Astronomy Answers: From the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

large numbers

Here is a list of British English and American English names of large numbers and small fractions, and the prefixes of units that go with them. In the title row, "om" stands for order of magnitude: this indicates how many zeros are involved. The prefix can be used with units: for example, a kilometer or km is equal to 1,000 meters, and a megameter or Mm is equal to 1,000,000 m or 1,000 kilometers.

Table 2: Very Large and Very Small Numbers

British American om prefix number
21 1.000.000.000.000.000.000.000
trillion 18 exa = E 1.000.000.000.000.000.000
quadrillion 15 peta = P 1.000.000.000.000.000
billion trillion 12 tera = T 1.000.000.000.000
billion 9 giga = G 1.000.000.000
million million 6 mega = M 1.000.000
thousand thousand 3 kilo = k 1.000
hundred hundred 2 hecto = h 100
ten ten 1 deca = da 10
tenth tenth −1 deci = d 1/10
hundredth hundredth −2 centi = c 1/100
thousandth thousandth −3 milli = m 1/1.000
millionth millionth −6 micro = ยต 1/1.000.000
billionth −9 nano = n 1/1.000.000.000
billionth trillionth −12 pico = p 1/1.000.000.000.000
quadrillionth −15 femto = f 1/1.000.000.000.000.000
trillionth −18 atto = a 1/1.000.000.000.000.000.000
−21 1/1.000.000.000.000.000.000.000