The following are some notes on tomorrow's spring equinox that may be of use to Utah educators. 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 >> for my topocentric observing point (o.p.) ( 111°53'27.6''W, 40°45'23.0''N ), shows Sun crossing 0 deg ecliptic longitude between 18:25 and 18:26 UT: 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 this SLC 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 - at the 19:34-19:35 UT (12:34-12:35 pm MDT) time mark. 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. - Canopus56 __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com