 When we drive over to visit a friend, we usually have directions with us, like, it's the third door on the left. Unfortunately, when you're going to the moon, you don't have signposts telling you where you are or where you're going. So how exactly did Apollo astronauts know where they were in space going to the moon? This one is pretty complicated, so let's start off with an analogy, putting. Flying to the moon isn't like putting. You can't just aim at the hole and putt. Flying to the moon is more like putting across a magical green with hills and valleys that move, aiming at a hole that is also moving. If you watch the magical green long enough, you might be able to see a pattern and putt in a way that anticipates all the movements and sinks the ball, in which case your brain is doing an incredible amount of calculations to get to that point. On Apollo, it was the spacecraft's computer brain that did those calculations, but constantly and in real time to get astronauts to the moon. But let's back up and start this story at the beginning, before Apollo 11 even launched. In the early 1960s with the inaugural Mercury program, NASA relied on ground control during the unmanned and manned phases of the program. The worldwide tracking and telemetry network was thus born. When it came to Apollo, however, there was some fear that the Soviets would jam any radio signals being sent to a crew on the way to the moon, and so NASA decided that onboard navigation was vital. This couldn't be tampered with. The onboard navigation system required nothing beyond the computer's own systems, so it could not be tampered with. It was the ultimate Cold War failsafe. But how did it work? Solving the navigation problem fell to the instrumentation lab at MIT, now known as Draper, for its founder, Dr. Charles Stark Draper. It was the men and women at this contractor's site who figured out the question of deep space guidance and navigation, how Apollo the golf ball would navigate through the moving hills and valleys of space to reach its target, the moving hole that is the moon. The basic navigation onboard Apollo wasn't much different than dead reckoning on a boat. When you're sailing by dead reckoning, you use a compass, a knot meter, and a clock to break the question of navigation into time, rate, and distance. Basically, you calculate your position at any point on your journey via a previously determined fixed position. Knowing your speed and how long you've been sailing and in which direction, since you last checked your position, you can determine where you are by how much your position has changed relative to that fixed spot. Sailors did this using a log and an hourglass. Knowing that dead reckoning doesn't account for currents, steering errors, etc., they would check it by using a sextant to find their position on the earth by observing the sun, moon, planets, or stars. That part was once or twice daily if possible, except of course when weather obscured the skies. Saving the stars is effectively the same thing, only in three dimensions instead of two and thousands of times faster, so the instruments are a little more sophisticated. Instead of a compass, a gyro-stabilized stable table kept track of direction. Instead of a knot meter, accelerometers on that table tracked every little change in speed. The electronic clock ticked a million times a second. Instead of water currents, gravity produced unfelt changes of course, so the computer ran the mathematical model of gravity as defined by Isaac Newton. Embedded in the Apollo Command Module opposite the hatch were two telescopes. The first was a single magnification spotting scope to give the astronaut a wide field of view. From that wide field, he'd pick a landmark or sometimes a star. Then he'd move over to the second instrument that was a space sextant, a disc and slit apparatus designed to measure angles. The sextant had two lines of sight, one fixed and one movable. The fixed was able to focus on a landmark on Earth, such as the San Francisco Bay, giving it the name Landmark Line of Sight, or LLOS. This was controlled by adjusting the attitude of the whole spacecraft to center the landmark on the LLOS. When such a landmark wasn't available, the spacecraft could put the LLOS on a point of the sunlit horizon on Earth or Moon, just like a sailor does at sea. In space, the correct point on that horizon is just one on a line between the star and the center of the Earth or Moon. The movable site, called the Star Line of Sight, or S-L-O-S, could be moved as far as 67 degrees from the fixed line of sight. What the astronaut did was focus on the star, then fiddle with the mirror inside the sextant to bring the image of that star into conjunction with the landmark or horizon in his fixed line of sight. When satisfied, he punched the mark button that told the computer to read the angle between the star and the horizon and note the exact time. Doing this again with a second star and sometimes a third gave the computer enough data to run it through the navigation program and determine where the spacecraft was. Now this might seem counterintuitive when you consider that everything in space, including the spacecraft, is moving. But of course, engineers figured out a way around that. The Apollo spacecraft had an inertial platform on board, sometimes called the stable table, and this kept a constant eye on how the spacecraft was oriented around the fixed axes of inertial space. This sophisticated gyroscope-based system allowed the crew to set its orientation with the RCS thrusters, and once that orientation was set, the spacecraft didn't rotate. The other thing engineers and astronauts used to their advantage was that for all intents and purposes, the stars are fixed. During an Apollo mission, they're unmoving long enough to take a guidance measurement. We also have to remember that even though Apollo 11 was the first mission to land on the moon, the crew wasn't going up blind. Apollo's 8 and 10 had preceded them, but more importantly, the engineers and scientists who built the onboard navigation system brought centuries of studying the sky to bear on the problem. The computer knew the positions of stars and bodies that would be used for sightings, namely the Earth and the moon, which made real-time navigation calculations a relative cinch. So how did all this fit into an Apollo mission? Navigation is the middle letter in the acronym GNC, Guidance Navigation and Control. Guidance means deciding what changes in velocity are required to make the spacecraft go where you need it to go. Navigation is what we just discussed, determining where the spacecraft is. Control refers to operating the engine hardware to get the spacecraft where it needs to go. On Apollo 11, like all Apollo missions, the crew first launched into Earth orbit, relying entirely on the Saturn booster's separate computer and inertial guidance to get them there. Then it came time for the TLI burn that would change their trajectory from an Earth Orbital 1 to one that would intersect the moon in three days' time. It aimed where the moon would be in its orbit when the crew got there. But no burn was ever perfect, and a tiny error in that burn could translate to a significant deviation 100,000 miles down the track. So the spacecraft computer combined readings from the inertial measurement unit with mathematical models of gravity to maintain its knowledge of the spacecraft's position and velocity. By the time of the Apollo 11 mission, the computer's software was so reliable that the crew only needed to take occasional readings with a sextant to account for drift or as a backup measurement. In either case, that information allowed the computer to determine whether or not the trajectory had to be adjusted. This is guidance, guiding the spacecraft to a correction and control, controlling the actual course adjustment burn. So really, the acronym should be NGC, but GNC just rolls off the tongue so well. Thanks to Draper's Hack the Moon initiative for making this video possible. To find out more about Draper's role in getting Apollo astronauts to the moon, check out wehackthemoon.com. The site chronicles the engineers and technologies behind the Apollo missions and is full of thousands of never-before-seen images, videos, and stories about the people who hacked the moon. Be sure to visit wehackthemoon.com and for more vintage space every day, be sure to follow me all across social media. If this is your first time seeing vintage space, welcome and please do subscribe so you never miss an episode. Thank you guys so much for watching and I'll see you next time.