# Definition of Dawn in Age of Sail
**URL:** https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547
**Category:** Factual Questions
**Created:** [April 30, 2017, 1:37am UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547 "2017-04-30T01:37:19Z")
**Posts on this page:** 11
**Page:** 1
### Author:  [@Trinopus](https://boards.straightdope.com/u/Trinopus)
#### Post date: [April 30, 2017, 1:37am UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/1 "2017-04-30T01:37:19Z")
I’m reading a bunch of lovely “Great Age of Nelson” fiction series – the Lewrie, Kidd, Ramage, and, of course, Hornblower books.
At some point, in some of these (and I can’t find it now!) “dawn” is defined. Something like the moment you can see another sail at half a mile. Something like that.
Can anybody help me?
(The Koran defines it as the moment you can tell a white thread from a black thread. That’s kind of fun to try, really early in the morning!)
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [April 30, 2017, 12:30pm UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/2 "2017-04-30T12:30:09Z")
I think that Jews (or at least, some Jews) use the thread thing, too. Except that in the Jewish calendar, days start at sundown, so it’s more about when you can’t tell them apart, than when you can.
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### Author:  [@bob\_2](https://boards.straightdope.com/u/bob_2)
#### Post date: [April 30, 2017, 1:06pm UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/3 "2017-04-30T13:06:15Z")
I am not sure that “dawn” has an exact definition, even in the RN. They would be much more likely to refer to “sunrise” which happens at an exact time, according to your latitude and longitude and that can be calculated or looked up in a table.
“Dawn” could also be “first light” which is likely to be half an hour to an hour before sunrise.
Edit - You might also like to read O’Brian - the acknowledged master of the RN in Napoleonic times.
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### Author:  [@Xema](https://boards.straightdope.com/u/Xema)
#### Post date: [April 30, 2017, 1:06pm UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/4 "2017-04-30T13:06:59Z")
> [@Trinopus](#):
>
> Something like the moment you can see another sail at half a mile.
This sounds notably vague:
- Distance would have been hard to judge precisely
- Sail visibility depends a lot on the size of the vessel
- And on whether the sail is east or west of you
- You rarely have a sail at exactly half a mile just when the sun is preparing to rise
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### Author:  [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)
#### Post date: [April 30, 2017, 3:10pm UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/5 "2017-04-30T15:10:27Z")
This [Twilight - Wikipedia](https://en.wikipedia.org/wiki/Twilight) includes the modern definition of nautical dawn. I know that’s not what the OP asked for, but the modern definition is \*probably \*a refinement of earlier informal or semi-formal practice.
I’d also be wary of taking works of fiction, even contemporary fiction, as authoritative.
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### Author:  [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)
#### Post date: [April 30, 2017, 3:30pm UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/6 "2017-04-30T15:30:00Z")
> [@LSLGuy](#):
>
> This [Twilight - Wikipedia](https://en.wikipedia.org/wiki/Twilight) includes the modern definition of nautical dawn. I know that’s not what the OP asked for, but the modern definition is \*probably \*a refinement of earlier informal or semi-formal practice.
>
> I’d also be wary of taking works of fiction, even contemporary fiction, as authoritative.
A little further down on that wiki page, “nautical twilight” is described as enough light to see the horizon, but not so much light that stars aren’t visible, allowing navigation by star siting.
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### Author:  [@Trinopus](https://boards.straightdope.com/u/Trinopus)
#### Post date: [April 30, 2017, 3:43pm UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/7 "2017-04-30T15:43:31Z")
> [@LSLGuy](#):
>
> This [Twilight - Wikipedia](https://en.wikipedia.org/wiki/Twilight) includes the modern definition of nautical dawn. I know that’s not what the OP asked for, but the modern definition is \*probably \*a refinement of earlier informal or semi-formal practice.
>
> I’d also be wary of taking works of fiction, even contemporary fiction, as authoritative.
Authoritative, perhaps not, but most of these novels are VERY well-researched. The grand-daddy of 'em all, the Hornblower novels, set an incredibly high standard, and most (!) of the authors who have followed have lived up to it.
I guess it’s an excuse to re-read the novels I’ve read so far… Thank’ee, all!
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### Author:  [@bob\_2](https://boards.straightdope.com/u/bob_2)
#### Post date: [April 30, 2017, 5:25pm UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/8 "2017-04-30T17:25:04Z")
I have been doing some reading. As you are probably aware, the Naval day begins at midday and the routine of the ship is based on that time. Dawn might be at 6am or 8am, but if the routine says that decks will be swabbed at 4 bells in the morning watch (0600), that’s when they get swabbed - daylight or not.
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### Author:  [@Trinopus](https://boards.straightdope.com/u/Trinopus)
#### Post date: [April 30, 2017, 6:07pm UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/9 "2017-04-30T18:07:08Z")
Another interesting one that gets mentioned by more than one author is that the ship (warship) is readied for action shortly before dawn, so as not to be surprised if the light reveals an enemy close at hand.
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### Author:  [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)
#### Post date: [April 30, 2017, 9:12pm UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/10 "2017-04-30T21:12:46Z")
That practice is still used by infantry today. Both in the defense and on the attack. Many’s the fire base or encampment that’s been greeted by attackers approaching the wire at the first hints of first light. Best to be ready.
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### Author:  [@Malleus\_Incus\_Stapes](https://boards.straightdope.com/u/Malleus_Incus_Stapes)
#### Post date: [May 1, 2017, 2:00am UTC](https://boards.straightdope.com/t/definition-of-dawn-in-age-of-sail/785547/11 "2017-05-01T02:00:49Z")
> [@Chronos](#):
>
> I think that Jews (or at least, some Jews) use the thread thing, too. Except that in the Jewish calendar, days start at sundown, so it’s more about when you can’t tell them apart, than when you can.
There are observances that can only be done before or after sun_rise_, so when you can tell them apart applies, too.
IIRC, it’s when you can (or can’t) tell blue from green. So Jewish dawn would be later than Muslim dawn, I guess.