[AstroPy] Effect of observer elevation on sunset time

[Sergio Pascual]

Hello,

I don't have my copy of Green's Spherical Astronomy with me, but I remember
there is as section about the effect of height on topocentric coordinates,
and hence, in sun set  and rise times. The affect is about a few minutes,
depending on height and latitude.

Regards

El sáb., 9 may. 2020 a las 20:25, Stuart P Littlefair (<
s.littlefair at sheffield.ac.uk>) escribió:

> 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
>>
>> _______________________________________________
>> AstroPy mailing list
>> AstroPy at python.org
>> https://mail.python.org/mailman/listinfo/astropy
>>
>
>
> --
> Adrian M. Price-Whelan
> Flatiron Institute, NYC
> http://adrn.github.io
> (he / him)
> _______________________________________________
> AstroPy mailing list
> AstroPy at python.org
> https://mail.python.org/mailman/listinfo/astropy
>
> _______________________________________________
> AstroPy mailing list
> AstroPy at python.org
> https://mail.python.org/mailman/listinfo/astropy
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/astropy/attachments/20200509/926dfdfe/attachment.html>