What happens when you’re lost and you don’t have your phone to give you directions? We all have that one road, building, or tree that helps us remember where we’re going. Without those kinds of landmarks, it might be harder to navigate an unfamiliar place. But what do you do when there’s nothing recognizable around you? Well, you look up.
The stars that we see in the sky today are the same ones that guided the ancient Egyptians in building their pyramids and maritime travelers in their travels around the globe. Many of the techniques that guided people in the past are similar to the ones that we use in our modern world. That’s because during any time of the year anywhere on Earth, the stars appear over the horizon at predictable heights and at measurable distances.
For example, Polaris — what most people know as the “North Star”— is an easy target when you’re navigating in the Northern Hemisphere. First thing you want to do is find “The Big Dipper.” Depending on where you are, and what time of day and year it is, the Big Dipper will be in a different part of the sky, but it’s always circling the North Star. Plus, it’s a pretty recognizable feature. Once you’ve located the Big Dipper, take a look at its “bucket.” On the front edge of this “bucket” are two stars, Merak and Dubhe. Draw an imaginary line from Merak, through Dubhe out to the bright star of Polaris. That’s the North Star! Now that you’ve found the North Star, you’ve also found the “Little Dipper.”
The reason we use the North Star is because it hardly moves, making it a reliable navigational point. However, the North Star isn’t much help if you’re in the Southern Hemisphere. You have to catch it at the right time and place. So, if you’re in the south, you’re going to need the constellation called the “Southern Cross.” You have to draw a line though the longest angle of the Cross, connecting the stars from the top-down, all the way to the horizon. Then you find the midpoint between the two “pointer” stars on the other end of the constellation and draw a line from that midpoint to the horizon as well. The area where those two lines intersect is the sweet spot we’re looking for and that points South. Now, obviously, humans travel in more directions than just North or South.
Even going way back to the 1802 American practical navigator and Nathaniel Bowditch, they would publish tables of some of the brightest stars in the sky. Generally there's about 80 to 82 or so bright stars, which are very easily identifiable because they also exist in the major constellations.
In order to get the latitude and longitude of your location, select one of these stars, measure its direction and height above the horizon, and the time it was when you took the measurement. Add a little spherical trigonometry and you’ve got your exact location. A tool that helps determine these calculations is called a sextant. This centuries-old device is a two-mirrored, doubly reflective instrument. One mirror keeps an eye on the horizon and the other is adjusted by a 60 degree arc to locate the constellation or celestial body. Depending on where you land, the arm has numerical notches that help the user calculate their location.
This tool can be helpful practically anywhere. In fact, Apollo astronauts had their vehicles equipped with a sextant that they could use in case they ever lost communication with Earth.
Apollo really used the Sexton throughout their whole mission. They would use it to periodically align their gyroscopes with the stellar inertial frame throughout the mission. In fact, a lot of times each crew had a designated navigator onboard who would be trained and taking the sites. And it was a little bit of a competition among the Apollo astronauts on who could actually get the most accurate Sexton sightings compared to the ground radio tracking. And Jim Lovell of course, was very famous for being a pretty good sighter. Frank Borman as well.
As recently as 2020, the crew aboard the International Space Station tested this Earth-based tool for it’s star-sighting techniques in space.
As part of the Sextant Navigation Investigation, researchers are studying whether this kind of astronavigation technique could be used for future missions, like NASA’s Orion spacecraft. Researchers are also exploring how to expand celestial navigation by using completely different stars: pulsars.
On the ISS, there is an instrument called the Neutron Star Interior Composition Explorer, or NICER, that looks at spinning ultra-dense left overs of exploding stars. These pulsating stars, known as pulsars could emit radiation blasts every few thousandths of a second. NICER collects time-stamps of these blasts and uses a software called Station Explorer for X-ray Timing and Navigation Technology, or SEXTANT— see what they did there? This software creates a system similar to GPS.
Because these pulses are so consistent and synchronized, it means we can predict their time of arrival in any place of our solar system. If scientists can get this right, we can navigate in deep space like we never have before. The experiment on the International Space Station — the NICER sextant — is really a quantum leap forward in the celestial navigation art form. And I really hope that they can get it working because that'll be a really cool technology to see once it comes online.