![]() The exponent is negative for numbers less than one and indicates dividing by that number of tens. Very small numbers can also be written using powers-of-ten notation. The same number could accurately be rewritten as 38.4 x 10 4 or 0.384 x 10 6, but the preferred form is to have the first number be between one and ten. So for example, the distance to the Moon of 384,000 km can be re-written as 3.84 x 10 5 km. In powers-of-ten notation, numbers are written as a figure between one and ten multiplied by a power of ten. ![]() All of the zeros are put in an exponent, which is written as a superscript, and indicates how many zeros you would need to write out the long form of the number. Astronomers use a system called powers-of-ten notation, which consolidates all of the zeros that you would normally find attached to very large or small numbers such as 1,000,000,000,000 or 0.0000000001. To describe such a huge range, astronomers need a way to avoid confusing terms like "a billion trillion" and "a millionth". The distances and sizes of of the objects astronomers study vary from very small, including atoms and atomic nuclei, to very large including galaxies, clusters of galaxies and the size of the universe. 1 pc = 3.09 x 10 13 km = 3.26 lyįor even greater distances, astronomers use kiloparsecs and megaparsecs (abbreviated kpc and Mpc). A parsec is the distance at which 1 AU subtends an angle of 1 arcsec. This is because its definition is closely related to a method of measuring the distances between stars. Many astronomers prefer to use parsecs (abbreviated pc) to measure distance to stars. This means light from Proxima Centauri takes 4.2 years to travel to Earth. ![]() Proxima Centauri is the nearest star to Earth (other than the Sun) and is 4.2 light-years away. Light travels at an approximate speed of 186,000 miles per second (300,000 kilometers per second), so this distance is just over 19 trillion miles (about 31 trillion. A light-year is the distance that light travels in a vacuum in one year: 1 ly = 9.5 x 10 12 km = 63,240 AU A parsec is an astronomical unit of measurement that is equivalent to 3.26 light years distance, or the distance photons will travel in vacuum over the period of 3.26 years. To measure the distances between stars, astronomers often use light-years (abbreviated ly). The distance from the Sun to the center of the Milky Way is approximately 1.7 x 10 9 AU. Jupiter is about 5.2 AU from the Sun and Pluto is about 39.5 AU from the Sun. An astronomical unit is the average distance between the Earth and the Sun: 1 AU = 1.496 x 10 8 km = 93 million miles ![]() However, the distances and sizes in the universe can be so big, that astronomers have invented more units to describe distance.ĭistances in the solar system are often measured in astronomical units (abbreviated AU). They often use meters for length, kilograms for mass, and seconds for time. Astronomers use many of the same units of measurement as other scientists. ![]()
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