BotEC: The Magnitude of Geologic Time
The oldest rock yet to be found on the earth is from Canada and is radiometrically dated at 3.8 billion years old. Various lines of evidence suggest that the earth is about 4.5 to 5 billion years old. A billion years is an immense length of time. To get a sense of the magnitude of geologic time, igure out how many years it would take you to count to one billion, assuming that each count takes one second.
References and Resources
This SERC page describes the use of Back of the Envelope Calculations
A View from the Back of the Envelope (more info) : This site has a good number of easy simulations and visualizations of back of the envelope calculations.
The Back of the Envelope : This page outlines one of the essays in the book "Programming Pearls" (ISBN 0-201-65788-0). The book is written for computer science faculty and students, but this portion speaks very well to back of the envelope calculations in general.