Expert: Tom Whiting - 6/7/2006

Question
Does the sun set at different speeds at different times of the year?  
I am getting married in October in Missouri.  I timed the sunset last night and i was wondering if i could use this same time frame to plan the time of the ceremony?
Also, does it set at a different spot and what directions would it be from the sunset last night?  
Thank you for your help

Answer
Hi Alicia,
Congratulations on getting married, but to your questions.

Since sunset is simply the Earth's rotation eastward, which is a very constant speed, the sunset "speed" remains the same all the time.....BUT it DOES set at different times during the year, AND a different, ever-changing place on the western horizon.

NO, you cannot use the current sunset time for October;
(Early March would be the same as mid-October.....)

This is June so the days are much longer (sunsets are much later) than October, and much farther north of due west....sunset occurs much earlier by October as that is well after the Autumnal Equinox on September 23rd and south of
due west instead of....north of due west as it appears in June.

So the sun sets a little south of due west in October.
On October 1st, the sun sets about 5 degrees (one-half a
fist's width at arm's length) south of due west, but by the end of the month, the sunset point is nearly 15 degrees (one whole fully extended hand span at arm's length) south of due west.  (For reference, the sunset point is *exactly* due west on the Equinox date of September 23rd, first day of autumn.)

I checked my skyglobe program for both St. Louis and
Kansas City, MO for October;  the 1st, 15th and the 31st, for sunset times, so here they are.

St. Louis, MO......10/1.........6:38 pm CDT
St. Louis, MO.......10/15.......6:17 pm CDT
St. Louis, MO........10/31.......4:56 pm CST

and for Kansas City, MO;

Kansas City, MO......10/1......6:56 pm CDT
Kansas City, MO.......10/15.....6:34 pm CDT
Kansas City, MO........10/31.....5:12 pm CST

Note that there is quite a change between the 1st and 31st
because we go off Daylight Saving time on Sunday,
October 29th, thus reducing the clock time by one hour.

Also note that Kansas City sunset is about 17 minutes later
than St. Louis because Kansas City is much farther west
in the Central time zone than St. Louis.  If you want a time
for the middle of the state, say Columbia or Jefferson City, just interpolate between the two given times and that will
be accurate to + or - 3 minutes.

And if you want an intermediate date, say October 8th in
Kansas City, just interpolate halfway between 6:56 and 6:34,
or  6:45 pm CDT....and this will be accurate to + or - 2 minutes.

Or in St. Louis on October 8th, it would be 6:27 pm CDT.

(If you furnish me the exact date and location, I could be
even more accurate with my skyglobe computer program,
but perhaps this is close enough for you....if you want to,
I'm at
bwhiting@velocity.net
for a direct answer rather than using AllExperts website.)

GOOD thing you checked on all this, because now in June
the sunset is like 8:30 pm....much much later than October!

Also, another factor you might consider, the twilight time
right after sunset now,  in June,  extends to a full 2 hours,
but by October, the amount of twilight after sunset slips to only 1.5 hours (90 minutes).  I don't know if this would be
a factor to consider or not, but you should be aware of it.

Hope all this helps you,
Clear Skies,
Tom Whiting
Erie, PA

FOLLOW UP:

Oops....the sunset times are correct, but as for twilight time,
I was quoting "astronomical twilight" or when total darkness
occurs, which you probably aren't interested in knowing...
I should have given you  "Civil Twilight" or that point where
you can no longer read a newspaper by twilight....
Civil twilight in June is about 45 minutes after sunset,
but in October, this time shrinks to about 30 minutes after sunset.

There are 3 types of twilight...Civil (sun 6 degrees below
the horizon), Nautical twilight when the brighter stars come
out (sun 12 degrees below the horizon), and Astronomical
twilight (sun 18 degrees below the horizon) when total
darkness occurs, and guys that photograph the night sky
can begin to use their cameras without fear of any light
ruining their pictures of the night sky.
For your purposes, Civil Twilight would be more appropriate.
Hope this helps,
Clear Skies,
Tom Whiting

ADDITIONAL FOLLOW UP
I know it seems "weird"now in June...how could the sunset
be so early in October, but just think back at the Halloweeners that hit the sidewalks around 6 or 6:30pm...it's
already getting dark!  And by 7 pm local time, the Trick or
Treaters are already in near-total darkness on Oct. 31!
So now you know that those are pretty accurate times...
Oh, and thanks very much for the nice comments on
the AllExperts website.

Good Luck and Clear Skies,
Tom