Lunar distancing

Okay, let’s say you’re in a rowboat at night with some friends—and you haven’t seen land for a while. You’re LOST. Nobody’s getting a signal on their cellphones, so you don’t know where you are. The strange old lady at the boat rental place left nothing but a weird navigational device; a map; and a book of star charts in the boat’s locker. Your friends are getting panicky and start blubbering. What do you do?

Because you’re a devoted reader of The Western Civ User’s Guide to Time and Space, you know exactly what to do. You tell your pals to stop their noise so you can concentrate. It’s a clear moonlit night, so you can see the moon, stars, and the horizon. You pick up the lovely brass sextant and set its sights on the moon—and a star, how about Regulus, just there to the left? You measure the altitude (how high above the horizon) of the moon; the altitude of Regulus; and the distance between them. You figure the angle of the 2 lines from you to the moon and you to Regulus. You do this measuring not in feet or miles but in degrees.

From the moon’s altitude you know what time it is (http://www.astrotulsa.com/page.aspx?pageid=27, scroll down)—and your latitude, too (http://www.lewis-clark.org/article/1268). Knowing the distance from the moon to Regulus, you pick up the book of star charts and find that lunar distance for your local time. Run your finger down the chart to find what time it is in Greenwich, England where it’s zero degrees longitude. The difference in time will tell you your longitude (15° for every hour, 1° for every 4 minutes). Find your latitude and longitude on the map and start rowing home. You don’t even need a compass—you keep Polaris, the North Star, above your right knee as you row.

You get safely back to land! Your friends can’t believe you saved the day with that stupid book. The lady at the boat rental gives you a wink and you all go home to bed.

This is how Nevil Maskelyne proposed finding your position while at sea.

https://theskylive.com/moon-info
https://www.rmg.co.uk/discover/behind-the-scenes/blog/time-solve-longitude-lunar-distance-method
I haven’t read these, but here’s a short list of books about ocean-going girls: https://books.google.com/books/about/From_Cabin_Boys_to_Captains.html?id=wBDWSAAACAAJ

Back to the beginning of The Western Civ User’s Guide to Time & Space

Eratosthanes and longitude

Old-time tv newsrooms had clocks on the wall set to local times of the big cities.

How did Eratosthanes or Ptolemy determine where the longitude lines should go? I got this from the History Stack Exchange site:

Longitude is calculated by comparing the elevation of an astronomical object to the pre-calculated (or observed) elevation of the same object at a reference location at the precisely simultaneous moment in time. Everything in the sky rotates once around that vast celestial sphere every 24 hours, so the more precisely one can establish simultaneity the more precise one’s measurement of longitude will be.

Whew! In other words: 2 people standing in 2 different places can measure the height in the sky of the moon, or the Sun, or the North Star to figure out how far east or west they are from each other. BUT—the measurement must be taken at exactly the same moment. Eratosthanes figured a way to find longitude without the measuring. Eratosthanes (in Alexandria) and an assistant (in someplace to the west—maybe Benghazi?) watched a lunar eclipse. They agreed to mark the exact local time the eclipse began. The local times won’t be the same, right? When it’s midnight in Benghazi, it will be 12:36 am in Alexandria. The difference in their times told Eratosthanes how many degrees apart from each other they were.

I’ll tell you how in the next post.

Back to the beginning of The Western Civ User’s Guide to Time & Space