# How Did They Measure Lat & Long? **URL:** \ **Category:** Factual Questions\ **Created:** [November 7, 2007, 12:11pm UTC](https://boards.straightdope.com/t/how-did-they-measure-lat-long/425530 "2007-11-07T12:11:30Z")\ **Posts on this page:** 6\ **Page:** 5
**Author:** ![The\_Stafford\_Cripps](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_stafford_cripps/32/7167_2.png) [@The\_Stafford\_Cripps](https://boards.straightdope.com/u/The_Stafford_Cripps)\ **Post date:** [April 5, 2018, 5:29pm UTC](https://boards.straightdope.com/t/how-did-they-measure-lat-long/425530/81 "2018-04-05T17:29:58Z")
> [@Chronos](#): > > Because marking noon, and comparing it to the time reported by your shipboard clock that’s still set to Greenwich, is what tells you your latitude. Longitude. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 5, 2018, 5:40pm UTC](https://boards.straightdope.com/t/how-did-they-measure-lat-long/425530/82 "2018-04-05T17:40:08Z")
> [@carnivorousplant](#): > > How does it come to pass that marking noon is an important event in all those Royal Navy books? Could you please quote the passage? Who was doing the marking, where, and how? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 5, 2018, 6:19pm UTC](https://boards.straightdope.com/t/how-did-they-measure-lat-long/425530/83 "2018-04-05T18:19:20Z")
> [@The\_Stafford\_Cripps](#): > > Longitude. Let’s think about it this way: if you observe the altitude of any heavenly body at some known time, that fixes your position as lying on a circle, or a line if you plot it on a chart. Whether that line slopes closer to a line of latitude or of longitude depends on the circumstances of the observation. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 5, 2018, 6:20pm UTC](https://boards.straightdope.com/t/how-did-they-measure-lat-long/425530/84 "2018-04-05T18:20:41Z")
Yeah, it was inevitable that I’d eventually make that mistake if this thread went long enough. Yes, longitude. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [April 5, 2018, 6:44pm UTC](https://boards.straightdope.com/t/how-did-they-measure-lat-long/425530/85 "2018-04-05T18:44:57Z")
If you know the local time (e.g., noon) and the time at Greenwich, then you know your longitude, and conversely. But if you also happen to measure the altitude of the sun around noon, that tells you your latitude. ---
**Author:** ![Timz](https://avatars.discourse-cdn.com/v4/letter/t/6a8cbe/32.png) [@Timz](https://boards.straightdope.com/u/Timz)\ **Post date:** [July 28, 2025, 7:20pm UTC](https://boards.straightdope.com/t/how-did-they-measure-lat-long/425530/86 "2025-07-28T19:20:33Z")
Somehow, people have the idea that navigators on ships would do a sun sight at local noon to determine their longitude. Far as we know, they didn’t. Noon sight was just for latitude. People seem to think navigators watched the sun with a sextant to see when it was highest above the horizon, figuring that at that point it was directly south/north of them. They think they noted the Greenwich Time when that happened and looked in the almanac to see where the sun was scheduled to be, at that Greenwich Time. Two reasons why navigators wouldn’t do that: The sun’s altitude is changing so slowly at noon that if the navigator thinks it has hit maximum altitude, and it’s actually one arc minute below its maximum altitude, the calculated longitude will likely be off be a degree or more. That’s if the ship is stationary. If it’s steaming southwest at a good clip – 25 knots, say – your calculated longitude could easily be a degree off even if you time the maximum altitude perfectly. An old Bowditch includes a few pages laying out the navigator’s tasks for a day, starting with the morning twilight sights, then a sun sight for longitude, ideally when the sun is directly east of the ship. The noon sight for latitude, then another sun sight for longitude when the sun is near west of the ship. The noon longitude isn’t measured – it’s calculated from the morning sights. [Previous page](https://boards.straightdope.com/t/how-did-they-measure-lat-long/425530.md?page=4)