[AstroPy] Effect of observer elevation on sunset time

[Stuart P Littlefair]

I should probably think about this more before I embarrass myself but - isn’t this right? 

The sun is 93 million miles away so a tangential movement of a few thousand metres is going to make a negligible angular distance, no?

Is the apparent difference in sunset times at high altitudes more of a refraction effect?

Stuart Littlefair
Dept. of Physics & Astronomy
Univ. of Sheffield, Sheffield, S3 7RH

email: s.littlefair at shef.ac.uk
Phone: +44 114 2224525
Sent from my iPhone

> On 9 May 2020, at 19:13, Adrian Price-Whelan <adrianmpw at gmail.com> wrote:
> 
> 
> Hi Eric,
> 
> I'm not sure yet what the root of the problem is, but it doesn't seem to be an astroplan issue - see the bottom of this notebook, which computes the difference in Sun altitude for the two elevations with astropy's AltAz frame (used internally by astroplan):
> https://gist.github.com/628c6ffed4f652d2278e970981f67854
> 
> Erik T. might have more thoughts?
> 
> best,
> Adrian
> 
>> On Sat, May 9, 2020 at 11:31 AM Eric Jensen <ejensen1 at swarthmore.edu> wrote:
>> Hi all, 
>> 
>> I was just working with astroplan to calculate some sunset times (actually nautical twilight in the example below), and I noticed that specifying the observatory elevation doesn’t seem to make any difference in the results, contrary to my expectations. 
>> 
>> Example code: 
>> 
>> from astropy.coordinates import EarthLocation
>> import astroplan
>> from astropy.time import Time
>> import astropy.units as u
>> 
>> location = EarthLocation.from_geodetic(-16.5097*u.deg, 
>>                                        28.3*u.deg, 
>>                                        2390*u.m)
>> # Same location, but zero elevation
>> location_sealevel = EarthLocation.from_geodetic(-16.5097*u.deg, 
>>                                                 28.3*u.deg, 
>>                                                 0*u.m)
>> teide = astroplan.Observer(location=location, 
>>                            name="Teide", 
>>                            timezone="Atlantic/Canary")
>> teide_sealevel = astroplan.Observer(location=location_sealevel, 
>>                                     name="Teide sea level", 
>>                                     timezone="Atlantic/Canary")
>> 
>> now = Time.now()
>> n = 1000
>> sun_set = teide.sun_set_time(now, which="next", 
>>                              horizon=-12*u.deg,
>>                              n_grid_points=n)
>> sun_set_sealevel = teide_sealevel.sun_set_time(now, which="next", 
>>                                                horizon=-12*u.deg,
>>                                                n_grid_points=n)
>> print("Sunset at altitude:  {0.iso}, JD: {0.jd}".format(sun_set))
>> print("Sunset at sea level: {0.iso}, JD: {0.jd}".format(sun_set_sealevel))
>> print("Difference: {}".format((sun_set - sun_set_sealevel).to(u.s)))
>> 
>> This yields the output:
>> 
>> Sunset at altitude:  2020-05-09 20:41:33.933, JD: 2458979.3621982955
>> Sunset at sea level: 2020-05-09 20:41:33.933, JD: 2458979.3621982983
>> Difference: -0.00024139881134033203 s
>> 
>> i.e. basically no difference.   There are three reasons I could think of: 
>> 
>> 1.  The effect of observer elevation simply isn’t implemented in astroplan. 
>> 2.  There’s something wrong with my code. 
>> 3.  There’s something wrong with my thinking that there should be a few minutes difference (later sunset / earlier sunrise) at a few thousand meters elevation vs. at sea level. 
>> 
>> My guess is #1, but I’m curious to hear if others have different thoughts.  I’m using astropy 4.0.1 and astroplan 0.6. 
>> 
>> Thanks, 
>> 
>> Eric
>> 
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> 
> 
> -- 
> Adrian M. Price-Whelan
> Flatiron Institute, NYC
> http://adrn.github.io
> (he / him)
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