from the sun than we are, its position in the sky seems to jump backwards. That backwards (westward) motion is retrograde motion.

Retrograde motion occurs because we're moving at different rates than the other planets. Sometimes, the planets appear to move backwards in the sky. This happens because the Earth is moving faster than they are, kind of like we're lapping them.

This example may help you picture retrograde motion. Imagine you're in a car going around the inside lane of a racetrack. Every five seconds, you look up to see where the car in the outside lane is.

  • The first time you look, the car is ahead of you.
  • The next time you look, the outside car is going around the turn, so it seems to have moved ahead to the left.
  • But now, the next time you look, you've gone into the turn, too, and since you're on the inside lane you make the turn faster, and you catch up to the outside car. When you look out the window, the outside car is right next to you. If you didn't know any better, you'd think the outside car moved backward.

That's how retrograde motion works. It's when something seems to move backwards because you're catching up to it and moving past it.

Discoveries by other Greek Astronomers

Aristarchus believed in a heliocentric universe, meaning that the sun was the center of the universe. But he couldn't prove it.

Eratosthenes, using trigonometry and careful observation, measured the circumference of the Earth within a few percentage points of its actual size.

Hipparchus discovered something called the "procession of the equinoxes." This refers to the fact that the earth wobbles like a top as it revolves. The wobble makes the north pole point at different places. The wobble is really slow, however. The north pole makes a full revolution and points back at the same spot every 26,000 years.

The wobble of the Earth causes the identity of the North Star to change. The North Star is the star that the north pole is pointing at. Right now the north pole points pretty close to Polaris, which is our North Star. The ancient Egyptians had a different North Star because the pole was pointing at a different place. The Egyptians' North Star was Thuban.

Before Copernicus

When the Roman Empire fell, Europe went into the Dark Ages. People in Europe were too busy warring on each other and getting the plague to do any science. The rest of the world kept working on astronomy.

Arabian astronomers in particular provided a vital link between ancient and modern astronomy. Many trigonomic formulas, and even the names of the stars, come from the work of Medieval Arabian astronomers. Renaissance scholars used the work of Arabian astronomers when they began working on astronomy again in the 1500s and 1600s.

The Copernican Revolution

The next great astronomer on our list is a guy named Nicolas Copernicus. Let's set the scene…

In 1250 AD, the king of Spain decided he wanted to know where all the stars and planets were in the sky, and where they would show up in the sky every year. So, he commissioned a group of astronomers to compile a star almanac. The Spanish astronomers used Ptolemy's model of the universe to compile their observations and make their predictions. (Ptolemy's model was the really complicated one that used planets rotating in smaller circles on bigger circles.)

The astronomers' star almanac failed to predict anything about the sky with any accuracy whatsoever. Except, of course, that the sky would be dark during the night.

Ptolemy's model was horribly complicated and hardly worked. Everybody knew that, but they tried to ignore it because no one had any better ideas.

In 1473, Nicolas Copernicus, a clergyman in a Roman Catholic cathedral, wrote a brief statement. Essentially, it said we live in a heliocentric solar system and that the sun is the center of everything. Now, if you remember, the Catholic Church said the Earth was the center of the universe.

Despite this, the church didn't seem to care that Copernicus was announcing his belief in a heliocentric solar system. Mostly, it was because his statement was poorly written and not many people paid attention to it. Besides, Copernicus was working on calendar reform, which the church really wanted done, so they figured they'd leave him alone until he was done with the calendar.

So Copernicus wrote a book about his ideas. In the book, he continued to argue that the Earth revolved around the sun. God would surely find a simple heliocentric universe more pleasing than the complex Ptolemaic model, he wrote. Besides, Copernicus argued, the Pythagorean model, with all the rotating celestial spheres, wasn't logical either. What makes more sense, that the Earth revolves around the sun, or that giant celestial spheres rotate around the Earth at astronomical speeds without breaking apart?

Enemies of Copernicus criticized his book. He didn't solve the parallax mystery. He couldn't explain why the Earth orbits the sun, and his model couldn't provide accurate data. And the church argued that the celestial spheres couldn't break apart, because the quintessence they were made of weighed nothing, and could therefore spin as fast as necessary.

Copernicus didn't get much credit while he was alive, but his idea started to catch on.

Tycho Brahe: Astronomical Jerk

The next important figure in astronomy was Tycho Brahe. Tycho was a rude, arrogant jerk. He was also a horrible swordfighter. This pair of traits cost Tycho his nose. Some guy cut it off Tycho's face during a duel. Youch!

Anyhow, Tycho was friends with the king of Denmark. The king gave Tycho a big observatory, which Tycho used to make very precise measurements of the stars and planets.

Tycho was the first to realize that, contrary to the teachings of the church, the sky did change and evolve. The church said the sky didn't change because God made the sky, and since God was perfect, He made it right the first time. Therefore, the sky did not change. Tycho realized the sky did change when he discovered a comet in 1572.

Tycho was good at measuring the paths of stars, but he was about as good at math as he was at swordfighting. So he hired a young mathematician, named Johannes Kepler, to help him with his studies.

Johannes Kepler

Kepler was set to work on calculating the orbit of Mars, which took a really long time. While Kepler was working on those calculations, Tycho died, and Kepler kept all of his data and equipment.

When he was alive, Tycho took accurate measurements of all the stars and planets. When Kepler got all the data, it was up to him to do the math to bring things together.

Kepler's Laws

Now, Kepler was a pretty smart guy in his own right. So smart, in fact, that he came up with three laws of planetary motion. Check this out.

Kepler lived from 1571 to 1630. In this time, he used Tycho's data to formulate three descriptive laws of motion. He didn't explain why things worked, just how.

Kepler based his studies on Copernicus' theories. But Kepler couldn't make Tycho's data match up with Copernicus' system of circular orbits. Finally, to make the math fit Tycho's observations, Kepler realized that he would have to change the Copernican model. Kepler was able to keep the sun as the center of the solar system, but to change the Copernican model enough to make the math work, he discovered that the orbits of the planets aren't circles at all. Planets orbit the sun in ellipses.

With this knowledge, Kepler discovered three laws of planetary motion.

Law One
The orbits of planets are ellipses, with the sun at one focus.

What's a Focus?

Try this activity. Put down two thumbtacks. Encircle the two thumbtacks and a pencil with a loop of string. Pull the string tight with the pencil, and draw a line around the thumbtacks. There's your ellipse. Each thumbtack is one focus of your ellipse. With the orbits of planets, the sun is at the center of one of those foci.

To sum up: the orbits of planets are ellipses, with the sun at one focus of the ellipse.

Law Two, the Law of Areas
An imaginary line from a planet to the sun will sweep over equal areas of the ellipse in equal intervals of time.

Huh?

That's kind of a confusing statement. What it really means, is that the planet moves faster in its orbit when it is closer to the sun. Kepler's law just explains things mathematically.

Kepler discovered an inverse relationship between how far a planet is from the sun and how fast a planet is traveling. As the planet's distance from the sun decreases, its speed increases and vice versa.

If a planet traveled in a perfectly circular orbit, the planet would always be the same distance from the sun and would always travel at the same speed. In an ellipse, though, the planet's distance from the sun varies, and, Kepler figured, so does its speed.

Just remember Kepler's second law in terms of this: a planet moves faster in its orbit when it's closer to the sun. When the planet is farther from the sun, it moves more slowly.

If you're wondering why all this happens, then remember that Kepler didn't know either. He just knew that it did happen because of his studies of the orbits of Mars and the other planets. We'll get to how it works when we learn about Newton.

Law Three
A planet's period squared is proportional to a cubed.

Formula: P2 is proportional to a3

What's a Period?

A period is the amount of time, in Earth years, that a planet takes to orbit the sun once. The period is measured in Earth years, not days, so the Earth's period is one year and not 365 days.

Then what's a?

A is the length of the semi-major axis of a planet's orbit. The semi-major axis is half the length of the longest diameter of the orbit, which is an ellipse. How's that work? Well, if we draw the longest possible line from one end of an ellipse to the other end, we have the major axis. If we cut that in half, we have the semi-major axis.

The only catch here is that you have to measure the semi-major axis in a unit called AU's. AU stands for "astronomical unit." One AU is equal to the length of the Earth's semi-major axis. We also use the semi-major axis to measure the Earth's distance from the sun.

The long and the short of Kepler's third law is that a planet's distance, in AU's, raised to the third power, is equal to the time it takes that planet to orbit the sun, squared.

How is that useful? Well, slap our pappy happy, we're gonna tell you. If you know the distance a planet is from the sun, you can figure out its period of orbit. Or, if you know its period of orbit, you can figure out its distance from the sun. If you know one, you can figure out the other.

Kepler's third law has come in especially handy, because it's easier to use Kepler's law and do a little math than to actually measure the distance between the sun and all the planets. I mean, think of what kind of measuring tape you'd need.

Example

Let's try a problem. Mars takes 1.9 years to complete one orbit of the sun. So the period of Mars's orbit is 1.9. How far away from the sun is Mars?

Using Kepler's third law, the distance cubed (a3) is equal to the period squared (1.92) squared. That gives us a3 = 3.61. Now we take the cubed root of both sides of the equation so we can find "a" To do that, you'll probably need a calculator.

The cubed root is just the opposite of raising something to the third power, so we just have to tell the calculator that. On most calculators, that's accomplished by hitting the inverse button, followed by the y to the x button to get the exponent, and then 3, for the third power. So, punch the following sequence into your calculator:

3.61; inverse; yx; 3.

The calculator should tell you "1.53 something," which we'll just round to 1.5. If we look in an astronomy book, we'll see the distance from Mars to the sun is 1.5 AUs. Just like Kepler said, the distance cubed is equal to the period squared.

Galileo

Next up on our tour of the history of astronomy is Galileo.

Galileo Galilei was the first scientist to actually experiment. For Aristotle, it was good enough to reason that heavy objects fell faster simply because they were heavy. Galileo went to the tower of Pisa and chucked things over the side. He wanted to see things for himself.

Galileo had heard about a Dutchman named Hans Lipperchet, who had invented a spy glass for looking at far off objects like boats.

Galileo instantly realized the importance of the invention. The spy glass could be used to look at the stars, the sun and the moon. So he whipped one up on his own, without ever actually seeing a spy glass. You can probably guess what Galileo called his new version of the spy glass: a telescope.

Just about everything Galileo looked at disproved something that Aristotle or Ptolemy had said. The moon was not smooth, like Aristotle claimed. It had ridges. The sun wasn't smooth either; it had blemishes and spots. Galileo observed different phases of Venus, leading him to think that the planet orbited the sun. He could see something strange about Saturn, and when Galileo observed Jupiter, he made perhaps his most startling discovery. There were moons orbiting Jupiter, like a miniature solar system!

Galileo's observations destroyed the geocentric model of the universe. The Earth was not the center of everything after all. Copernicus and Tycho and Kepler had been arguing this the whole time, but now Galileo had convincing evidence.

Galileo published explanation of how his observations supported Copernican theory in a book called Sidereus Nunces. In Galileo's time, scholars were the only people who knew Latin, and they all wrote in Latin. Galileo wrote his book in Italian, hoping that many people, and not just scholars, would be able to read his work.

The book received widespread attention and was quickly banned, and church officials criticized Galileo's work. In 1616, the church ordered Galileo to stop teaching Copernican theory.

In 1632, Galileo defied the church, and published the book Dialogue Concerning the Two Chief World Systems. The book portrayed the followers of the Ptolemaic system as dim-witted simpletons, and Galileo's own Copernican thinkers were enlightened and witty. The public loved the book, and the church hated it. The church was so angry, in fact, that they arrested the seventy-year-old Galileo, and forced him under threat of torture to denounce his own works. Galileo did, but by that time it was too late, the word was out. Scholars continued to pursue the theory of a heliocentric system.

Trivia tidbit for you: Galileo died on Christmas Day, 1642, the same day that Isaac Newton was born.

Newton

That's some coincidence, especially since Sir Isaac Newton eclipsed even Galileo with his contributions to astronomy, mathematics and science in general. Now, let's look at the life and work of Sir Isaac Newton.

Newton went to college at Cambridge, but in 1665, the school was closed to help prevent transmission of the plague. Newton spent the next year in his uncle's farmhouse, out in the middle of nowhere.

In the year and a half he spent in the farmhouse, Newton performed some of the most incredible intellectual feats of human history.

He created calculus.

He discovered the laws of motion.

He began to formulate his ideas on the universal law of gravitation.

In that same period of time, Newton also performed one of the most bone-headed feats in human history. He didn't publish his work, or tell anyone that he was doing it, and then he lost all his notes and documentation.

Twenty years later, Newton's friend Edmund Halley stopped by to visit. Halley and his buddies in London had been trying to figure out what force could possibly hold a planet in an elliptical orbit. Halley asked Newton to help them figure that out. Newton casually remarked that he had figured the problem out two decades ago!

As we're sure you can imagine, Halley was a tad bit surprised. Through several months of convincing, Halley persuaded Newton to publish his work. Newton remembered what he could and published it in a book called Principia.

Principia contained all of Newton's work, and we'll go over the major parts here. Except for the calculus, that is. (Aren't we nice?)

Newton's Universal Law of Gravitation

Newton's law of gravitation was the culmination of the work of Copernicus, Tycho, Kepler, and Galileo. It explained how the heliocentric system worked. Gravity is the force that can hold planets in elliptical orbits around the sun. Gravity is the force of attraction between two objects due to their mass.

Newton's law of gravitation states that the force of gravity between any two objects in the universe is equal to the mass of the first object (m1), multiplied by the mass of the second object (m2), multiplied by a gravitational constant (G), all divided by the square of the distance between the two objects.

Formula: Gravitational force = (m1 x m2 x G) / (distance2)

Okay, that's the textbook definition. What it means to you and me is that bigger objects have more gravity, and you feel more gravity the closer you are to an object.

Newton's Laws of Motion

Kepler's laws explain how planets move in their elliptical orbits, but Newton's laws of motion explain why the planets move the way they do. There are three laws of motion.

Law One

All objects at rest stay at rest. All objects in motion stay in motion, in a straight line and at a constant speed, unless acted upon by a force.

Law Two

Force equals mass times acceleration, or F = m x a.

A force is an action on an object that causes the object's movement to change. We measure force in newtons. One newton is the force it takes to change the motion of one kilogram of something by one meter per second every second.

Law Three

For every force one body exerts on a second, the second exerts an equal and opposite force on the first.

So how do these laws explain how the planets keep orbiting the sun?

We know that Newton's law of gravitation explains how planets maintain their orbits through the force of gravity. But, do you remember Kepler's second law? That's the law that says a planet moves faster the closer it is to the sun, and slower the farther it is away from the sun. Well, Kepler knew that, but he could never say why it happened. Newton's law of gravitation and the second law of motion explain the why for Kepler's how.

The law of gravitation says that the gravity between large bodies increases as those bodies get closer together. For example, as the sun and the Earth get closer together, the gravity (the force between them) increases. And Newton's second law of motion says that force equals mass times acceleration.

Well, the planet's mass isn't going to change, so if the force goes up, the acceleration goes up and the object goes faster. The closer a planet is to the sun, the faster that planet will travel, and vice versa. Newton explained why Kepler's equations work. Using calculus, Newton was able to derive all of Kepler's laws. And the good thing was, Newton didn't have to take any calculus courses because he had invented it.

Spectrum of Visible Light

Newton was also the first person to understand the spectrum of visible light. Newton found that if you shine a beam of light through a triangular piece of glass called a prism, the light is refracted, or bent. Some of the light gets bent more than other light, so the beam that comes out the other side is broken up into different colors, just like a rainbow.

We call that rainbow of light a spectrum. A spectrum ranges in color from red to orange to yellow to green to blue to indigo to violet.

If you have more than one spectrum, you call them spectra. Spectra is the plural of spectrum.

An easy way to remember the order of light in a spectrum is to remember the name Roy G. Biv. The letters of his name correspond to the light in the spectrum: Red, Orange, Yellow, Green, Blue, Indigo, and Violet.

Newton also did a couple of other cool things. He invented the first reflecting telescope, formulated the laws of reasoning, and probably a bunch of other stuff that he didn't tell us about.

Conclusion

Alas, our intergalatic journey into astronomy has come to an end. Thanks for reading the Standard Deviants' guide to astronomy.

Now that you've read All About Astronomy, test your knowledge with our Sample Test.

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