Note 4 — Chapter I · Exploring the Sky
Reading edition
The notation 6 × 10²¹ stands for the number formed by a 6 followed by 21 zeros, this shorthand notation being essential, in the interests of brevity, in discussing astronomical numbers. A million is 10⁶, a million million is 10¹² and so on.
A similar notation is needed to express very small numbers. The expression 10⁻²¹ is written for 1/10²¹ and so on. Thus 6 × 10⁻⁶ stands for 6/1,000,000 or 0·000006.