What Columbus used instead of the North Star

What Columbus used instead of the North Star – Signore Galilei

Christopher Columbus didn’t navigate by the North Star. At least, not directly.

Columbus is a controversial figure from a modern perspective, and he undeniably made many mistakes. But this wasn’t one of them. Mariners sailing for Spain and Portugal in the 1490s just didn’t use the North Star as true north. Because back then, it wasn’t.

Polaris, the North Star, is the brightest star in the constellation Ursa Minor, the little bear. It’s also the closest bright star to the North Celestial Pole: the point in the sky right above the North Pole of the Earth. As the Earth spins about its axis, all the stars in the sky appear to rotate around the North Celestial Pole, for an observer in the Northern Hemisphere. The angle between the North Celestial Pole and the horizon is equal to your latitude.

Polaris isn’t exactly at the North Celestial pole – it’s about two thirds of a degree away from it. So if you were to treat Polaris as true north for celestial navigation, you could be sailing up to two thirds of a degree north or south of where you think you are. If you’re aiming for Miami, Florida, you could wind up in Florida Keys instead.

Now, the Earth’s axis wobbles around relative to the sky, taking about 26,000 years for one complete wobble. This is called “axial precession”, and it changes which star is the “North Star” and how close it is to true North. We’re actually pretty lucky that Polaris is so close to the North Celestial Pole right now: it’s not the brightest star in the sky, but it is in the top 50, and no brighter star will be quite as close to the pole for the rest of this 26,000 year cycle (though Vega, the 5th brightest star in the night sky, is set to get decently close about 12,000 years from now).

For a task as precise as celestial navigation, the effect of axial precession matters even over the span of a few centuries. In Columbus’s day, Polaris was about 5 times farther away from the pole: 3.5 degrees. So if you were aiming for Miami in 1492, you could wind up in Cuba!

The solution was called “The Regiment of the North Pole”.1 Using the other bright stars in Ursa Major as a guide, a navigator could determine the rough direction Polaris was from the true pole. Then they could add or subtract the appropriate correcting factor to their latitude calculation. Iberian navigators used the “guard stars” Kochab and Pherkad, which were then between a quarter and a third of a turn counterclockwise around the pole from Polaris. So if the guards were e.g. up and to the right of Polaris, then Polaris must be directly below the pole itself, so Columbus would have needed to add 3.5 degrees to his measured latitude.

By the 1700s, when Polaris was still 2 degrees away from the Pole, star charts and navigation equipment had improved dramatically, and these rules of thumb gave way to more precise measurements. But even with modern GPS, these guidelines still find their niche. Amateur astronomers still use the line between Polaris and Kochab to align telescopes with equatorial mounts.2 And in the Southern Hemisphere, where there’s currently no bright pole star, there are plenty of tricks for amateurs to find the South Celestial Pole which are on par with anything from the Age of Sail.

Coming soon: You’ll never guess who made the first wireless telephone

1 https://archive.org/details/sim_geographical-journal_1931-10_78_4

2 http://www.weasner.com/lxd/techtips/polar_alignment.html

What Columbus used instead of the North Star

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