Calculate daytime hours based on location

[vincentneo]

Hi,

I am currently trying to implement a feature in my app that calculates the daytime hours at a specific coordinate and date. Is it possible to do so using this library? I'm not really good with astronomy stuff and algorithms, hence, I am not sure if this is the right tool for the job, or that its hidden somewhere, and I couldn't find it.

Thank you in advance!

[onekiloparsec]

Hi. Yes, you can use SwiftAA for that. Grab the Earth object, and compute the twilights times. You will get precise times of sunrise, sunset etc. You must choose the precise altitude of the sun (you may start with 0):

For instance:

let paris = GeographicCoordinates(positivelyWestwardLongitude: Degree(.minus, 2, 21, 07), latitude: Degree(.plus, 48, 51, 24))
let jd = JulianDay(year: 2017, month: 6, day: 21)
let times = Earth(julianDay: jd).twilights(forSunAltitude: 0, coordinates: paris)

[vincentneo]

Alright! Thank you

[vincentneo]

Hi @onekiloparsec

So I tried with this code:

let singapore = GeographicCoordinates(CLLocation(latitude: 1.290, longitude: 103.852))
let today = JulianDay(year: 2020, month: 9, day: 6)
let todayTimes = Earth(julianDay: today).twilights(forSunAltitude: 0, coordinates: singapore)

let timeFormatter = DateComponentsFormatter()
timeFormatter.unitsStyle = .short
timeFormatter.allowedUnits = [.hour, .minute, .second]
timeFormatter.allowsFractionalUnits = false

print(timeFormatter.string(from: todayTimes.set!.date, to: todayTimes.rise!.date)!) 
// 11 hr, 58 min, 43 sec

and noticed that the daytime hours to be a little off, compared to https://www.timeanddate.com/sun/@1.290,103.852, which gave 12:07:37 instead of 11:58:43.

Did I do something wrong, or are the values online inaccurate?

Thank you.

[onekiloparsec]

Hello Vincent. You did perfectly correct, but it all depends on the details. For instance, for the standard sunAltitude (ArcMinute(-50).inDegrees in the Sun class), you get a result that is 6 minutes different (11 hr, 52 min, 1 sec).

It is hard to say which one is correct. I have no idea how timeanddate.com compute their values. It would be nice to discuss with them and compare (there is no reason at all theirs should be "wrong").

Please note that due to the recent issue #95 , I have made 2 changes that will impact you.

• I have homogenized return times of twilights and rise, transit, set times. Hence you will need to use .riseTime! instead of .rise! for example.
• A Swift Playground is added to the project, and you'll be able to play directly inside it.

[vincentneo]

Alright then, thanks for the guidance!

[vincentneo]

Looks like if I were to use TwilightSunAltitude.upperLimbOnHorizonWithRefraction or -0.833, I would get similar results from online sources, just about 1 minute away.

[onekiloparsec]

Wonderful, thanks for sharing. I'll try to add another unit test then to validate that in the code.