SUSPENDED SUMMER ---------------
 John Pazmino
 NYSkies Astronomy Inc
 nyskies@nyskies.org 
 www.nyskies.org
 2016 September 4

Introduction
 ----------
    Observers have long mentioned a curious effect with the summer 
stars that occurs each fall. The topic comes up, for instance, at the 
NYSkies Astronomy Seminars when we discuss the summer and autumn sky. 
    Altho the days roll on thruout the fall, the sky maintains about 
the same aspect at nightfall for many weeks. It is clearly NOT caused 
by the daylight-standard time shift in autumn.  Astronomers who keep 
standard time all-year also experience the effect. 
    The prime example of this 'suspended summer' effect is the 
lingering of the Summer Triangle, becoming an 'Autumn Triangle'. Near 
yearend it finally scoots away into twilight. 
    I never knew a general name for this effect, not even from 
seasoned observers. When they speak about it, they describe it in many 
sentences. For this article I call it 'suspended summer' but there 
could e a crisper term on the street.. 

What happens
 ----------
    Note well that the effect is that the sky stays about stable in 
attitude when viewed at nightfall, NOT at a stipulated clock hour. 
That is, when the sky gets dark enough for observing, there's the 
Summer Triangle hanging up there week after week. 
    If nightfall came at the same clock hour all thru the year, as it 
does at the equator, there is no suspended summer effect. Nightfall is 
at about the same clock hour every day. At nightfall, at this same 
hour, the sky slides westward by one degree per day. 
    In autumn in the mid north latitudes, where still the bulk of 
observers are, the clock hour of nightfall is a bit earlier each 
night, by a couple minutes. This offsets the westward seasonal drift 
of the stars. It gets dark a little earlier to catch the stars before 
they slide westward, causing the suspended summer effect. 

Counter effect 
 ------------
    This lagging of the stars in autumn can not continue indefinitely. 
There must be an other part of the year when the stars seem to zoom 
away quicker at nightfall. The two cancel out over a year to complete 
the annual cycle of seasonal drift. 
    There is such a speed-up period in spring when Orion, as a typical 
winter constellation, seems to whoosh away within a few weeks. 
    In spring nightfall comes later each day and adds to the westward 
drift of the stars. The stars at nightfall are farther along in there 
seasonal movement, pulling the stars into twilight more quickly. 

Awareness
 -------
    Awareness of the two effects, with the autumn one far the more 
discussed, comes in the 21st century. It was strengthened by GLOBE at 
Night and New York City's autumn Milky Way sighting window. GaN uses 
Orion for assessing sky transparency.  In fall thru early spring Orion 
is conveniently placed in the evening and night sky. As spring 
approaches, Orion starts to scoot westward into twilight, ending its 
service for GLOBE at Night. 
    The Milky Way season is peculiar to New York. The Milky Way is 
routinely dismissed as a feature of the City's sky. For sure it never 
shows as the band across the sky. When the City's weather shifts from 
summer to winter and vice versa, the sky can be dark and clear enough 
to allow the brightest segments of the Milky Way to shine thru. We 
then spot a patch or two of the summer reach of the Milky Way. City 
observers keep watch of the sky in May-June and in September-November 
for the rare instance of a Milky Way sighting. 
    During the autumn window, while regularly inspecting the sky, 
astronomers witness the suspended summer effect. For the Milky Way In 
the spring, observations are far less frequent because the Milky Way's 
brightest segments are in the predawn sky. A counter effect of a 
speeded-up shift of stars is less noticeable. 

Nightfall
 -------
    Conventionally it gets dark enough after sunset for astronomy work 
when astronomical twilight ends. For the City, where the sky can not 
become as dark as that of a darksky location, full night is achieved 
when nautical twilight ends. Yet the sky, as described above, can 
under certain weather conditions allow the brightest parts of the 
Milky Way to be seen. otherwise, the sky is thoroly dark for general 
observing, even of deepsky targets. 
    I assembled the times in evening when nautical twilight ends as 
the time of nightfall in New York. 

Suspended stars
 -------------
    The effect is that the stars are in about the same place over the 
horizon at nightfall. Their altitude and azimuth stay about the same, 
or, what amounts to the same parameter, their arc of visibility from 
nightfall to setting is about the same. 
    For the autumn effect I picked Altair as a representative summer 
star, being one corner of the Summer Triangle. For the effect in 
spring i picked Procyon, one corner of the Winter Triangle. 
    By chance these two stars are about 12 hours apart in right 
ascension and near the celestial equator. Each rises more or less when 
the other sets. 

Calculation
 ---------
   With ephemeris software I produced three tables: time of evening 
nautical twilight, setting of Altair, setting of Procyon. A five-day 
step gives a good resolution to bring out the two effects.  All hours 
are standard time and may differ by a minute or two from other 
computation methods. 
    These tables I merged into the composite table below, with columns 
added for the visibility arc and the change of this arc from date to 
date. The star is worked on only when its visibility arc is less than  
07:00. It shrinks to 00:00 when the star sets before nautical 
twilight. 

    SKY SHIFT EFFECTS WITH ALTAIR AND PROCYON 
    -----------------------------------------
                 | Altair            | Procyon 
                 +-------------------+---------
     Date  Naut  | Set    Arc    Diff| Set   Arc    Diff 
     ---- -------+ -------- --- -----+---------  --- ---
    01 Jan 17:44 | 19:34  1:50  0:25 | 07:12  -----
    06 Jan 17:48 | 19:15  1:27  0:23 | 06:52  -----
    11 Jan 17:52 | 18:55  1:03  0:24 | 06:32  -----
    16 Jan 17:57 | 18:35  0:38  0:25 | 06:13  -----
    21 Jan 18:03 | 18:16  0:13  0:25 | 05:53  -----
    26 Jan 18:08 | 17:56  ----  ---- | 05:34  -----
    31 Jan 18:13 | 17:36  ----  ---- | 05:14  -----
    05 Feb 18:19 | 17:17  ----  ---- | 04:54  -----
    10Feb  18:24 | 16:57  ----  ---- | 04:35  -----
    15 Feb 18:30 | 16:37  ----  ---- | 04:15  -----
    20 Feb 18:36 | 16:18  ----  ---- | 03:55  -----
    25 Feb 18:41 | 15:58  ----  ---- | 03:36  -----
    02 Mar 18:46 | 15:38  ----  ---- | 03:16  -----
    07 Mar 18:52 | 15:19  ----  ---- | 02:56  -----
    12 Mar 18:57 | 14:59  ----  ---- | 02:37  -----
    17 Mar 19:03 | 14:39  ----  ---- | 02:17  07:14  --- 
    22 Mar 19:08 | 14:20  ----  ---- | 01:57  06:49  0:25 
    27 Mar 19:14 | 14:00  ----  ---- | 01:38  06:24  0:25 
    01 Apr 19:21 | 13:40  ----  ---- | 01:18  05:57  0:27
    06 Apr 19:25 | 13:21  ----  ---- | 00:58  05:33  0:24 
    11 Apr 19:32 | 13:01  ----  ---- | 00:39  05:07  0:26 
    16 Apr 19:38 | 12:41  ----  ---- | 00:19  04:41  0:26 
    21 Apr 19:44 | 12:22  ----  ---- | 23:57  04:13  0:28 
    26 Apr 19:50 | 12:02  ----  ---- | 23:36  03:46  0:27 
    01 May 19:57 | 11:42  ----  ---- | 23:16  03:19  0:27 
    06 May 20:03 | 11:23  ----  ---- | 22:56  02:53  0:26 
    11 May 20:10 | 11:03  ----  ---- | 22:37  02:27  0:26 
    16 May 20:16 | 10:43  ----  ---- | 22:17  02:01  0:26 
    21 May 20:22 | 10:24  ----  ---- | 21:57  01:35  0:26 
    26 May 20:28 | 10:04  ----  ---- | 21:38  01:10  0:25 
    31 May 20:33 | 09:44  ----  ---- | 21:18  00:45  0:25 
    05 Jun 20:37 | 09:25  ----  ---- | 20:58  00:21  0:24 
    10 Jun 20:41 | 09:05  ----  ---- | 20:39 -----   ----
    15 Jun 20:43 | 08:46  ----  ---- | 20:19  -----  ----
    20 Jun 20:45 | 08:26  ----  ---- | 19:59  ----   ----
    25 Jun 20:46 | 08:06  ----  ---- | 19:40  -----  ----
    30 Jun 20:45 | 07:47  ----  ---- | 19:20  -----  ----
    05 Jul 20:41 | 07:27  ----  ---- | 19:00  -----  ----
    10 Jul 20:39 | 06:07  ----  ---- | 18:41  -----  ----
    15 Jul 20:37 | 06:48  ----  ---- | 18:21  -----  ----
    20 Jul 20:33 | 06:28  ----  ---- | 18:02  -----  ----
    25 Jul 20:27 | 06:08  ----  ---- | 17:42  -----  ----
    30 Jul 20:21 | 05:49  ----  ---- | 17:22  -----  ----
    04 Aug 20:14 | 05:29  ----  ---- | 17:02  -----  ----
    09 Aug 20:07 | 05:09  ----  ---- | 16:43  -----  ----
    14 Aug 19:59 | 04:50  ----  ---- | 16:23  -----  ----
    19 Aug 19:51 | 04:30  ----  ---- | 16:03  -----  ----
    24 Aug 19:42 | 04:10  ----  ---- | 15:44  -----  ----
    29 Aug 19:34 | 03:51 -----  ---- | 15:24  -----  ----
    03 Sep 19:25 | 03:31 -----  ---- | 15:05  -----  ----
    08 Aep 19:16 | 03:11 -----  ---- | 14:45  -----  ----
    13 Sep 19:07 | 02:52 -----  ---- | 14:25  -----  ----
    18 Sep 18:58 | 02:32 -----  ---- | 14:06  -----  ----
    23 Sep 18:49 | 02:12 -----  ---- | 13:46  -----  ----
    28 Sep 18:41 | 01:53 -----  ---- | 13:26  -----  ----
    03 Oct 18:33 | 01:33  7:00  0:12 | 13:07  -----  ----
    08 Oct 18:25 | 01:13  6:48  0:12 | 12:47  -----  -----
    13 Oct 18:17 | 00:54  6:37  0:11 | 12:27  -----  ----
    18 Oct 18:10 | 00:34  6:24  0:13 | 12:08  -----  ----
    23 Oct 18:03 | 00:14  6:11  0:13 | 11:48  -----  ----
    28 Oct 17:57 | 23:51  5:56  0:15 | 11:28  -----  ----
    02 Nov 17:51 | 23:31  5:40  0:16 | 11:09  -----   ----
    07 Nov 17:46 | 23:11  5:25  0:15 | 10:49   -----  ----
    12 Nov 17:42 | 22:52  5:10  0:15 | 10:29   -----  ----
    17 Nov 17:38 | 22:32  4:56  0:14 | 10:10  -----  ----
    22 Nov 17:36 | 22:13  4:37  0:19 | 09:50  -----  ----
    27 Nov 17:34 | 21:53  4:19  0:18 | 09:30  -----  ----
    02 Dec 17:33 | 21:33  4:00  0:19 | 09:11  -----  ----
    07 Dec 17:33 | 21:13  3:40 |0:20 | 08:51  -----  ----
    12 Dec 17:33 | 20:54  3:21  0:19 | 08:31  -----  ----
    17 Dec 17:35 | 20:34  2:59  0:22 | 08:12  -----  ----
    22 Dec 17:37 | 20:15  2:38  0:21 | 07:52  -----  ----
    27 Dec 17:40 | 19:55  2:15 0:23  | 07:33  -----  ----
    01 Jan 17:44 | 19:35  1:51 0:24  | 07:13  -----  ----
    -----------------------------------------------------

Interpretation 
 ------------
    In absence of the suspended summer effect, or speeded spring 
effect, the stars would be in sight for 20 minutes less each date,4 
minutes per day, from nightfall. to setting. For Altair this span is 
less, order 15 minutes. Altair moves westward only 15 minutes between 
dates, not 20. This is the lingering we see in the Summer Triangle in 
late fall. 
    Procyon moves westward by order 25 minutes between dates, not 20. 
It, with the Winter Triangle, seems to speed ahead toward a quicker 
departure into twilight. 
    I remind that this analysis applies to the latitude of New York, 
40.7d N. The table will be significantly different for other latitudes 
away from this one. Never the less the 'suspended summer' is mentioned 
by most skilled observers in the middle belt of the United State form 
35dN to 45dN. 

COnclusion 
 --------
         found it curious how so obvuiys a begavuir if tge sjtm najubg 
tge suspendde summer and spring ahead effect, apparently was not 
explained way much earlier thn now. Against a claim the calculations 
rrequire a computer software, I must point out that this analysis of 
suspended summer is an excellent exercise for the classical astrolabe. 
It would have to be one constructed for the present epoch latitude 
with resolution on its scales of one degree. 
    Some astronomers suggested that tracking sidereal time for the 
moment of nautical twilight should demonstrate the suspended summer 
effect. It does, altho I did only a spot check for a couple dates. 
Sidereal time at nautical twilight would increase by less than 4 
minutes per day in autumn; more than 4, spring. 
    Everything here is for New York's latitude. Can a reader run the 
analysis for a remote latitude/