Given the latitude and longitude, how can I tell if it's daylight?

Question

I'm writing a Python program that needs to determine whether it's post-sunrise or not, given only the current UTC time and the target latitude and longitude. I see apps do this sort of thing all the time, but I have no idea how it's done. Any ideas?

That was the first link I found, but it wasn't enough for someone with limited calculus background like me to work with. — Daniel Quinn, Sep 12 '11 at 10:04
related: [Calculating dawn and sunset times using PyEphem](http://stackoverflow.com/q/2637293/4279) — jfs, Nov 07 '15 at 06:46
related: [Sunrise / set calculations](http://stackoverflow.com/q/2538190/4279) — jfs, Nov 07 '15 at 06:47

score 4 · Accepted Answer · answered Sep 12 '11 at 09:17

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Here's someone's implementation of sunrise/sunset calculation in Python: http://michelanders.blogspot.com/2010/12/calulating-sunrise-and-sunset-in-python.html

answered Sep 12 '11 at 09:17

Acorn

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smulholland2 · Answer 2 · 2011-09-12T09:07:24.030

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Just doing a quick google search I found this. The calculations in the NOAA Sunrise/Sunset and Solar Position Calculators are based on equations from Astronomical Algorithms, by Jean Meeus. The sunrise and sunset results have been verified to be accurate to within a minute for locations between +/- 72° latitude, and within 10 minutes outside of those latitudes.

A detailed explanation of the calculation details can be found from that page and also here.

edited Sep 12 '11 at 09:07

answered Sep 12 '11 at 08:55

smulholland2

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Single-link answers can **easily** bit-rot. You should *at least* provide a short summary of the link in your answer. – Joachim Sauer Sep 12 '11 at 08:57
Posters are also supposed to have at least run their questions through a search engine. I was trying to relay that requirement in as helpful a manner as I could. – smulholland2 Sep 12 '11 at 09:01
If you think that the question is bad, then a bad answer is *not* the appropriate response. – Joachim Sauer Sep 12 '11 at 09:01
Actually, I'd run into that link in my own search, and you're right it's exactly what I needed, just in Javascript. I was hoping that there was a Python module out there for something like this rather than having to rewrite it based on that. – Daniel Quinn Sep 12 '11 at 10:03

score 1 · Answer 3 · answered Sep 12 '11 at 08:57

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A complete layout of the algorithm is here

Not a lot to add really. The 10 points in the document quoted break it down into simple to reproduce steps that can be implemented in any language really.

answered Sep 12 '11 at 08:57

nimbusgb

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score 0 · Answer 4 · answered Feb 10 '12 at 22:42

You need something that just works™? Use the superior language. And you know it'll always work. How can you trust 122 lines of advanced astrophysics? You can't. But you can always trust PHP.

>>> def is_day(lat, lon): # optinally adjust zenith (currently 96) at the end of the line
...   return subprocess.check_output(["php","-r","""date_default_timezone_set("GMT");\n$a=date_sunrise(time(),SUNFUNCS_RET_DOUBLE,{0},{1},{2},0);\n$b=date_sunset(time(),SUNFUNCS_RET_DOUBLE,{0},{1},{2},0);\n$t=date("H")+date("i")/60+date("s")/3600;\necho($a<$b?($t>$a&&$t<$b):($t>$a||$t<$b))?"day":"night";""".format(lat,lon,96)])=="day"
...   #                                                                                                                                                                                                                                                                                                                             ^^ optionally adjust zenith here
... 
>>> is_day(-33,151) # Sydney
True
>>> is_day(0,0) # Some hundred km off the coast of Africa
False
>>> __import__("datetime").datetime.utcnow()
datetime.datetime(2012, 2, 10, 22, 29, 30, 365019)
>>>

Given the latitude and longitude, how can I tell if it's daylight?

4 Answers4