Basic Astronomy Facts

Whenever you look up at night you see what astronomers thousands of years ago saw. This section tells some history, defines terms, explains telescopes and other tools that astronomers use and gives observing hints. When you are ready, click back to return to the Student Center or click below to explore again.


For most of recorded history the Earth was the center of the universe and never moved. The constellations were named and stories were told about them. The Greek Aristotle wrote in the third century BC that the Earth was a sphere because different stars could be seen as you move north and south and the shadow of the Earth on the moon during an eclipse was curved. A hundred or so years later, Eratosthenes used the length of the shadow cast by the sun to determine the circumference of the Earth.
In the second century AD Ptolemy gathered star catalogs and astronomical writings. Astronomers were called astrologers at the time and tried to predict events using the stars. Ptolemy proposed that the solar system was like a huge carousel with the Earth as the center and the planets, stars, sun and moon riding where the horses would be.
After about 1400 years Nicolaus Copernicus used mathematics to place the sun in the center of the "carousel" to better allow for how the sun, planets and stars appear to move. Johannes Kepler expanded on the idea using elliptical orbits to predict planetary motion.
Soon after the invention of the refracting telescope, Galileo Galilei used one to view craters on the moon and watch the moons of Jupiter circle the planet. This validated the idea that the sun was in the center of the solar system and the Earth orbiting it. In addition, Isaac Newton invented the reflecting telescope that used a mirror instead of a glass lens and proposed the Theory of Gravity which explained how the planets were held in place. At about this time Gian Cassini measured the distances between the planets using a method called parallax shift. This is similar to watching the thumb on your outstretched arm "move" from side to side when you close one eye and then the other.
This is, of course, a very brief and narrow view of history but will give you a few clues for further study. There are many contributions by women and non-European cultures that would make a great report project and Internet search practice. See the links below.
Here is a list of other contributors to astronomy you may want to research.
Hipparchus, Tycho Brahe, Edmond Halley,
William and Caroline Herschel, Edwin Hubble,
Henrietta Swan Leavitt, Harlow Shapley, Percival Lowell,
Vesta Slipher and Albert Einstein.


Here are some basic terms used in astronomy.
     A named group of stars that is part of a constellation, the Big Dipper is one.
     Small, rocky world. Most asteroids are between the orbits of Mars and Jupiter.
Astronomical Unit
      The average distance between Earth and the sun, 1.5 x 108 km.
     The glowing light from solar particles interact with Earth's magnetic field.
Celestial Equator
     The imaginary line around the sky directly above the Earth's equator.
Celestial Pole
     Points above the Earth's north and south poles.
Celestial Sphere
     An imaginary sphere surrounding the Earth where the sun, planets and stars are, a 3 dimensional      map of the universe.
     One of the small, icy bodies that orbit the sun that make tails of gas and dust when they get close      to the sun.
     A pattern of stars usually named after animals or people in stories.
     The path the sun seems to follow in the sky.
Emission Nebula
     A cloud of glowing gas.
Globular Star Cluster
     A group of stars that look like a shape of a ball.
Light Pollution
     Wasted light from city and outdoor lights that makes it hard to see the stars at night.
     The distance light travels in a year.
     Small rocks or sand making a bright trail through the sky as it burns in the atmosphere.
     A meteor that has landed on the Earth.
     A glowing clouds of gas or dust reflecting the light of nearby stars.
Open Star Cluster
     A group of stars that look close together in the sky.
     The path an object takes as it moves around another object.
Planetary Nebula
     An expanding ring of gas around a star.
Reflecting Telescope
     A telescope that uses a mirror to focus light.
Reflection Nebula
     Dust and gas reflecting light from stars close by.
Refracting Telescope
     A telescope that uses a lens to focus light by bending it.
     An object spinning about its center.
     The Search for Extra-Terrestrial Intelligence.
Solar Eclipse
     The name of the event when the moon comes between the sun and Earth.
     The point in the sky directly overhead.


Telescopes come in different sizes, designs and styles. The usefulness of the power of the instrument is limited by the size of the lens or primary mirror. The table below summarizes the three main types of optical telescopes. Radio telescopes use the reflecting design, however, they focus radio waves instead of light.
Galileo used the refracting type of telescope to discover the moons of Jupiter. Most large observatories use the reflectors. These are sometimes known as "Newtonian" telescopes named after the inventor Isaac Newton.

                               Telescope Types



Telescope Types

The incoming light is focused by the objective lens and directed down the tube to the eyepiece.
The focused image is then magnified by the eyepiece.

The incoming light is focused by the primary mirror and directed up the tube to the secondary mirror. The light travels to the eyepiece. The focused image is then magnified by the eyepiece.

The incoming light is adjusted by the corrector plate before it travels down the tube. Then the light is focused by the primary mirror and directed back up the tube to the secondary mirror. Finally, the light travels back down the tube through a hole in the primary mirror to the eyepiece. The focused image is then magnified by the eyepiece.

Telescopes are attached to various types of mounts. Some are mounted on tripods like a camera and some, called Dobsonian mounts, rest in boxes that move up and down and around like a lazy susan. Equatorial mounts can be aligned with the axis of the Earth to help keep an object centered in the eyepiece. Some newer mounts have built-in electronics and computers to point to any celestial object in the night sky.

                       From left to right are the Dobsonian, Equatorial and Fork mounts.

The diameter of the telescope primary lense or mirror determines how much light is focused in the eyepiece and how much detail you can see. Telescopes as small as 60 mm in diameter with the right lenses can view the craters of the moon, the rings of Saturn, cloud bands on Jupiter, and split binary stars. Most observers, however, prefer 4 inch or more in size.

Observing Hints

There are a number of things you should keep in mind when observing the night sky.
First, learn the sky using your eyes and binoculars. Find out where each constellation is located and how to find out what time of year they can be seen. Learn to estimate how big a constellation is in the night sky. Learn the names of the brighter stars. This preparation makes it easier to find objects because when you are looking through a telescope you see only a small part of the sky.
Soon you can learn how to read star maps and how to find binary stars, nebulae, galaxies and the planets. Keep a log of your adventure.
Dress for cooler temperatures, bring a chair or blanket to sit on and let your eyes adjust to the darkness. Sometimes you can just lay on your back and look for meteors, satellites and the aurora. Bring snacks and beverages and make it a fun family time discovering the cosmos.
For more information on observing go to the Astronomical League pages describing their viewing programs. The site below will open up in a new window. Close the window to return to Kid's Cosmos.