People have wondered about the skies for thousands of years.  Today, this wondering has a name, astronomy.  The study of space and all the objects in it is called astronomy.  It is one of the oldest sciences.  There are ancient records dating back over 2,000 years that predict eclipses of the Sun.  The planets Mercury, Venus, Mars, Jupiter, and Saturn have been known about for over 5,000 years.  Even the word planets, comes from an ancient Greek word “planetes” which means wanderer.  You see, the stars are ALWAYS in the exact same positions compared to each other, but the planets are always changing their positions compared to the other “stars”.  We get our word, planets, from the Greeks because these “stars” seemed to wander.  Today, we know that the planets are so much closer to us than the stars are and that the planets are actually orbiting the Sun, just like we are.  By the way, none of the other planets were discovered until 1781!

The ancient folks used astronomy to predict the seasons, especially the farmers.  Sailors depended on the positions of the stars to help them navigate at sea.  Have you ever heard of the North Star?

Let’s take a short historical look at astronomy over the last 2,500 years or so.  Of course all of these ancient observations were made using nothing but the human eye.  Between 600 B.C. and A.D. 140, the Chinese, Egyptian and Babylonian civilizations observed and recorded the positions of the Sun, Moon, and first five planets.  Ancient Greek astronomers attempted to explain the changing positions and motion of the planets.  They came up with a geocentric model of the solar system in which the Earth was the center of a giant sphere and the Sun, Moon, stars, and the other planets were revolving or orbiting around the Earth.  About 500 years B.C., Anaxagoras, a Greek philosopher (a dude that likes to tell people what he has been thinking about) explained that the Moon was actually reflecting Sunlight and he explained why the Moon has phases.  He also tried to explain what caused eclipses and rainbows.  In the 3rd century B.C., Aristotle figured out that the Earth was round.  In the 2nd century B.C., Aristarchus used geometry to calculate the relative distances from the Earth to the Sun and the Moon.  He was also the first to suggest that the Earth revolves around the Sun, a heliocentric model.  Unfortunately, Aristotle believed in the geocentric model and because he was more famous, most of the world believed in the geocentric model for the next 2,000 years!  About this same time, another Greek astronomer,  Hipparchus, recorded the location of 850 stars and measured the length of the year to within minutes of what we know it to be today.

Around A.D. 140, another Greek astronomer, Ptolemy, wrote a set of books that included most of the information about astronomy known at that time.  Ptolemy believed in the geocentric model and explained it in great detail.  His ideas were accepted by the church and everyone was expected to believe what the church believed.  This belief in the geocentric model lasted for 1,500 years. 

In the 1500’s, Copernicus, a Polish astronomer, decided that the heliocentric model of the solar system was correct.  He said that Aristarchus had been correct.  Copernicus also said that the Earth was rotating on it’s axis and that this would help explain the motions of the sky.  Remember, the Church believed in the geocentric model and that anyone who said anything else was a terrible sinner.  Copernicus was so afraid of being punished by the Church that he wouldn’t allow his ideas to be published in a book until he was dying.

Later in the 1500’s and early 1600’s, two other astronomers, Tycho Brahe and Johannes Kepler continued to study this idea of a heliocentric model.  Brahe discovered that the planets orbits were elliptical and Kepler came up with an explanation as to how the planets orbited the Sun.  Most people and the Church still believed in the geocentric model and it was still not a good idea to dispute the geocentric model.  In the early 1600’s, Galileo Galilei, an Italian scientist, used a telescope to prove that the heliocentric model was correct.  He was the first person to use a telescope to study the sky.  He studied the movement of moons around Jupiter and the phases of Venus as evidence that the Earth and everything else orbited the Sun.  Galileo wrote and published a book saying the geocentric model was wrong and that the heliocentric model was correct.  He was put on trial by the Church, sentenced to house arrest for the rest of his life and threatened with death if he ever discussed the heliocentric model again!

Before Galileo, philosophers and scientists would just think about problems and try to figure out solutions by thinking about them. Galileo came up with the idea of the Scientific Method. He thought about the problem, came up with a quick answer, called a hypothesis, but then he would devise an experiment or conduct research until he came up with a conclusion that would either prove or disprove his hypothesis. There are five general steps to the Scientific Method: 1. Start with a question or problem, 2. develop a hypothesis (quick answer) based on what you already know or have observed. 3. Develop and conduct an experiment or research that will help prove or disprove your hypothesis. 4. Analyze your findings or results and reach a conclusion. 5. Communicate your findings or revise your hypothesis and repeat the steps until your findings support your hypothesis.

Sir Isaac Newton, an English scientist, used the writings of Galileo, Copernicus, Brahe, and Kepler as inspiration to try to explain how the heliocentric model would work.  In the late 1600’s, Sir Isaac Newton came up with three laws of motion and the law of universal gravitation.  He explained that gravity is the force that is responsible for the orderly motion of the solar system.  He said that if the Sun had no gravity, the planets would take off into space in a straight line away from the Sun.  Newton’s law of universal gravitation says that the closer you get to the Sun, the stronger the gravity becomes and that if two objects are moved twice as far apart, the gravity between them is FOUR times less instead of two times less.  Because of this, the closer an object is to the Sun, the faster it must orbit in order to keep from being pulled into the Sun.  This is why Mercury is actually moving much faster than Pluto or even the Earth. Newton also invented a new type of telescope that he used in his studies.

The new telescopes led to the discovery of nebulae.  A nebula is a cloud of rotating dust and gases.  In the 1700’s, a German philosopher, Kant, first suggested that gravity could cause a nebula to slowly contract and condense into stars and planets.  This idea that stars and solar systems form from nebula came to be known as the nebular hypothesis.

Since the time of Newton and Galileo, telescopes have been getting better and better.  These better telescopes led to the discovery of Uranus, Neptune, and Pluto.  They have also led to the discovery of many objects the earlier scientists could only dream of.  Today, we have large radio telescopes that listen for energy being radiated from other stars and galaxies.  We also have the world’s first telescope in outer space, the Hubble Space Telescope.  It was launched into orbit around the Earth in April of 1990.  You can find amazing photos taken by the HST all over the internet. 

Universe Lesson

The Universe is composed of galaxies, planetary systems, planets, stars, and other matter.  It is infinite in size.  When you look up into the night sky, the first thing you notice are the bright, twinkling dots that we know as the stars.  BUT, not all of them are stars.  Most are stars, but some may be planets, nebulae, galaxies, globular clusters, and rarely, a comet.  We know what planets are, so let’s briefly talk about nebulae and galaxies.  A nebula is a huge cloud of dust and gas out in space.  It is where stars are created.  You can see the Orion Nebula with your eyes and many others are easy to see through binoculars and telescopes.   

Galaxies are huge and contain billions and billions of stars.  Our galaxy, the Milky Way Galaxy contains about 100,000 billion stars.  It is about 100,000 light years in diameter.  The Milky Way is a spiral galaxy and is sort of shaped like a pinwheel with a rounded center.  If you look up into the night sky, you can see a faint, milky colored band of light.  You are looking into the center of our galaxy, the Milky Way.  Our Solar System is located out in one of the arms of the Milky Way, about 2/3rds of the way from the center. 
There are three different types of galaxies, spiral galaxies, elliptical galaxies, and irregular galaxies.   Check the notes for details about these types of galaxies.