Expert: Tom Whiting - 8/6/2006

Question

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Followup To

Question -
I have found a few resources about time keeping and how people do it. The most obvious measure of time is the earth turning causing days and nights. I would like to know the best way to measure the number of days in a year. I assume that a year is the most complete cycle the earth goes through as that we observe all the seasons etc, and start again, and very little else changes? I've read up that there are 365.242 civil days, 365.2564 solar days, and 365.24237 civil days (Iranian calender). If I were to come up with my own calender how would I derive the most accurate measure of the number of days in a year?

Answer -
Hi James,
Time is no longer measured with respect to the Earth rotating,
or by the Earth's revolution around the sun.....

Actually since around 1968, nothing in astronomy rules the
length of time; Physics and chemistry does.    Cesium atomic clocks came into their own and actually now we define a second as  9,192,631,700 cycles of the cesium atom using
a cesium atom fountain clock.   And that in turn defines our day (86,400 seconds) and in turn the length of a year.  You can read all about it at
http://tf.nist.gov/cesium/fountain.htm

Actually, all that calendar work has already been done,
and approved by all nations through International
Agreements....probably because it is so very accurate today.
Atomic clocks are accurate to one part in 10 to the 16th
power, and vary by one second every 60 million years.
And the current calendar is accurate to one day every
3000 years or so, and if a minor change is made around
current year 4000 AD,  we can increase the accuracy to one day every 60,000 years.  (How much more accurate would
one want??)

For any calendar system, the name of the game is to get
the calendar in as close as perfect step as possible to
the true, or mean tropical year, of 365.24199 days.
(Irrespective of calendar system...this is the astronomical
figure that has to be met).  
At least in our lifetimes....realizing that the dinosaurs
65 million years ago had about a 20 hour day; that's how
much the rotation rate of the planet has slowed in 65 million
years.

And actually, that's already been done to a very high
degree of accuracy with the current Gregorian System
which is accurate to one day every 3000 years.
This system has a Feb. 29 in it for every year divisible
by 4,  and for end of century years have to be divisible
by 400.....thus 1600, 2000, and 2400 have a Feb 29th,
but 1700, 1800, 1900, 2100, 2200, and 2300....do not.
This eliminates 3 "leap days" every 400 years, and keeps
us in step with the tropical year to one day every 3000 years.

{The end of century rule was initiated by Pope Gregory
in 1572 or thereabouts, as a very superior improvement
on the Julian Calendar, which contained  3 too many days every 400 years, by simply having a Feb. 29th every 4th year......the Gregorian system eliminates those 3 extra
days every 400 years.}

The only possible way to make this system even more
accurate is to go out to a longer time frame, subtracting
one more day every 4000 years, but since not that many
years have yet passed, no one as yet seems to be worried
about it.

So fortunately, all that work has already been accomplished,
and really can't be improved upon.....for quite a number of
years down the road....much longer than our lifetimes.

Hope all this helps,
Clear Skies,
Tom  Whiting
Erie, PA


So what is the 365.24199 day year based upon? Even with our seconds based specifically on 9,192,631,700 cycles of the cesium atom how does that compute towards the changing of the seasons?

Answer
Hi James,
Oh, thanks for the very nice comments on Allexperts, I really
appreciate that.

As for your question, the mean tropical year is based upon
the sun's accurate position from Vernal equinox to Vernal
Equinox,....typically on March 21, the so-called First
day of Spring  (for the N. Hemisphere, autumn for the S.
Hemisphere) also called the First point of Aries in astronomy.

Today, we can actually view the sun directly, all the time,  with the SOHO (Solar and Helio Observatory)  satellite pictures.....see
http://sohowww.nascom.nasa.gov/

So we can plot the sun's position against the background
stars and planets to a very accurate degree.  I personally
don't know exactly how they time a tropical year....that's
for the professionals....but I do know it can be done today
using the SOHO to a very high degree of accuracy....
Hey, if we can hit a comet, Tempel 1,  out there some 100 million miles, with an 820 pound copper bullet...see
http://www.planetary.org/explore/topics/asteroids_and_comets/tempel1.html
then I wouldn't question their timing on the mean tropical
year.

As far as seasonal changes, that is almost strictly the Earth's
axial tilt of 23.5 degrees that accounts for the seasons.

In the N. Hemisphere, the direct rays of the sun striking
the N. Hemisphere more directly in May, June, July, give
us our summer, and not so directly in Nov, Dec, and Jan.
our cooler winter months.
Of course, the reverse occurs in the S. Hemisphere.
Near the equator, there is very little seasonal change.

Since we are at perihelion (closest to sun, 91.5 million miles) in January and farthest in July (94.5 million miles),
this is only a 3% change from the average of 93 million miles,
thus accounting for only 3% of the seasonal change, so
it's the Earth's axial tilt that accounts for 97% of the
seasonal changes, and it has nothing to do with the length of
a second, length of a day or year, or cesium clock used for timings.  If our Earth's axis wasn't tilted 23.5 degrees from
the vertical, there would be no seasonal changes at all.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA