Page 10 - 2008 DT 12 issues

It’s About Time up a little bit short every year. time that brings the observed positions
This is where things get a bit of the celestial bodies into accord with
‘If you knew Time as well as I do,’ complicated. Earth’s orbit is elliptical, the Newtonian dynamical theory of
said the Hatter, ‘you wouldn’t talk carrying us varying distances from the motion.” Phew! For the year 1900, the
sun, faster and slower as we go along. tropical (solar) year was measured as
about wasting it.’
The sun’s gravitational pull, the tidal ef- 31,556,925.9747 seconds. Divide that
fects of the moon, earthquakes, etc., all number by 86,400 (mean solar day) and
ime. If only we could stretch it serve to affect our journey. If we didn’t you’ll get 365.24 days! Our days are still
just a little. We could certainly leap to make up the unaccounted for 24 hours long by our clocks (civil time),
Tuse the extra sleep. For each of time, we’d be one whole day (minus a but a bit longer in ET.
the last four years, we’ve been deprived few minutes) short in our solar year over Even more precise timekeeping be-
of the almost-quarter-day to which we four years. There are boring exceptions came available with the development of
are entitled. This year we’re in luck. Be- to when to leap or not. Most years that the first atomic clock in 1949. In 1967,
cause 2008 is a leap year, we’re finally can be divided by 4 are leap years, ex- the atomic second (our current standard)
going to cash in those accumulated extra cept for century years, unless they can be made its official debut, defined by the
hours and get to sleep-in for one whole divided by 400. We leapt oscillations of two closely spaced energy
day if we choose! Why not? in 2000, but won’t leap levels of the cesium-133 atom. Atomic
What can you do with in 2100. (Keep your clock time, expressed as Coordinated
365.2422 (or so) days anyway? scorecard handy.) Universal Time (UTC), is uniform over
After 100 years, you’d have an Forgetting all those a period of millions of years and is the
additional 24 days added to rules, the world is seri- current basis for civil time. This relation-
the calendar, but the seasons ously dependent upon ship between the atomic second and the
wouldn’t quite match up. On knowing exactly what ephemeris second has been restated to
the bright side, you’d have 24 time it is. The second make their values equivalent, because
more days to exchange those is the basis for mod- Earth’s rotation is decelerating, due to
Christmas presents you didn’t ern timekeeping. Until the breaking actions of ocean tides, by
want. By keeping the calendar 1954, the second had roughly 1.4 milliseconds per day per
year at a convenient 365 days been defined as 1/86,400 century. The length of a mean solar
and ignoring the extra 5 hours, 48 min- of a mean solar day or the average time day based on the Earth’s rotation is now
utes, 45.51 seconds it actually took Earth it takes the Earth to make one complete measured at 86,400.02 seconds, which
to come back to “zero position” each of rotation on its axis over a period of adds up over a year to about a minute.
the last four years (including the last leap years—not precise enough as a standard In order to keep the cumulative time
year), you get to squander all those accu- of time. difference between the two time systems
mulated hours in one lump on Leap Day, A new standard for the second, to 0.9 seconds or less, one leap second
February 29, 2008. The time is yours to known as the ephemeris second, was is inserted into the UTC time scale once
spend any way you want. Enjoy! defined in terms of the solar or tropical or less per year, usually after the last
A solar year represents the time year, the “period of Earth’s revolu- second of December, or alternatively,
it takes Earth to make one complete tion around the sun for a particular after the last second of June.
revolution around the sun. It is measured epoch.” Astronomical observations of Since the first leap second was
as the mean time between two vernal the Earth’s motion for the epoch year inserted in 1972, there have been 23
equinoxes, when the sun is directly of 1900 defined the ephemeris second leap seconds added in the 34 years up
above the Earth’s equator, crossing as 1/31,556,925.9747 of a solar year. to January 2006. You may be happy (or
from the southern hemisphere to the Historic observations of the motion of not) to learn that we did not have to add
northern hemisphere. Each 365 days, the sun with regard to the motions of a leap second on December 31, 2007 to
one complete revolution around the other celestial bodies over the last two our atomic clocks. Adding leap seconds
sun is supposed to land us in exactly the centuries provided a formula to establish has become controversial. Concerns
same position we started with at the last the mean length for the solar year in over complications with GPS monitor-
vernal equinox. Unfortunately, we come ephemeris time (ET), the “measure of ing and other ultra-precise equipment

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