Talk:Twilight

Civil, Nautical, & Astronomical - What are the Reasons?

What is the purpose for the three different "versions" to twilight? The article mentions that nautical twilight is used in military planning; but is that it's origin? If so, what about the other two? Any addition of this info would greatly help both me, and the article in general. — Preceding unsigned comment added by 97.124.76.228 (talk) 19:23, 4 May 2013 (UTC)

Civil twilight is bright enough that normal daytime activities can be performed without additional lighting. For example, a car can be driven safely without using its headlights. Many jurisdictions have laws that compel drivers to turn their headlights on at civil dusk. Defining the time when this occurs is therefore necessary for the implementation of these laws.

Nautical twilight is essentially bright enough to allow a ship to be navigated safely without relying on navigational aids such as lighthouses. In warfare, lighthouses cannot be relied on, so planners must ensure that activities will occur when nautical twilight is present.

Viewing very faint astronomical objects such as distant galaxies can be done only if the sky is almost perfectly dark. Knowing the times of astronomical dusk and dawn allows astronomers to plan their activities appropriately.

DOwenWilliams (talk) 20:00, 4 May 2013 (UTC)

The reference given is a US one. Are these 3definitions US only, or are they recognised internationally? I'vebeen a seaman for 40 years and never heard of them. In the UK lighting-up time is legally defined in terms of minutes after sunset, not solar elevation. The two methods will diverge widely during the year, especially in Shetland -User:Brunnian — Preceding unsigned comment added by 195.137.63.170 (talk) 08:01, 18 February 2014 (UTC)

Interesting question. As far as I can see from http://astro.ukho.gov.uk/nao/miscellanea/birs2.html, the British government uses the same definitions of the three kinds of twilight as we do here. There is also a period called "Hours of Darkness" which extends from 30 minutes after sunset to 30 minutes before sunrise. Laws concerning use of car headlights, etc., are written in terms of hours of darkness. DOwenWilliams (talk) 21:53, 18 February 2014 (UTC)

Beginning and end of the various twilights

Quote: "Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon." It seems like it begins in "the morning", lasts the whole day, and ends in the evening at a specified sun angle. Same with astronomical twilight.

It is not explicitly stated in the main sections for nautical twilight and astronomical twilight when they end in the morning and when the begin in the evening. It is possible for a reader like me to wonder whether nautical twilight in the evening starts at sunset or at civil dusk, etc.

I suggest that these problems can be solved by writing out fully in the sections for nautical and astronomical twilights when they begin and when they end.

Gbwi (talk) 12:22, 5 October 2013 (UTC)

Possible photo

Possible photo for twilight.

For possible inclusion in article.--Tomwsulcer (talk) 13:01, 22 December 2013 (UTC)

• Hello Tom. I also am a boomer (read your user page), and made a living as a photographer for the past 35 years. You have some nice photos on your user page, I enjoyed looking at them. In the case of this photo, in my opinion you have a "Dusk" photo. Hard to get because of the lighting. Also, the composition of the photo needs some editing. If you still have the original, balance the photo by coming lower, and showing the base of the garage. There will still be plenty of sky left to see. Also, spin the photo so the objexts sit at 90 degrees to better balance it, by simply making sure the base of the garage is perfectly horizontal. Then, put it in the Dusk discussion page, and we'll have another peek at it. Thanks-Pocketthis (talk) 17:53, 22 December 2013 (UTC)

• • Hey thanks Pocketthis. I deleted the original (actually in my computer trash bin but it would take me an hour to search through). Or maybe I'll try to redo the shot when lighting is similar outside. Cool you're a photographer; I'm only an amateur photographer but I carry my camera wherever I go and sometimes get some good shots. Good idea about Dusk page; I'll post on its talk page.--Tomwsulcer (talk) 18:50, 22 December 2013 (UTC) Btw just saw your user page. WOW -- great photos. You're a pro!--Tomwsulcer (talk) 18:52, 22 December 2013 (UTC) Also I'd like to post one of your photos on my user page here that is assuming you have no objections.--Tomwsulcer (talk) 19:07, 22 December 2013 (UTC)

• • • Post away my friend, I'm flattered you'd like to display a photo of mine. You're not so bad yourself. My photos these days are far from technologically accurate. I sold all my pro equipment when I retired and just take 'point and shoots' with my digital Leica. I still strive for composition, which is just as important as the technical aspects of a photo. In fact, I think composition may be more important. Happy Holidays.-Pocketthis (talk) 23:54, 22 December 2013 (UTC)

Shrunk the Gallery

I thought the gallery was getting out of control, so I deleted and replaced, and moved around some shots, and cut the Gallery to one line. Any of you whose photos didn't make the cut, I promise you that I removed at least as many of my own shots as others when editing. If you want to scream at me, this is the place...:-)→Pocketthis (talk) 17:52, 30 May 2015 (UTC)

Article contradicts itself on twilight definitions

Diagram shows twilight as...

Civil Twilight: Sun 0-6º below the horizon

Nautical Twilight: Sun 6-12º below the horizon

Astronomical Twilight: Sun 12-18º below the horizon

The text in the article shows twilight as...

Civil Twilight: Sun 0-6º below the horizon

Nautical Twilight: Sun 0-12º below the horizon

Astronomical Twilight: Sun 0-18º below the horizon

The diagram is right, not the text. See this article by Cambridge University.

http://www.ast.cam.ac.uk/public/ask/2445

See Bowditch's The American Practical Navigator, pp. 227-228 & table 1516, which agrees with the definition as given in the text. AstroLynx (talk) 14:16, 11 August 2015 (UTC)

Further note that the Cambridge web article cited above does not cite a reliable source but only WP (circular reasoning). AstroLynx (talk) 14:20, 11 August 2015 (UTC)

The American Practical Navigator is a good source you have there. It's possible that it's right. It shows twilight as...

Civil Twilight: Center of sun 0º50' - 6º below the horizon

Nautical Twilight: Center of sun 0º50' - 12º below the horizon

Astronomical Twilight: Center of sun 0º50' - 18º below the horizon

However, the Glossary of Marine Navigation which is put out by the same agency says differently.

http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf#22

“ civil twilight. . 1. The period of incomplete darkness when the upper limb of the sun is below the visible horizon, and the center of the sun is not more than 6° below the celestial horizon.”

http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf#82

“ nautical twilight. . 1. The time of incomplete darkness which begins (morning) or ends (evening) when the center of the sun is 12° below the celestial horizon. The times of nautical twilight are tabulated in the nautical twilight are tabulated in the Nautical Almanac; at the times given the horizon is generally not visible and it is too dark for marine sextant observations. See also FIRST LIGHT.”

http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf#9

“ astronomical twilight. . 1. The period of incomplete darkness when the center of the sun is more than 12° but not more than 18° below the celestial horizon. SEE ALSO CIVIL TWILIGHT, NAUTICAL TWILIGHT.”

Evidently, there is much confusion about the definitions (even among those who should know). Perhaps it is best to mention this in the article, the reader can then decide which definition s/he finds most practical. AstroLynx (talk) 11:21, 16 August 2015 (UTC)

That sounds like a good plan.

@Appple please sign your postings by typing four tildes at the end so that I know to whom I am responding. AstroLynx (talk) 08:06, 17 August 2015 (UTC)

The Astronomical Almanac may not make it clear, but Whitaker's Almanac does:

To assist observers, the durations of civil, nautical and astronomical twilight are tabulated at intervals of ten days. To find the beginning or end of any particular twilight, subtract or add the duration given to the tabulated times of sunrise or sunset.

We have a similar problem at Greenwich Mean Time. It contains the surprising statement that Greenwich Mean Time is different onshore and on shipboard. This is completely unreferenced. I haven't scrutinised the history, but this is almost certainly the work of one editor who has been removing references confirming that both versions are identical to UT1 to promote his POV on the topic. This is the same editor who said that British Summer Time (one hour in advance) was called Greenwich Mean Time and in the Gregorian calendar article that years which are exactly divisible by 400 are not leap years. If editors could go over there and restore order it would be appreciated. I hardly need to point out that "Whitaker's Almanac" is used as much by the general public as navigators or astronomers, and the Greenwich Mean times of nautical, astronomical, and civil twilight ("lighting - up time"), sunrise and sunset, moonrise and moonset and the phases of the moon given there and in the press and other almanacs (including "Foulsham's Old Moore's Almanac") correspond exactly with the UT1 times tabulated in the Astronomical Almanac. Both this and Whitaker's plus multiple reliable sources confirm that these are one and the same. 5.150.92.20 (talk) 09:15, 17 August 2015 (UTC)