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  1. #1
    canopus56's Avatar
    canopus56 Guest

    Default Notes on the spring equinox for science teachers



    The following are some notes on tomorrow's spring equinox that may be
    of use for science teachers. These notes explain how to relate the
    altitude of the Sun to a student's latitude. You may need to run more
    detailed ephemeris << http://aa.usno.navy.mil/ "Complete Sun and Moon
    Data for One Day" or http://ssd.jpl.nasa.gov/?ephemerides >> for your
    location.

    The easiest method of finding the time of solar transit discussed below
    is to go to the USNO Astronomical Applications page
    << http://aa.usno.navy.mil/ >> and their "Complete Sun and Moon Data
    for One Day" web application. In USNO "Form A" select your state and
    type in your nearest large city. A typical report display looks like
    this:

    ============
    SUN
    Begin civil twilight 6:04 a.m.
    Sunrise 6:31 a.m.
    Sun transit 12:35 p.m.
    Sunset 6:40 p.m.
    End civil twilight 7:07 p.m.

    ============

    The time to measure the Sun, as discussed below, is the "Sun transit"
    time.

    The spring equinox is tomorrow, 3/20/2006 at 18:26 UTC (11:26 MDT).

    The NASA/JPL Horizons Ephemeris << http://ssd.jpl.nasa.gov/?ephemerides

    Date/Time Az/Alt Eclip Long Lat Local
    solar time
    2006-Mar-20 18:25 154.2263 46.2549 359.9995381 -0.0018950 10 49
    00.7171
    2006-Mar-20 18:26 154.5682 46.3370 000.0002209 -0.0018920 10 50
    00.7313

    Local transit of the Sun (our topocentric equinox) occurs later at
    19:34-19:35 UT (12:34-12:35 pm MDT):

    Date/Time Az/Alt Eclip Long Lat Local solar
    time
    2006-Mar-20 19:34 179.6279 49.2600 0.0466343 -0.0016771 11 59
    01.7169
    2006-Mar-20 19:35 180.0111 49.2608 0.0473167 -0.0016739 12 00
    01.7312
    2006-Mar-20 19:36t 180.3942 49.2604 0.0479991 -0.0016706 12 01
    01.7456

    At this point, that this observing point's latitude (~ 41N degs) is
    equal to 90 degrees (zenith) minus the altitude of the Sun, e.g. -

    41 degs = 90 degs - 49 degs

    Due to the effect of precession, from my topocentric o.p., the Sun does
    not cross the celestial coordinate spring equinox until 20:22-20:23 UT
    (1:22-1:23pm):

    Date/Time RA/Dec Alt/Az
    2006-Mar-20 20:22 23 59 59.85 -00 00 07.1 197.6971 47.8966
    2006-Mar-20 20:23 00 00 00.00 -00 00 06.1 198.0597 47.8387

    These angles can be demonstrated to students using a protractor with a
    string and hanging weight (like a bolt or nut) run through the center
    hole. The protractor is held so that the string hangs through the 90
    degree mark. Then stick a pencil perpendicular to the protractor into
    the center hole. A fuzzy shadow will be cast by the Sun. To reduce
    the chance of eye injury, advise students to not sight along the
    protractor at the Sun. Measure angles indirectly using the pencil's
    shadow.

    An accurately aligned north-south building edge (check with a compass)
    can also be used. Use the protractor to make a large graduated quarter
    circle on a piece of paper. Using the protractor with plumb bob, tape
    the graduated circle to the north-south side of the building. Hold a
    pencil perpendicular to the building and take a shadow measurement.

    Degree of accuracy for such methods is ~ 1.0 - 3.0 degrees.

    Thus, you can demonstrate the relationship between latitude and the
    equinox transit of the Sun. ( geo latitude = 90 - equinox transit
    altitude )

    At the equinoxes, the Sun rises and sets at azimuth 90 degrees (due
    East) and 270 degrees ( due West). Using an accurately aligned
    east-west edge of a building, or two stakes driven in the ground, that
    the sun sets at due west can be illustrated.

    Your magnetic declination ( the degrees you subtract or add to a
    compass heading to find the geographic direction, can be estimated from
    magnetic declination maps at this web site <<
    http://disitron.com/freeresources/ >> )

    Another demonstration that can be done on the equinox uses a gnomon, a
    single stick driven in the ground. Or you can use any existing
    feature, like a basketball backboard, a street lamp or a flag pole.

    The equinoxes are the only day of the year that the shadow cast by a
    gnomon moves in a straight east-west line. Have students go out at one
    or two hour intervals and mark the location of the shadow of the top of
    the gnomon on the ground with chalk. The line traced by the shadow
    will be a straight east-west line. If repeated in few weeks, the line
    traced by the gnomon's shadow will be curved.

    - Canopus56


  2. #2
    oriel36's Avatar
    oriel36 Guest

    Default Notes on the spring equinox for science teachers

    I quite understand,with some regret,that you wish to remain
    quasi-geocentrists and reckon the Equinox by sunrise and sunset and
    the 'motion' of the Sun.

    For those who wish to reach the level of astronomers and
    astronomy,there is always the gift of discerning what the Equinox means
    in terms of pure heliocenticity by using the motions and orientations
    of the Earth to generate enjoyable and productive data and working
    principles.


  3. #3
    Marty's Avatar
    Marty Guest

    Default Notes on the spring equinox for science teachers

    Gerald K. was saying

    YES ~ YES ~ YES !!! You've GOT it!! You can give up on us now!!! I'm
    sure some other group would be MUCH more appreciative of your efforts.
    Thanks for trying! Sorry to see you go!!
    Marty


 

 

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