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When was twilight officially defined?

  1. Aug 1, 2016 #1
    Does anyone know when the twilights (civil, nautical, astronomical) were officially scientifically defined?

    My internet searches have come up with nothing. Any suggestions appreciated!
     
  2. jcsd
  3. Aug 1, 2016 #2
    I think that you must define "scientifically defined" before anyone can give you an answer. Even today, different countries have different definitions of such common terms as sunrise, sunset, and twilight. "Science" made its way into different cultures at different times in history, and with greater and lesser degrees of acceptance. Even today, there are countries and cultures where scientific definitions of times of day must legally yield to religious ones.

    An approximate answer might be the general (but by no means universal) adoption of the International Standard Units of Measurement in 1960. Some might argue that it dates from the Metre Convention of 1875. In all likelihood, what you are looking for does not exist.
     
  4. Aug 1, 2016 #3
    There are standard definitions for civil twilight (geometric center of the Sun is 6° below the horizon), nautical twilight (Sun is between 6° and 12° below the horizon) and astronomical twilight(Sun is between 12° and 18° below the horizon). Surely there must be a record of when these standards were first adopted?
     
  5. Aug 1, 2016 #4
    The difficulty is that there exists no single organization that pronounces on what is and is not “scientifically acceptable” and whose pronouncements are generally accepted. Instead, we have a wide variety of national and sub-national academic organizations whose definitions have varied over the years ever since the founding of the Royal Society around 1660. No single universally-accepted organization has been give the authority to define what is scientifically meant by “sunrise, sunset, twilight, etc.”. Consequently, we are left to the mercies of lesser authorities.

    Yes, there are “standard definitions” that you can find on the internet and in any number of astronomical textbooks and navigational guides. But who said that they were “standard” and when did they say it. That, of course, is what you want to know; but it is my position that the question has no meaningful answer.

    I think that you will find that these “standards” that you enumerated far predate any attempt at universal standards. I believe that they were first proposed by some European academic society or other and then gradually adopted by other academicians using similar astronomic and navigational guides. I would start with the Royal Society, as the eldest of the bunch.

    In any case, the argument is largely academic. When navigators want to determine their location, they don’t really care how far the sun is below the true horizon. As long as they can get a fix on the apparent horizon and the star of their choosing, they are generally content. The exact position of the sun below the horizon at any given time has little practical value and only minor academic interest.
     
  6. Aug 1, 2016 #5
    Firstly, civil twilight is often used legally to define "the hours of darkness" and, in the UK at least, lighting up time. That's "of minor interest", is it?

    Secondly, it may be of little interest to you, but you have absolutely no idea why I am looking for this information and what my interest is in it. I'm at a loss to understand why you seem intent on belittling my request for someone who knows something about this area to help me out.
     
  7. Aug 1, 2016 #6

    jim mcnamara

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    Staff: Mentor

    klimatos is trying to say something. He is not in any way belittling your position. If you have to know, you will have to accept it on a per-country basis. There is no single world wide authority on this. If you do not like this answer please do not claim I am trying to denigrate your position. That is not helpful at all. Nor is it correct.

    Let's pretend you are in the US. Then, consider:
    The USNO (Naval Observatory which works in concert with Britain's similar agency) defines time and standards that are used in many agencies

    Example: the stratus 0 clocks are maintained by US Naval Observatory. These are used to synchronize computer time in many, many places.
    You seem to be asking ask when exactly did the current definition come into effect? You do realize this has changed over historical time. As your question stands I cannot tell what you really need, exactly.

    Start here: http://www.usno.navy.mil/USNO
    Publication of things like this here have an effective date - meaning that was when the definition went into effect. You want 'Astronomical Applications'. I think.

    Twilight is measured as a position of the sun: based on the sun angle, local time, latitude, longitude, local sun time (sunrise & sunset times). JPL maintains a long ephemeris :
    https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris
     
    Last edited: Aug 1, 2016
  8. Aug 1, 2016 #7
    Thank you both for your replies. I seem not to be making myself clear, for which I apologise. What I'm after is someone who knows the history of the terminology in this area, like a scientific historian. For example, if I asked when the plum pudding model of the atom was first proposed, people would say it was around the early 1900s.

    For instance, I've now discovered that the definition of astronomical twilight as "the Sun being 18° below the horizon" can be found in Chaucer's "Treatise on the Astrolabe”, written in the 14th century. Chaucer apparently used the same definition as the Jewish-Persian astronomer Mashallah ibn Athari, who wrote a description and directions for use of the astrolabe back in the 8th century. And as the earliest astrolabe was probably made by Hipparchus in around 150BC, it's possible that this definition goes all the way back to Ancient Greece. So I now have an answer for astronomical twilight.

    Nautical twilight used to be regarded as the point when a ship's mast was visible on the horizon, but over time this was replaced by the definition we use today. And at some point we started using the concept of civil twilight as well. What I'm asking is whether anyone knows roughly when, just by century if necessary, people started using the two definitions that we use today, along the lines of the answer I've now found for astronomical twilight. Or failing that, just the terms themselves. If I must attach a country to it, then it could be England or the US. All I've found out so far is that nautical and civil twilight were first officially tabulated in the UK in 1937, 12 years after astronomical twilight, but the concepts themselves may go back much further than that.

    I hope this makes my question clearer.
     
  9. Aug 2, 2016 #8

    jim mcnamara

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    Staff: Mentor

    You do need a historian, and probably one who specializes in astronomy. I cannot help you much. In fact this question probably belongs in another forum.
    However consider this as an anecdote:
    Keres speaking Pueblos in the US Southwest have/had good astronomers for as long time. Read about Fajada butte here:
    https://www.exploratorium.edu/chaco/HTML/fajada.html
    One practice involves the spirits of the dead. Between the autumnal equinox and the following midpoint (samhain, dziady, All Souls, Dia de los Muertos) which corresponds best to Halloween in the American tradition, the spirits are out at night. You really do not want to be outside then. Night is defined by the end of twilight. It ends with the start of twilight before dawn. I'm not clear on how twilight is defined precisely.

    The point being twilight and various definitions transcend cultures, this tradition is pre-Columbian. The end of the period is celebrated widely in Albuquerque:
    http://muertosymarigolds.org/ You have a lot of digging to do, don't 'set your sights too close to home'.
     
    Last edited: Aug 2, 2016
  10. Aug 3, 2016 #9
    Nisse:

    I am glad to see that you have more clearly specified just what it is that you are looking for (post #7). That should help any readers of this thread who have the necessary expertise to assist you in your efforts. I wish you luck, but I agree with Jim McNamara in saying that you need an historian, not a physicist.

    A small quibble: “Lighting up time” (post #5) in the UK has nothing to do with the position of the sun below the horizon and everything to do with the times of sunset and sunrise as defined by the local authorities. Here in California, even those times give only part of the picture. We must use our headlights (not just the “running” lights) whenever our windshield wipers are in operation.

    An anecdote: Readers who have driven (or ridden) automobiles after dark in the Middle East can testify that drivers extinguish all automotive lights (including headlights) when approaching or passing another vehicle. This action is motivated by local concepts of courtesy and the desire not to blind oncoming drivers. This sudden darkness can induce cases of severe sphincter-tightening on narrow two-lane roads on the part of participants who do not share Middle Eastern fatalistic beliefs. Westerners do not find expressions such as, “We are all in the hands of God” to be particularly comforting.
     
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