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Is it possible to calculate when dawn/dusk will be if I have the time for sunrise, sunset and the longitude and latitude of a particular location?

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  • I've been able to google how to calculate sunrise and sunsets but haven't been able to find how to long dawn and dusk will be :/ – Siva Nov 19 '14 at 19:14
  • Which definition of dawn do you want? [Wikipedia Dawn](http://en.wikipedia.org/wiki/Dawn). – Andrew Morton Nov 19 '14 at 19:19
  • The duration of dawn and dusk varies according to the time of year because they're affected by the angle that the sun moves relative to the horizon. When the angle is high, dawn and dusk will be shorter. – Alnitak Nov 19 '14 at 19:19
  • @Strawberry right, except that your description makes no sense - it's 12 or 18 degrees _below the horizon_, not _from your position_. Hence my point above, which is that with the sun taking 2 minutes to move its own diameter, the amount of time it takes to clear those 12 or 18 degrees varies depending on how close the sun rises to "vertically". – Alnitak Nov 19 '14 at 19:24
  • @Sivapriyan do you happen to have the ability to calculate the azimuth and altitude of the sun at any arbitrary location and time? – Alnitak Nov 19 '14 at 19:30
  • @Alnitak Unfortunately no, but I have the exact location of the place. I would assume there is a formula that would be able to achieve this. – Siva Nov 19 '14 at 21:09
  • This question appears to be off-topic because it is about definitions used in astronomy rather than about programming. – High Performance Mark Nov 20 '14 at 22:19

1 Answers1

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there are some simplified equations for that but the real thing is this:

if you want easy solution then obtain sunrise calendar for your location

it should look something like this:

sun calendar

Just pick the closest latitude or generate the right diagram for your location. The picture was taken from here so there should be also the generation applet somewhere. Can't find any image of the astronomy sun calendar on the WEB so here is scan from really old book of mine:

astro sun calendar

Booth are the same thing but the scan is just for single long/lat geo-location.

  • x axis is the date

  • y axis is the time sun reach (sun rise/set or dusk/dawn,...)

  • Noc means night and Den means day

  • all the marks are unimportant for you

  • dash dash line is sun rise/set (leading visual edge at mathematical horizont)

  • dash dot line is the civil twilight (visual center of sun is 6 deg below horizont)

  • filled line is astronomical twilight (visual center of sun is 18 deg below horizont)

    Diagram is for latitudes 48,49,50 deg, nautical twilight (visual center of sun is 12 deg below horizon) is not on the image. When you approximate this curve by polynomial or piecewise polyline then you can easily compute what you need per any date unless you want to use this for thousands of years then you can take this as periodic function so you need just one year table and all the other years are the same.

[Notes]

visual

  • means geometric position + atmospheric refraction + aberation + precession/nutation + speed of light
  • near horizon is the difference little more then 0.5 degree !!! (one Sun disc)
  • and the time difference is around 8 minutes
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