[astronomyed] "Planetary Wonderings" December 2007
- From: Mary-Frances Bartels <ki0dz@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- To: astronomyed@xxxxxxxxxxxxx
- Date: Thu, 06 Dec 2007 16:32:44 -0500
*Planetary Wonderings*
*December Focus: Winter Solstice Surprises*
*By Mary-Frances Bartels, NASA Solar System Ambassador*
What and when is the winter solstice? The winter solstice is the day of
the year with the least amount of daylight, and when the sun is the
lowest in the sky. It also marks the start of winter, which this year
occurs at 1:08 AM EST on December 22.
Did you know that the day of the solstice, which has the least amount of
daylight, has */neither/* the latest sunrise */nor/* the earliest
sunset? At first, this seems to make no sense. But, when examined in
more detail, one can see why the "shortest day of the year" has neither
the latest sunrise, nor the earliest sunset of the year. In fact, these
three events occur on three different days! Two phenomena conspire to
make the latest sunrise and earliest sunset on different days than the
solstice; the Earth's tilt on its axis and location in its orbit around
the sun, particularly with respect to perihelion and aphelion.
Let's start with the easier part of the sunrise/sunset/solstice
quandary. As everyone knows the earth's tilt on its axis is what causes
our seasons. Scientists call this tilt /obliquity/. The sun's rays are
most direct and high in the sky on the summer solstice, while the
opposite is true on the winter solstice. Daylight starts to lengthen on
the winter solstice and shorten on the summer solstice. The speed of
the sun with respect to the equator throughout the year changes because
of the tilt. It travels faster (along an east-west line) at the time of
the solstices. The rest of the year it travels northward (heading from
winter to summer solstice) or southward (heading from summer to winter
solstice) in addition to its eastward movement. At the equinoxes the
sun's northward or southward travel is the greatest. (A way you can
check this out for yourself is by observing where the sun rises and sets
each day throughout the year. You will find that it rises and sets
around the same place for a few days around the solstices, but changes
considerably more around the time of the equinoxes.) This accounts for
the change in the length of daylight around the solstices to be
practically zero (discussed in /PW/, Dec. 2005) and to be greatest at
the equinoxes. Because the sun's eastward movement is not at a constant
"speed" this causes "local noon," the time when the sun is highest in
sky on a given day, to be either later or earlier depending on the
season (technically the switch would occur at the midpoints of the
seasons). At the latitude of central Ohio the earth's tilt is primary
in determining sunrise, local noon, and sunset times from June 28 until
December 8, and from January 5 until June 14. These dates can vary
depending on latitude of the observer.
Next, let's examine the problem presented by the earth in its orbit
around the sun. Scientists call this /ellipticity/. If the earth's
orbit were circular with the sun at the center, our planet would travel
at the same speed in this orbit throughout the year. The orbit is not a
perfect circle, however, but is an ellipse with the sun at one of the
ellipse's foci. Kepler discovered that objects in orbit travel faster
when they are closer to the sun (or whatever body the satellite is
orbiting) and slower when farther from the sun. It just so happens that
the earth is closest to the sun around the time of the solstice. The
next perihelion occurs January 2 at 7:00 PM EST. So, what does that
have to do with the length of a day? Usually when people think of the
length of a day they, possibly unconsciously, count it as the time it
takes for the sun to return to about the same place in the sky from one
day to the next, for example from one local noon to the next.
Throughout the year, this time *averages *about 24 hours. However, when
the earth is near perihelion, since it is traveling noticeably faster in
orbit, it has to rotate further before the sun is back to nearly the
same point in the sky. In reality the length of day near perihelion is
greater than 24 hours, as much as 30 seconds longer by late December.
Since we do not correct for this on our clocks, this keeps pushing the
sunrise and local noon times later and later, even past the winter
solstice. Ellipticity pushes local noon later from October to April
(the half year the earth is closer to the sun) and earlier from April to
October. *//*
As you can see, the reasons for the difference in latest sunrise and
earliest sunset with respect to the solstice are based on the fact that
our clocks run on a constant 24 hours per day basis. This works out
fairly well for living everyday life, but not so well when understanding
apparent contradictions such as this one. Here's a brain-buster
question: If we told time using a sundial, would the latest sunrise and
earliest sunset ever occur on one day? If so, what day would that be?
Feel free to e-mail me your answer and how you arrived at that conclusion.
Times in table are for Centerburg, OH and in Eastern Standard Time
* *
*Date*
*Sunrise** Time*
*Sunset Time*
*Local **Noon***
*Sunrise** Position*
*Sunset Position*
*Hours Daylight*
*Comment*
Dec. 8
7:41 AM
5:04 PM
12:23 PM
120°
240°
9:24
Earliest Sunset
Dec. 22
7:50 AM
5:08 PM
12:29 PM
121°
239°
9:18
Solstice
Jan 5
7:54 AM
5:18 PM
12:36 PM
120°
240°
9:24
Latest Sunris
Earliest "local noon" is November 2 - 4 at 12:14 PM EST
Latest "local noon" is February 11-12 at 12:46 PM EST
*Resource of the Month:* /Windows to the Universe/ e-newsletter,
dedicated to Earth and Space science education. It is sponsored by NCAR
(National Center for Atmospheric Research), NASA, CISM (Center for
Integrated Space Weather Modeling), National Earth Science Teachers
Association, CMMAP (Center for Multi-Scale Modeling of Atmospheric
Processes). Explore the Windows to the Universe website at
http://www.windows.ucar.edu/ . Information on subscribing to the
newsletter may be found in the middle of the page at "Newsletter for
Teachers."**
*Activity of the Month:* My explanation of the sunrise/sunset/solstice
problem is a bit oversimplified. If you feel so inclined, examine this
issue further. Find out what the following terms mean and how they
relate to this phenomenon: analemma, angular distance, apparent motion,
celestial equator, sidereal time, equation of time, declination, sun
transit, azimuth, ecliptic, and conservation of angular momentum. How
much closer is the earth to the sun during perihelion than at aphelion?
More advanced students might want to find the actual equations that are
used to determine sunrise time and the graph showing the equation of
time throughout the year. Discover the time differences between mean
local noon and clock noon as contributed by ellipticity or obliquity
separately. From the information given in this article, can you
determine how many minutes central Ohio is from the center of the
Eastern Time Zone?
Suggestions, questions, and comments about "Planetary Wonderings" are
welcomed and may be directed to stargazer @ keeplookingup.net (remove
spaces). Past columns may be found at www.keeplookingup.net
<http://www.keeplookingup.net/> (all past columns, click on "Planetary
Wonderings" on the right side of opening screen) and at
//www.freelists.org/archives/astronomyed/ (columns from Jan. 2007
to the present).
*Remember to /keep looking up!/*
*/ /*
Sources (not already mentioned):
http://www.chiff.com/home_life/holiday/winter-solstice.htm
www.usno.navy.mil <http://www.usno.navy.mil/> US Naval Observatory
http://www.arachnoid.com/lutusp/sunrise/ Solar Computer
http://curious.astro.cornell.edu/question.php?number=208
/Kaluach 3/ calendar software from www.kaluach.org
<http://www.kaluach.org/>
--
M R Bartels ki0dz@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx(((#))) ^ ^
http://www.qsl.net/ki0dz Central Ohio | ^ - ^
Watkins Products www.watkinsonline.com #092389 ------- (o o)
Internet $9.99/month --- Free trial |ooOoo| >{ | }<
^ http://continue.to/internet ------- RRR)*
Other related posts:
- » [astronomyed] "Planetary Wonderings" December 2007
