Introduction For thousands of years human cultures have had a deep interest in the skies above them The motions of objects in the sky told the people of North America many things These things included when to plant when to harvest when to hunt and when to celebrate Researchers think England’s Stonehenge structure was a temple designed to follow the sun’s movements In Africa a stone circle built 7,000 years ago tracked the summer solstice and the arrival of the rainy season Many cultures saw gods and goddesses moral lessons and hints of things to come in the night sky As humans learned People began building Stonehenge thousands of years ago 4
In this diagram the blue line represents the Kármán line The red line shows the altitude of a space station about 250 miles 400 km above the surface more about the natural world they began to discover the scientific nature of the things they saw in the sky There is no distinct boundary above Earth where space begins The atmosphere gradually becomes thinner and thinner as the altitude increases But one commonly accepted point for this transition is the Kármán line Named for scientist Theodore von Kármán it is located at an altitude of 62 miles 100 km above the planet’s surface 5
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
