Hi Wolfgang:
A friend of mine has an observatory in his back yard and the computer controlled telescope can point to a star within a
few arc seconds accuracy.
The key inputs are where the telescope is located (determined by pointing the scope at a number of stars) and the time
from the internet. Accuracy is improved by determining the errors in the mechanical parts. Note a time error of 1
second is an angle error of 15 arc seconds.
In the late 1960s and early 1970s atomic clocks were found to be more stable than the rotation of the Earth so the
definition of time was changed from the period of the Earth's rotation to a number of vibrations of a Cesium atom at sea
level. UTC is based on atomic clocks and UTC1 adds a correction that's less than 1 second to get to the Earth's
rotation. In order to keep the UTC1 correction below a second leap seconds can be added or subtracted as needed, but
since the Earth is slowing down lately they seem to all be adding a second.
The Gregorian calendar has the years' length as 365.2425 after you take into account the occurrence of leap years every
(4, 100 or 400 years. Which is pretty close to the time of a year. But. . . . a sundial would need to take this into
account in order to be very precise. For example where in the fastest 4 year leap year cycle you are would effect how
you interpret the position of the sun. It's my understanding that a sundial accurate to a second would need to be
customized for the year of operation.
An astronomical telescope like described above has a pointing accuracy of a few arc seconds which corresponds to a time
accuracy on the order of milliseconds. But the astronomical "seeing" will limit the accuracy to much less. I looked
into measuring the time for a star to cross my local meridian and so measure the earth's sidereal day length, but the
"seeing" is greater than the day to day variation in the earth's period.
http://www.prc68.com/I/StellarTime.shtml
The zero meridian is not going to change. It's a political decision where on the earth that is and GPS depends on it
staying put. The idea of navigation using the time difference between zero and your longitude will still work after
you take into account the time at zero will creep. Note plate tectonics comes into play since pretty much every
location is moving a few mm every year in some direction.
The leap second causes problems for today's communication systems so they want to get rid of it. If you remember the
Y2K fiasco that was caused by people using a 2 digit year number a much bigger fiasco would happen if leap seconds are
eliminated because at some point a correction would be needed because the time would be noticeably out of sync with the
sun. Pretty much everyone would not know how to handle the large adjustment that would accumulate.
I think the ultimate limit will come about because of "earth tides". Like the sun and moon causes sea level to move up
and down it also causes the elevation of granite mountain to move up and down, maybe by a foot or so. Because of
relativity that means the time shown by an atomic clock will vary, but maybe when the clocks are accurate enough to show
that difference they will be corrected by a gravity meter.
I'm on the "Time Nuts" mailing list where very precise time and time
measurements are the topic.
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Mail_Attachment --
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
Wolfgang R. Dick wrote:
ITU-R has decided not to decide anything now on the future of
leap seconds, but has postponed a decision to 2023 - see below.
By the way, am I right that all sundials would show wrong time
earlier or later if no leap seconds would have introduced?
(I mean at least the precise sundials which correct for the
equation of time and show time with a precision of minutes or even
better.)
A second question: What are the most precise sundials? In Furtwangen
I saw a large sundial with a claim to disply time to a second,
but I could not recognize this precision on the sundial itself.
And a third question: Are there already thoughts how to construct
a precise sundial in a world without leap seconds, when the zero
meridian for time will no longer be coincide with the geographic
zero meridian and will be shifted eastwards with time?
Best regards,
Wolfgang
Coordinated Universal Time (UTC) to retain "leap second"
New reference time scale to be considered by World Radiocommunication
Conference in 2023
Geneva, 19 November 2015 - The ITU World Radiocommunication Conference
(WRC-15), currently in session in Geneva from 2 to 27 November, has
decided that further studies are required on the impact and application
of a future reference time-scale, including the modification of
coordinated universal time (UTC) and suppressing the so-called "leap
second".
Leap seconds are added periodically to adjust to irregularities in the
earth’s rotation in relation to Coordinated Universal Time (UTC), the
current reference for measuring time, in order to remain close to mean
solar time (UT1). A leap second was added most recently on 30 June 2015
at 23:59:60 UTC. The proposal to suppress the leap second would have
made continuous reference time-scale available for all modern electronic
navigation and computerized systems to operate while eliminating the
need for specialized ad hoc time systems.
The decision by WRC-15 calls for further studies regarding current and
potential future reference time-scales, including their impact and
applications. A report will be considered by the World
Radiocommunication Conference in 2023. Until then, UTC shall continue to
be applied as described in Recommendation ITU-R TF.460-6
[https://www.itu.int/rec/R-REC-TF.460-6-200202-I/en] and as maintained
by the International Bureau of Weights and Measures (BIPM).
WRC-15 also calls for reinforcing the links between ITU and the
International Bureau of Weights and Measures (BIPM). ITU would continue
to be responsible for the dissemination of time signals via
radiocommunication and BIPM for establishing and maintaining the second
of the International System of Units (SI) and its dissemination through
the reference time scale.
Studies will be coordinated by ITU along with international
organizations such as the International Maritime Organization (IMO), the
International Civil Aviation Organization (ICAO), the General Conference
on Weights and Measures (CGPM), the International Committee for Weights
and Measures (CIPM), the International Bureau of Weights and Measures
(BIPM), the International Earth Rotation and Reference Systems Service
(IERS), the International Union of Geodesy and Geophysics (IUGG), the
International Union of Radio Science (URSI), the International
Organization for Standardization (ISO), the World Meteorological
Organization (WMO), and the International Astronomical Union (IAU).
"Modern society is increasingly dependent on accurate timekeeping," said
ITU Secretary-General Houlin Zhao. "ITU is responsible for disseminating
time signals by both wired communications and by different
radiocommunication services, both space and terrestrial, which are
critical for all areas of human activity."
"The worldwide coordination of time signals is critical for the
functioning and reliability of systems that depend on time," said
Francois Rancy, Director of the ITU Radiocommunication Bureau. "ITU will
continue to work with international organizations, industry and user
groups towards providing coherent advice on current and potential future
reference time-scales."
Source: ITU Press Release,
http://www.itu.int/net/pressoffice/press_releases/2015/53.aspx
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