[INTRO ♫].

Apollo 13 went down in history—and not because it went great. Well, the movie was very successful, but the mission not so much.

The spacecraft had two liquid oxygen tanks that were supposed to provide air for the astronauts and power the spacecraft on the way to the Moon. And then one of those oxygen tanks exploded, damaging the other one in the process. So the astronauts were left with very little air and power, and they needed to get home.

If you've seen the movie, you probably remember that they used the gravity of the Moon to slingshot themselves back to Earth. A nice, no-power solution! But to get in the right position for that, and to stay on course the whole way home, they still needed to navigate.

Fortunately, the astronauts had another nice, no-power solution, one that humans have been using for almost 300 years: a sextant. In the olden days, sailors used sextants to navigate the seas using the stars. Centuries later, when technology was advanced enough to send humans to the Moon, astronauts still used sextants onboard for navigation.

And they weren't just useful. They were literally lifesavers. Sextants are these fancy telescopes attached to what sort of looks like a protractor.

They were a major technological innovation in the 1700s that made it possible for explorers to chart extremely precise journeys. To use one, you'd look through the eyepiece and line up the crosshairs with the horizon. Then, you'd use the triangular arms to measure the angle between the horizon and the Sun or a particular star.

After that, you'd use a chronometer, the most accurate type of timepiece available back in the day, to get the time. And finally, after consulting a bunch of almanacs and doing some math, you'd know exactly where you were on the globe and how to chart your course. The Apollo sextant worked basically the same way, but it was souped up for the space age.

It lived on top of the command module (the part of the spacecraft that held the crew) and the astronauts operated it from inside. It still had crosshairs like an old-timey sextant, but in space, there's no horizon. So astronauts had to align the sextant with a set of stars with known positions that NASA used as a reference.

Then, to get the angular positions of the stars, there was no protractor-type thing, but there was an Archimedes' spiral drawn over the crosshairs. Archimedes' spiral is special because each point is at a different distance from the center. So if you trace the spiral for 360 degrees, the line of the spiral is at a different radius for each degree.

That means once you see where a star intersects the spiral, you can use the radius of that intersection to figure out the angular coordinates of the unknown star. It's basically like sailors finding the angle between a star and the horizon, except this works in space. Finally, for the very last step, instead of consulting almanacs, astronauts would punch their coordinates into the on board computer, and they'd know exactly where they were in space and how to chart their course!

The Apollo sextant was great for these missions because it was light, super simple, and it didn't have a bunch of moving parts that could break. And, aside from the last step, it purposely used no power. NASA even had a totally analog backup plan that let astronauts navigate without a computer by lining the sextant up with Earth's day-night line.

And it turns out, an analog system was a good idea! Not just for Apollo 13, either. In fact, the first time the sextant saved the day was two years earlier, on Apollo 8.

Jim Lovell was the navigator on that mission, and when the crew was re-entering the atmosphere,. Lovell was using the sextant. But as he punched the numbers into the computer, he accidentally erased all the navigational data.

Don't you hate when that happens right when you're plunging toward your planet at tens of thousands of kilometers per hour? So Lovell asked ground control what to do, and they told him to try navigating totally manually. So he lined the sextant's crosshairs up with the Earth's day-night line, and used the sextant readings, along with communication from ground control, to adjust the tilt of the spacecraft on the fly.

Talk about calm under pressure. And it worked! The crew landed safely, and they knew that this backup method of re-entry would work in another emergency.

Which was pretty lucky. Because that wasn't Apollo's last, or worst, emergency. So, Apollo 13 happens.

The crew slingshots around the moon, they jerry-rig an air filter out of a sock and duct tape (different story, watch the movie) and then they get back to Earth. It's time to re-enter the atmosphere, and they hear from ground control that they're coming in too shallow. They'll skip off the atmosphere like a rock on a pond if they can't correct their course!

Except, they have very little power left, so they can't use the navigational computer with the sextant. Lucky for them, the mission commander just happens to be the one guy in the whole world who's done this before. Jim Lovell is sitting in that spacecraft, and he says, “I got this.” Once again, Lovell lined the sextant up with the line between day and night.

Ground control told the crew exactly when to fire the thrusters to realign themselves. Then all three astronauts on board handled the navigation: two worked the thrusters to tilt the spacecraft the right way, and a third timed everything. And they made it!

They landed safely in the ocean, where boats were waiting to retrieve them and take them home. Fortunately, that was the last time that landing procedure had to be used, because future Apollo missions went a lot smoother. But that wasn't the last time sextants were used in space.

NASA wants astronauts on future deep space missions to have the ability to navigate completely on their own, in case communications with the ground go down. So, maybe someday, this old tool could make yet another comeback. Also, if you learned nothing else today, remember:.

You always have to have a plan B! And you should pay attention in geometry class! Angles save lives!

Thanks for watching this episode of SciShow Space! While you're here, I want to tell you about February's SciShow pin of the month. Every month, we here at SciShow design a new, space-themed pin, and we've just released the one for February!

This one is of the Mir Space Station, and you can only get it this month. Check it out at dftba.com or in the description below. [OUTRO ♫].