Hi there PNE,
To start off, here is a table with solstice and equinox times for 2003
through 2050 from the U.S. Naval Observatory, Astronomical
Applications Department. Times are listed as UTC, which stands for
?Coordinated Universal Time?. (Sounds to me like they need to
coordinate their acronym, as well...)
http://www.crh.noaa.gov/ind/seasons.txt
UTC is equivalent to the civil time for Iceland, Liberia, Morocco,
Senegal, Ghana, Mali, Mauritania, and several other countries. During
the winter months, UTC is also the civil time scale for the United
Kingdom and Ireland.
http://aa.usno.navy.mil/faq/docs/UT.html
Solstices and equinoxes are not defined by time but by the orbital
relation of the Sun and Earth. The solstices are the times of the
year when a spot on the Earth known as the sub-Solar point lies
directly on one of the two tropics, either the Tropic of Cancer in the
northern hemisphere or the Tropic of Capricorn in the southern
hemisphere. The solstices occur *around* the 21st of June for the
summer, and *around* the 21st of December for the winter. The
phenomena of the solstices occur because the Earth's axis is tilted
at 23.5º in relation to its orbit. On or about June 21st, the north
pole points at its furthest extreme toward the sun, making the
sub-Solar point lie directly atop the Tropic of Cancer at 23.5º North
Latitude. On or about December 21st the south pole points at its
greatest extreme toward the sun, and the sub-Solar point lies directly
on the Tropic of Capricorn at 23.5º South Latitude. The equinoxes are
the mid-points of the sub-solar spot's 47º migration route across the
face of the Earth, and occur when the spot lies directly atop the
equator. This migration of the sub-solar point is also directly
responsible for the Arctic and Antarctic circles. At Winter Solstice,
direct rays from the sun fall only as far north as 66.5º, (90º ? 23.5º
= 66.5º) North Latitude, which defines the Arctic Circle. The
opposite situation defines the Antarctic Circle.
The solstices, (and equinoxes too, for that matter), occur at
differing dates because of aberrances in the Earth's orbit, and
because the actual length of the year, the time it takes for the Earth
to make a complete revolution around the sun, is not *precisely* 365
days but about 365.24, hence the existence of leap years. The
following is from this site:
http://www.tondering.dk/claus/cal/node2.html
and explains the issue fairly clearly:
?The astronomical ``tropical year'' is frequently defined as the time
between, say, two vernal equinoxes, but this is not actually true.
Currently the time between two vernal equinoxes is slightly greater
than the tropical year. The reason is that the earth's position in its
orbit at the time of solstices and equinoxes shifts slightly each year
(taking approximately 21,000 years to move all the way around the
orbit). This, combined with the fact that the earth's orbit is not
completely circular, causes the equinoxes and solstices to shift with
respect to each other.?
There is also a good discussion of the issue of time and solsticial
and equinoctial phenomena here:
http://www.nmm.ac.uk/site/request/setTemplate:singlecontent/contentTypeA/conWebDoc/contentId/3843/navId/00500300f00h
The ?shortest day? of the year is also the longest day of the year
depending on which hemisphere you happen to be in at the time. The
solstices are the longest and shortest days of the year, the second
shortest days are the days immediately preceding and succeeding the
soltices...technically. The sub-solar point actually ?stands still?,
or more correctly moves imperceptibly for about a week before and
after the solstices, making the issue of day length a bit academic.
The following is from this web site:
http://www.cix.co.uk/~starling/FAQ004.htm
? The longest day will be the one in which the summer solstice occurs.
If the solstice occurs during the day then that day is obviously the
longest. If the solstice occurs during the night, then it will be the
day before if the solstice is before midnight and the day after if the
solstice occurs after midnight. If the solstice occurs exactly on
midnight then the days either side will be equal length and there will
be two longest days that year.?
Another good web site with information on this topic is:
http://www.sunspot.noao.edu/sunspot/pr/answerbook/sunrise.html
From that site:
?Near the shortest and longest days the lengths of the daytime and
nighttime periods vary slowly. This means that we have to be specific
in which definition we use for sunrise and sunset.
The moment of true sunrise (when the top part of the Sun becomes
visible for the first time, assuming no clouds hide the Sun) depends
on the conditions of the atmosphere, and on whether things such as
mountains or buildings block part of the sky in the direction where
the Sun rises. Variations in the conditions of the atmosphere may
change the moment of sunrise by a minute or so from day to day, and
likewise for sunset, so if you take that into account then the longest
day and shortest night may not be adjacent, and their exact dates
cannot be predicted.
If you disregard the variations in the atmosphere and assume that the
horizon is clear in the directions of sunrise and sunset, then the
dates of the longest day and shortest night are influenced only by the
variations in the Equation of Time (which defines the time of true
noon) and in the length of the daytime period. The former varies by at
most about 0.3 seconds per day near the solstices (longest and
shortest days in the temperate and arctic zones), and the latter by an
amount that is about 5 seconds per day just outside the arctic circles
and decreases to zero at the equator.
So the variation in the length of the daytime period dominates, except
possibly close to the equator (where there are no great variations in
the length of days and nights anyway). Therefore I expect that the
longest idealized day and shortest night are adjacent in the temperate
zones of the globe, and likewise for the longest night and shortest
day.?
A good discussion of the aberrances of Earth's rotation and its effect
on day length can be found here:
http://www.jpl.nasa.gov/earth/features/longdays.html
The US Naval Observatory has this to say about lengths of day and night:
http://aa.usno.navy.mil/faq/docs/equinoxes.html
In the northern hemisphere, at latitude 5 degrees the dates of equal
day and night occur about February 25 and October 15; at latitude 40
degrees they occur about March 17 and September 26. On the dates of
the equinoxes, the day is about 7 minutes longer than the night at
latitudes up to about 25 degrees, increasing to 10 minutes or more at
latitude 50 degrees.
Here is a table with the longest days and nights for several different
latitudes, as well as a discussion of the factors contributing to the
phenomena:
http://aa.usno.navy.mil/faq/docs/longest_day.html
The day of earliest sunset occurs about two weeks before the winter
solstice, and the day of latest sunrise happens about two weeks after
the winter solstice. More on this can be found at:
http://starryskies.com/articles/2002/12/sunriseeq-time.html
I hope I've covered all of your questions. If you need clarification
on anything, don't hesitate to click on the ?Request Clarification?
button. Thanks for an intriguing question!
--Kutsavi
SEARCH TERMS:
solstice dates length day
length day shortest longest
date latest sunrise
shortest day year
latitude tropic capricorn arctic circle
utc time |
Request for Answer Clarification by
pne-ga
on
25 Dec 2003 11:32 PST
Hello,
apologies for not getting to this question earlier.
Thank you for the background information and the links you found me
which talk about these connected phenomena of daylight length,
solstices/equinoxes, and sunrise/sunset times.
However, I believe you have not answered two of my questions:
"What is the date of the latest sunrise? I've heard that this does not
coincide with the date of shortest daytime. How much difference is
there? Is it always a fixed period of time before/after the date of
shortest daytime? What does it depend on?
What is the date of the earliest sunset? Is it the same day as the
date of the latest sunrise? Or perhaps "on the other side" of the
solstice from the date of the latest sunrise?"
I believe I've understood, from the material you've presented, that in
the Northern Hemisphere, the date of earliest sunset is before the
winter solstice and the date of latest sunrise is after the winter
solstice (because the time of transit is becoming later at that time
of the year faster than the length of daylight is changing and so this
factor dominates the sunrise/sunset times), but not how much.
You cite http://starryskies.com/articles/2002/12/sunriseeq-time.html
as saying that "The day of earliest sunset occurs about two weeks
before the winter solstice, and the day of latest sunrise happens
about two weeks after the winter solstice." but this is qualified by
saying "The exact date varies with latitude" without stating the
latitude of the writer of that article.
Apparently, sunrise and sunset times are determined from the time of
transit as well as the length of daylight;
http://www.sunspot.noao.edu/sunspot/pr/answerbook/sunrise.html says
this as "To determine the times of sunrise or sunset, you need to know
how long the Sun is above the horizon (which depends on your latitude
and on the season). Sunrise is equal to transit minus half of the
length of the daylight period, and sunset to transit plus half of the
daylight period. So, the time of sunrise varies because the length of
the daylight period changes (with the seasons and with latitude), but
also because the time of transit changes (with the seasons)."
So the time of transit changes in the same manner everywhere, but
earliest sunset/latest sunrise depends on the interaction of this with
daylight length, which depends on the latitude.
What, then, are the dates of earliest sunset and latest sunrise at
53°30' N? (And 10° E, though I believe that doesn't matter in this
connection.)
I realise that these dates change slightly from year to year (+/- one
or two days), so a rough or "average" day would be fine, as would be a
value for a particular year. Dates for 50° N would also be acceptable
if you cannot find (or calculate) data for 53°30' N.
(http://www.sunspot.noao.edu/sunspot/pr/answerbook/expl-5.html#eot is
also interesting in this connection; the sections "Equation of Time"
and "Times of Earliest and Latest Sunrise and Sunset" give formulae
depending on latitude but the page is pretty maths-heavy.)
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Clarification of Answer by
kutsavi-ga
on
27 Dec 2003 07:44 PST
Hello again PNE, and thanks for asking for the clarification. I hope
your holiday was happy. Now, let's see:
Due to the variation of the dates, I had considered the following an
answer to the question of dates for the latest sunrise/earliest
sunset, but I think I see what information you want, now:
?The day of earliest sunset occurs about two weeks before the winter
solstice, and the day of latest sunrise happens about two weeks after
the winter solstice. More on this can be found at:
http://starryskies.com/articles/2002/12/sunriseeq-time.html?
You said:
?What is the date of the latest sunrise? I've heard that this does not
coincide with the date of shortest daytime. How much difference is
there? Is it always a fixed period of time before/after the date of
shortest daytime? What does it depend on??
And:
?What is the date of the earliest sunset? Is it the same day as the
date of the latest sunrise? Or perhaps "on the other side" of the
solstice from the date of the latest sunrise??
The actual dates do vary from year to year, but the date for the
latest sunrise, disregarding your latitude for a moment, is around
the 5h of January, while the date for the earliest sunset is around
the 7th of December. Here's a great site by an amateur astronomer
about the earliest sunrise and latest sunset. It offers a very
detailed look at the reasons why the dates of the latest and earliest
sunrise and set don't coincide with the solstice. The dates he has on
the table are from 2001 and 2002, but the reasons behind the dates are
the important stuff, and the times listed for each rise and set are
close enough to use for general comparison from year to year:
http://members.aol.com/jwholtz/analemma/analemma.htm
Now, specifically addressing your situation, because Hamburg is so far
north, the actual time of sunrise is the same from December 25th
through January 4th, so you essentially have about 11 days of the
latest sunrise, and 4 or 5 days of the earliest sunset. I found a
table listing the times for sunrise and sunset throughout the year
that is too large to fit here, but you can reproduce my results by
visiting this site:
http://aa.usno.navy.mil/data/docs/RS_OneYear.html
And entering the following:
Year: 2003 or 2004
Type of Table: sunrise/sunset
Place Name: Hamburg Germany
Longitude: 10 degrees 0 minutes
Latitude: 53 degrees 30 minutes
Time Zone: east of Greenwich
Of course the reason behind there being several days of the same
sunrise/sunset time in Hamburg is the relative alignment of the
Earth's axis in relation to the Sun and your far-northern orientation
on the surface of the planet. The Sun's relative position on the
horizon in Hamburg doesn't change appreciably over the weeks
surrounding the winter solstice, so the times for the events remain
the same. In other words, the relative position of the sun on your
horizon has reached its southernmost limit before the actual date of
the latest sunrise/earliest sunset, and so the actual changes, and
hence differences in times, are invisible from your location. (To
appreciate this further, refer to my original answer regarding the
sub-solar point and the position of the Arctic Circle.)
If you ever have the occasion to appreciate the same period of time in
more southern climes, it would be interesting for you to observe the
changes first hand via photographing, from the same location, the
rise and set of the sun in relation to the horizon on several
different days. This is how most ancient cultures kept track of the
unfolding year, by marking the rising and setting points of the sun
against known or constructed landmarks on the horizon.
I hope this went further toward addressing your questions. If you
need further clarification, please request it! Happy New Year and
Happy Latest Sunrise!
--Kutsavi
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