Re: length of day

In article <H56dnalyy_lMueHfRVn-og@xxxxxxxxxx>, jeff-n@xxxxxxxxxx
says...
>Paul: Thank you for responding. Here are some data, as you requested:
>
>2005
>Vernal equinox is Mar. 20. Sunrise is 7:21, sunset 7:29, interval is 12:08.
>Autumnal equinox is Sept. 22. Sunrise is 7:04, sunset 7:16, interval is
>12:12.
>
>2006
>Vernal equinox is Mar. 20. Sunrise is 7:22, sunset 7:29, interval is 12:07.
>Autumnal equinox is Sept. 23. Sunrise is 7:05, sunset 7:15, interval is
>12:10.
>
>2007
>Vernal equinox is Mar. 21. Sunrise is 7:20, sunset 7:30, interval is 12:10.
>Autumnal equinox is Sept. 23. Sunrise is 7:05, sunset 7:15, interval is
>12:10.
>
>2008
>Vernal equinox is Mar. 20. Sunrise is 7:21, sunset 7:29, interval is 12:08.
>Autumnal equinox is Sept. 22. Sunrise is 7:05, sunset 7:16, interval is
>12:11.
>
>2009
>Vernal equinox is Mar. 20. Sunrise is 7:21, sunset 7:29, interval is 12:08.
>Autumnal equinox is Sept. 22. Sunrise is 7:04, sunset 7:16, interval is
>12:12.
>
>Jeff Nadir

First, a quote of your original question:

# A table of sunrise/sunset times for Toronto shows that the interval
# between sunrise and sunset is always a few minutes longer on the first
# day of autumn than on the first day of spring. Why?

Your own figures above show that this does not ALWAYS happen, not even
in the table you used. It didn't happen in 2007 for instance...

Next, we need the longitude, latitude and time zone of Toronto. From
a world atlas I got:

Lon 79d 20' W
Lat 43d 39' N

Timezone Eastern Time which is UT-5h winter time and UT-4h summer Time

Next we check when the equinoxes did occur. In UT we get:

March eq. September eq.

2005 March 20, 12:34 Sept 22, 22:23
2006 March 20, 18:26 Sept 23, 04:03
2007 March 21, 00:07 Sept 23, 09:51
2008 March 20, 05:48 Sept 22, 15:44
2009 March 20, 11:44 Sept 22, 21:19

Let's convert this to Toronto time by subtracting 4 or 5 hours (it's
DST on the September equinox but not on the March equinox):

2005 March 20, 08:34 Sept 22, 17:23
2006 March 20, 14:26 Sept 22, 23:03
2007 March 20, 20:07 Sept 23, 04:51
2008 March 20, 01:48 Sept 22, 10:44
2009 March 20, 07:44 Sept 22, 16:19


I computed rise/set times myself for Toronto for your dates, and got
these results (your figures appear within parentheses):

2005
Mar 20 Sunrise 6:20 (7:21) set 18:30 (7:29) interval is 12:10 (12:08)
Sep 22 Sunrise 7:05 (7:04) set 19:14 (7:16) interval is 12:09 (12:12)

2006
Mar 20 Sunrise 6:21 (7:22) set 18:30 (7:29) interval is 12:09 (12:07)
Sep 23 Sunrise 7:05 (7:05) set 19:13 (7:15) interval is 12:08 (12:10)

2007
Mar 20 Sunrise 6:21 (----) set 18:29 (----) interval is 12:08 (-----)
Mar 21 Sunrise 6:19 (7:20) set 18:31 (7:30) interval is 12:12 (12:10)
Sep 23 Sunrise 7:05 (7:05) set 19:13 (7:15) interval is 12:08 (12:10)

2008
Mar 20 Sunrise 6:20 (7:21) set 18:30 (7:29) interval is 12:10 (12:08)
Sep 22 Sunrise 7:05 (7:05) set 19:14 (7:16) interval is 12:09 (12:11)

2009
Mar 20 Sunrise 6:20 (7:21) set 18:30 (7:29) interval is 12:10 (12:08)
Sep 22 Sunrise 7:05 (7:04) set 19:14 (7:16) interval is 12:09 (12:12)

As you can see, in my computations the effect you reported vanishes
and gets replaced with an opposite effect - the interval appears to
be smaller during the "first day of autumn", but only by a minute
so it could very well be a round-off effect or something. The only
exception is the year 2007 - but in the year 2007 you selected March 21
as "the first day of the spring" even though the March equinox occurred
on March 20, Toronto time (but on March 21, Universal Time). If I in
2007 use March 20 instead, I get an exact match to the minute!


So my explanation to the effect you reported is that it's due to some
fairly small but systematic errors on the computation of the sunrise/set
time in the table you consulted. What kind of errors? I don't know, it
could be a lot of different things. And to try to find that out, you
must contact the people responsible for those computations. If you
don't want to trouble yourself with that, just consider that table
somewhat unreliable.



>--------------------------
>
>
>"Paul Schlyter" <pausch.NO@xxxxxxxxxxxx> wrote in message
>news:1115405917.357764c13b81d150dff7eb0e9661b432@xxxxxxxxxxx
>> In article <LZCdnQKm9KoL5ebfRVn-pw@xxxxxxxxxx>, jeff-n@xxxxxxxxxx
>> says...
>>>Ray,
>>>
>>>Thanks for responding to my post so quickly and clearly.
>>>
>>>I know about the equation of time. I think I understand how the variation
>>>between sundial time and regular time affects the times of sunrise and
>>>sunset given in the tables.
>>>
>>>What I don't understand is why the equation of time should affect the
>>>INTERVAL between sunrise and sunset. If sundial and regular times differ
>>>by
>>>x minutes on the day of the vernal equinox shouldn't this just shift the
>>>times of sunrise and sunset equally by x minutes in the same direction
>>>leaving the length of the day unaffected? If sundial and regular time
>>>differ

--
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at saaf dot se
WWW: http://stjarnhimlen.se/
http://home.tiscali.se/pausch/
.

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