Introduction The solar system’s planets vary greatly in appearance Traveling Outward Earth’s nearest neighbor in space is the moon It is the only natural object orbiting Earth Earth is the third of eight planets orbiting the sun Several of these planets have their own moons Smaller objects such as asteroids and comets also orbit the sun The sun planets moons and other objects all make up the solar system Discussing distances in space can be difficult because the distances are so large One useful tool is the speed of light Light always moves at the same speed Saying how long it takes light to travel from place to place is a simple way to give 6
distances For example the moon is about 1.3 light-seconds from Earth In other words it takes 1.3 seconds for a beam of light to make that trip Light takes about four hours to go from the sun to Neptune the most distant planet in our solar system The sun is one of about 100 billion stars in the Milky Way galaxy These stars are very far apart The nearest star to our sun is called Proxima Centauri It is about 4.25 light-years away The Milky Way is not the only galaxy There are at least 100 billion other galaxies in space The nearest is the Canis Major Dwarf galaxy It is about 25,000 light-years away Together all the galaxies make up the known universe Over time scientists have learned a great deal about space Spacecraft and telescopes have unlocked many secrets of galaxies stars planets and more But with such a vast frontier in the night sky there is still much more to discover With telescopes scientists have learned a lot about the universe around us 7
The Sun For much of human history many people believed Earth was fixed in the center of space They thought the other planets and the sun revolved around it This concept is now known as the geocentric model Thinkers such as the Greek scholar Ptolemy who worked in the 100s CE were strong supporters of this idea Early thinkers believed Earth was at the center of the universe 8
Copernicus challenged existing notions of the structure of the universe Things began to change in the 1500s The Polish scholar Nicolaus Copernicus proposed the idea that the sun was at the center and Earth was simply one of several planets revolving around it This concept is the heliocentric model It came into conflict with some religious teachings relating to the nature of the sun and Earth For decades after Copernicus published his idea it was not widely accepted 9
