Error in obtaining twilight time for site

[simontorres]

I've found that issues #464 and #533 present a similar problem.
I'm using astroplan version 0.10.1
I'm running the following code for testing:

import astropy.units as u
from astropy.time import Time
from astropy.coordinates import EarthLocation
from astroplan import Observer

observatory = 'SOAR Telescope'

geodetic_location = ['-70d44m01.11s', '-30d14m16.41s', 2748]

longitude = geodetic_location[0]
latitude = geodetic_location[1]
elevation = geodetic_location[2]

timezone = 'UTC'

description = 'Soar Telescope on Cerro Pachon, Chile'


soar_loc = EarthLocation.from_geodetic(
    longitude,
    latitude,
    elevation * u.m,
    ellipsoid='WGS84')

soar = Observer(name=observatory,
                location=soar_loc,
                timezone=timezone,
                description=description)

date_obs = [
    '2024-12-08T19:12:58.281',
    '2024-12-08T19:14:00.814',
    '2024-12-08T19:14:13.016',
    '2024-12-08T19:14:25.218',
    '2024-12-08T19:14:37.423',
    '2024-12-08T19:14:49.627',
    '2024-12-08T19:15:01.826',
    '2024-12-08T19:15:14.025',
    '2024-12-08T19:15:26.223',
    '2024-12-08T19:15:38.421',
    '2024-12-08T19:15:50.620',
    '2024-12-08T19:16:02.819',
    '2024-12-08T19:26:51.024',
    '2024-12-08T19:27:01.016',
    '2024-12-08T19:27:10.980',
    '2024-12-08T19:27:20.945',
    '2024-12-08T19:27:30.939',
    '2024-12-08T19:27:40.904',
    '2024-12-08T19:27:50.873',
    '2024-12-08T19:28:00.841',
    '2024-12-08T19:28:10.807',
    '2024-12-08T19:28:20.773',
    '2024-12-08T19:31:11.133',
    '2024-12-08T19:32:03.088',
    '2024-12-08T19:32:14.580',
    '2024-12-08T19:32:26.074',
    '2024-12-08T19:32:37.592',
    '2024-12-08T19:32:49.092',
    '2024-12-08T19:33:00.585',
    '2024-12-08T19:33:12.073',
    '2024-12-08T19:33:23.569',
    '2024-12-08T19:33:35.067',
    '2024-12-08T19:35:16.249',
    '2024-12-08T19:37:07.451',
    '2024-12-08T19:39:44.707',
    '2024-12-08T19:41:30.004',
    '2024-12-08T19:41:55.606',
    '2024-12-08T19:44:42.483',
    '2024-12-08T19:45:43.977',
    '2024-12-08T19:45:59.987',
    '2024-12-08T19:46:15.949',
    '2024-12-08T19:46:31.938',
    '2024-12-08T19:46:47.928',
    '2024-12-08T19:47:03.930',
    '2024-12-08T19:47:19.913',
    '2024-12-08T19:47:35.907',
    '2024-12-08T19:47:51.902',
    '2024-12-08T19:48:07.926',
    '2024-12-09T00:36:05.258',
    '2024-12-09T00:42:48.890',
    '2024-12-09T00:45:59.575',
    '2024-12-09T00:49:47.609',
    '2024-12-09T00:51:15.288',
    '2024-12-09T00:54:38.605',
    '2024-12-09T01:02:21.778',
    '2024-12-09T01:13:07.968',
    '2024-12-09T01:23:34.241',
    '2024-12-09T01:35:06.785',
    '2024-12-09T01:43:37.992',
    '2024-12-09T01:45:02.019',
    '2024-12-09T02:04:56.958',
    '2024-12-09T02:15:06.920',
    '2024-12-09T02:26:06.814',
    '2024-12-09T02:32:23.438',
    '2024-12-09T02:42:33.399',
    '2024-12-09T02:52:43.364',
    '2024-12-09T03:03:35.575',
    '2024-12-09T03:09:17.902',
    '2024-12-09T03:15:02.153',
    '2024-12-09T03:20:44.887',
    '2024-12-09T03:32:59.756',
    '2024-12-09T03:38:22.151',
    '2024-12-09T03:44:21.035',
    '2024-12-09T03:50:33.126',
    '2024-12-09T03:55:54.897',
    '2024-12-09T04:01:39.901',
    '2024-12-09T04:07:31.277',
    '2024-12-09T04:17:57.604',
    '2024-12-09T04:19:47.582',
    '2024-12-09T04:22:57.544',
    '2024-12-09T04:31:11.036',
    '2024-12-09T04:41:21.359',
    '2024-12-09T04:51:37.185',
    '2024-12-09T05:02:19.628',
    '2024-12-09T05:08:44.461',
    '2024-12-09T05:18:54.425',
    '2024-12-09T05:29:43.797',
    '2024-12-09T05:35:50.238',
    '2024-12-09T05:46:00.214',
    '2024-12-09T05:56:16.908',
    '2024-12-09T06:07:08.788',
    '2024-12-09T06:10:44.439',
    '2024-12-09T06:11:54.360',
    '2024-12-09T06:12:44.865',
    '2024-12-09T06:21:21.403',
    '2024-12-09T06:26:31.429',
    '2024-12-09T06:32:43.745',
    '2024-12-09T06:38:25.124',
    '2024-12-09T06:43:51.164',
    '2024-12-09T06:49:01.142',
    '2024-12-09T06:54:58.791',
    '2024-12-09T07:03:49.460',
    '2024-12-09T07:08:59.467',
    '2024-12-09T07:14:41.059',
    '2024-12-09T07:25:59.400',
    '2024-12-09T07:36:09.377',
    '2024-12-09T07:46:19.382',
    '2024-12-09T07:58:00.468',
    '2024-12-09T07:59:00.468']

for _t in date_obs:
    print(f"{Time(_t)} Evening: {soar.twilight_evening_astronomical(Time(_t), which='nearest').isot} Morning: {soar.twilight_morning_astronomical(Time(_t), which='nearest').isot}")

And I obtain the following output, I removed some of them for brevity:

2024-12-08T19:12:58.281 Evening: 2024-12-09T01:12:33.691 Morning: 2024-12-08T07:57:58.333
2024-12-08T19:14:00.814 Evening: 2024-12-09T01:12:33.739 Morning: 2024-12-08T07:57:58.146
...
...
2024-12-09T00:51:15.288 Evening: 2024-12-09T01:12:33.764 Morning: 2024-12-09T07:57:59.287
2024-12-09T00:54:38.605 Evening: 2024-12-09T01:12:33.682 Morning: 2024-12-09T07:57:59.567
2024-12-09T01:02:21.778 Evening: 2024-12-09T01:12:33.505 Morning: 2024-12-09T07:57:59.343
2024-12-09T01:13:07.968 Evening:                     ——— Morning: 2024-12-09T07:57:59.620
2024-12-09T01:23:34.241 Evening: 2024-12-09T01:12:33.691 Morning: 2024-12-09T07:57:59.560
2024-12-09T01:35:06.785 Evening: 2024-12-09T01:12:33.718 Morning: 2024-12-09T07:57:59.270
2024-12-09T01:43:37.992 Evening: 2024-12-09T01:12:33.774 Morning: 2024-12-09T07:57:59.368
2024-12-09T01:45:02.019 Evening: 2024-12-09T01:12:33.672 Morning: 2024-12-09T07:57:59.278
...
...
2024-12-09T07:36:09.377 Evening: 2024-12-09T01:12:33.771 Morning: 2024-12-09T07:57:59.298
2024-12-09T07:46:19.382 Evening: 2024-12-09T01:12:33.689 Morning: 2024-12-09T07:57:59.309
2024-12-09T07:58:00.468 Evening: 2024-12-09T01:12:33.629 Morning:                     ———
2024-12-09T07:59:00.468 Evening: 2024-12-09T01:12:33.791 Morning: 2024-12-09T07:57:59.451

There are these two points where the output is not correct, they seem to be random, in my application I use the first and the last value to get the corresponding twilight for evening and morning. In this case the last value was failing so I just copied it and added one minute and a a proper value is obtained.

Thanks in advance!

[pavolgaj]

The problem is probably caused by the fact that for these times the Sun's altitude is very close to -18deg (the limit for astronomical twilight).

Running soar.sun_altaz(Time('2024-12-09T07:58:00.468')).alt gives altitude of -17.998091718201977 deg.

However, soar.twilight_morning_astronomical(Time('2024-12-09T07:58:00.468'),which='previous').isot works fine. But, which='next' fails. In the case of which='nearest', both values are calculated and compared according to the given time.

Due to the change in the day length, the next morning twilight is 2024-12-10T07:58:01.967 which is 24 hours + 1.5 seconds later. Also, this warning tells something similar:

WARNING: TargetNeverUpWarning: Target with index 0 does not cross horizon=-18.0 deg within 24 hours

The issue can be fixed using the only values not masked in comparison - soar.twilight_morning_astronomical(Time('2024-12-09T07:58:00.468'),which='next').masked is True. Probably here in astroplan code:

astroplan/astroplan/observer.py

Line 1032 in 0968b73

if which == 'nearest':

.