Expert: Tom Whiting - 6/26/2011

Question
If someone were to ask me how to determine the day of the vernal equinox using simple means, how should I describe it to them? For example, is the following accurate? If not, what is wrong with it and what is the better simple means? Please answer both questions. Thanks.

Well, the old adage that the sun always sets in the west is true; but, what is not true is that the sun always sets in the same place. What you can do is to set up a wooden frame whose top is parallel to and level with the western horizon as you stand and look at it, always from the same spot, about 10 feet to the east of the frame. Then, every third day, you can drive a small finishing nail part way into and sticking up from the top board to mark the position at which the sun went down. You’ll need about 122 nails to cover a full year. Then, the halfway point between the nail farthest south and the nail farthest north marks the day of the equinox.

Thanks.

Answer
Hi Dave,
Yeah, that sounds good to me, except I prefer the "due West setting" or due East rising, so I don't have to wait all year. Plus, realize that the Vernal Equinox is a moment in time, and not always on the same date.  In 2010 it was at 1733 Hr UT on March 20th and in 2011 it was at 2321 Hr UT on March 20th, which of course is March 21st east of Greenwich, England... so depends on where you are on the Earth's surface as to what day it is.  And it also depends in which century you are in.
Spring began while the pyramids were being built in late April, or maybe even in May (Gregorian Calendar).  Due to precession of the Earth's axis, the Vernal Equinox date (and all other sky established dates like first day of winter, date of the Perseid meteor shower, etc)... will change by one day every 71 years.  (25,800 year cycle divided by 365 days). Then of course the addition of a Feb. 29th every 4 years has an effect on the date too... but that's our calendar, not the Earth's orbit around the sun.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA