Finding Longitude: Shadows, Sundials and Chronometers
For three centuries the hardest question at sea was "where am I?" - and the answer turned out to be a clock. Why longitude is really a measurement of time.
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The hardest question at sea 🧭
Latitude was never the problem. Measure how high the Sun or the Pole Star climbs and you know how far north or south you are. **Longitude was different**, and ships were lost over it for three hundred years. There is nothing to measure: every line of longitude looks exactly like every other. The answer, when it came, was not a better telescope. It was a **clock**.
Longitude is time ⏱️
The Earth turns once, 360 degrees, in 24 hours. Divide one by the other and you have the whole topic: **15 degrees of longitude for every hour** of time difference. **4 minutes of time for every single degree.** So if you know what time it is **where you are**, and what time it is **at Greenwich** at that same instant, the gap between them tells you how far around the world you have travelled. **That is why it needed a clock.** Not to tell you your own time - the Sun does that - but to carry Greenwich time with you, accurately, for months, on a rolling ship.
Minutes per degree 🔢
An interactive activity.
How many zones? 🌍
An interactive activity.
The two times you need 🗂️
Finding longitude means knowing two times at the same moment. These are the terms the question will use:
East or west? ❓
Your local noon happens LATER than noon GMT. Where are you?
- West of Greenwich, because the Sun reaches you after it reaches Greenwich
- East of Greenwich
- On the Prime Meridian itself
- It cannot be told from the time alone
Find the longitude 📐
An interactive activity.
Two ways to carry Greenwich with you ⚓
Both methods were real, and the specification names both. Each is a way of knowing GMT while nowhere near Greenwich.
Order the method 🪜
An interactive activity.
What do you actually need? ✅
Select the TWO things without which longitude cannot be found by this method.
- A way of finding your own local time, such as the Sun and a shadow stick
- A way of knowing the time at the Prime Meridian at that same instant
- Your latitude
- A telescope powerful enough to see the coast
On the deck 🧪
An interactive activity.
The summary 📝
The Earth turns _____ degrees in 24 hours, so every hour of time difference is _____ degrees of longitude and every single degree is _____ minutes of time. To find your longitude you need your own local time and the time at the _____ Meridian at the same instant. And if your local noon happens after noon GMT, you are _____ of Greenwich.
Your turn ✍️
An interactive activity.