# In Their Own Words: Bill Robertson on Gravity

Bill Robertson worked in computer science, calculating gravity constants and coding routines necessary for the Apollo missions.

"I'm William M. Robertson and, back in the Apollo days, I was responsible for the routines which calculated where the spacecraft was and where the spacecraft would go. I was the keeper of the constants, which means the Earth's gravitational constant and the strength of gravity at the launch pad. The gravity of the Earth influences the path of the satellite to the moon and then the gravity field of the moon influences how the spacecraft will approach the moon and where it will land. It was all programmed in the onboard computer of Apollo beforehand, so those constants were known and they were used in these routines which computed the spacecraft trajectories. It was important to understand the amount of gravity on the earth and at the moon in order to get the trajectory of the spacecraft correct. If the calculations were wrong, then the spacecraft would not land on the moon in the right place that had been planned.

We knew we had to make changes in the gravity field of the moon and in the model that we had of the gravity field of the Moon to add in some more terms that NASA wanted us to add into the on board spacecraft computers. I computed what those terms were and did the coding to have those terms into the model so that the landing would be more precise. I was confident with my routines and with the whole program because I had run hundreds of simulated test cases on my routines and stress test cases. I would propose an orbit different from the orbits that I knew would actually fly just to stress test their routines to see if they would work in those extreme cases. Back in those days, my office was an inside room and I spent all day working in the inside room, working on the computer, or going down to the computer room. Then one evening, after a long day's work, I came out of my office and walked outside to go to the subway and there in the sky above me was the real full moon itself and I was just kind of overwhelmed to think that that's where we're going. And I have a part in it. It's kind of so amazing to see."