SHORT KNOW

 Astronomers now recognize eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), five dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris) , more than 150 moons, and many, many asteroids and other small objects (See the figure on the following page). These objects move in regular and predictable  paths around the Sun.  Distances in the Solar System Distances in the solar system are often measured in astronomical units (AU) . One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km (93 million miles).  Listed below is the distance from the Sun to each planet in AU (Tablebelow) . The table shows how long it takes each planet to spin once on its axis. It also shows how long it takes each planet to complete an  orbit. Notice how slowly Venus rotates! A day on Venus is actually longer than a year on Venus! Our solar system is about 100,000 AU across from the Sun to the Oort Cloud or 1.87...

 After discovering that there are galaxies beyond the Milky Way , Edwin Hubble went on to measure the distance to hundreds of other galaxies. His data would eventually show how the universe is changing, and would even yield clues as to how the universe formed. Redshift . If you look at a star through a prism, you will see a spectrum, or a range of colors through the rainbow. The spectrum will have specific dark bands where elements in the star absorb light of certain wavelengths. By examining the arrangement of these dark absorption lines, astronomers can determine the composition of elements that make up a distant star.  In fact, the element helium was first discovered in our Sun—not on Earth—by analyzing the absorption lines in the spectrum of the Sun. While studying the spectrum of light from distant galaxies, astronomers noticed something strange. The dark lines in the spectrum were in the patterns they expected, but they were shifted toward the red end of the spectrum, ...