Of course the designers also used digital computers, but in the 1960s computers were giant machines that you programmed with punched cards, and they were strictly reserved for only the most complex mathematical calculations. As the 40th anniversary of the Apollo 11 mission approaches, we are constantly reminded of how incredible that voyage was. Add to the incredulity the slide rule: the basic mathematical tool that helped get the astronauts to the moon and back.

The visitors who gathered around the Command Module on Space Day generally fell into two camps. Older visitors told me that they used a slide rule in school but hadn't seen one in years, and they had completely forgotten how it worked. The younger visitors (i.e., those under 40!) had never seen one before, although a few had heard of them. I belong to the former group, having once been quite proficient while in high school. For this presentation, I got out the manual and taught myself all over again how to use it. It was not easy.

The National Air and Space Museum has preserved a few slide rules, including one carried by Apollo 13 astronauts on their April 1970 journey. The Museum also has on display the slide rule owned by Wernher von Braun, who headed the Marshall Spaceflight Center in Huntsville, Alabama during the Apollo era. It shows signs of heavy use. One other favorite of mine is the "Space Vehicle Pocket Designer," a specialized circular rule that computes spacecraft payload and range, based on fuels and rocket engine efficiency. It was given to me by a mathematician who had just retired from a northern Virginia technology firm. When he gave it to me, the retiree said, "Congratulations, Paul, you are now officially a rocket scientist!" If only it were that easy.

Apollo astronauts carried slide rules, but by the time of the last mission to the Moon in 1972, the pocket calculator had been invented. On the Apollo-Soyuz mission in 1975, the last to use Apollo hardware, the crew carried a Hewlett-Packard pocket calculator that had more power than the on-board Apollo Guidance computer.