So why oh why do they say .63? The book says (and I quote):

"We can calculate any rising exponential curve's time constant, where a value of 0.63 times the total voltage change cuts the time axis at a time equalling the curve's time constant."

WTF??!! Does this mean? Where the hell did they get this? Then it says:

"...when the capacitor voltage is about 5V7 (that is .63 times the total voltage change of 9V)..."

Please help me understand this concept of measuring capacitors and graphing them. Thank you for your time.