Time and Time Code

Summer Time Start and End Dates

In the UK, as with all the other countries that are part of the European Union, we now always change the clocks at 01:00 UTC on the last Sundays in March and October.  Summer Time ends, and the clocks will therefore be going back an hour (you gain an hour’s sleep!), on the night of Saturday 24th and Sunday 25th October 2009.  Summer Time starts again on the night of Saturday 27th and Sunday 28th March 2010.

Note that from 2007, the U.S.A. extended its Daylight Saving period to save energy.  Their DST will end on Sunday the 1st of November (at 02:00 local daylight time), starting again on Sunday the 7th of March 2010 (at 02:00 local standard time).  A useful website that shows the current date and time for most countries of the world, including if an when they change to Daylight Saving Time, can be found at: Time and Date.com.

In February 2001, the European Union agreed to stay with this last Sunday in March and October timing for the five years from 2002 to 2006, and required all member states to abide by this decision.  The British Parliament formerly adopted this in late May 2001 - see the UK’s “Department for Business Enterprise & Regulatory Reform” web page.  The following table shows the start and end dates for the current decade. 

Leap-Seconds

Many factors affect the rotational speed of the Earth, including tides, and the gravitational pull of the Sun and Moon, etc.  Since the creation of the Caesium atomic clock in the 1950's, it has been appreciated how relatively irregular the Earth's rotation actually is.  Atomic clocks are now used throughout the World, but they need correcting at irregular periods in order that they match the Earth's rotation.  See, for example, the UK National Physical Laboratory's leap-second web page.

In 1967 the second was defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.  However, since then the Earth has been turning, on average, a little slower.  Extra seconds - "leap-seconds" - are inserted into our time when it becomes too far ahead of the Earth.  This timescale is called UTC - "Coordinated Universal Time", and is kept within 0.9 of a second of the Earth's actual rotational time by the International Earth Rotation and Reference Systems Service.  Leap-seconds are usually only added after the last second of the last day of June or December, but in exceptional circumstances could also be added in March or September.

The IERS have recently announced that there will be no need for a leap-second at the end of June this year, and their predictions currently show that there will be no need for one at the end of December either.  (The last leap-second was on December 31st 2008.)

Enjoy them while you can - a proposal is still before the ITU to eliminate leap-seconds!  The idea is to keep a timescale called UTC that is based on Atomic clocks, but only kept within one hour of UT1 (the actual mean solar time) instead of one second as at present.  Apparent solar time already varies by about plus and minus fifteen minutes (see "Sundial Errors" below) which go largely unnoticed, and we practically all wake up after sunrise, and stay up well beyond sunset.  So it is perceived that there would be no problem having a timescale that doesn't accurately reflect mean solar time, and many advantages from maintaining a constant timescale over a much longer period.

For more information on leap-seconds, and time in general, the sites listed on my links page go into a LOT(!) more detail, but you may be interested first to read John Chambers' perspective on the subject in an email he sent me, and then look at the pages of Dr John Stockton who has a tremendous amount of useful information on the subject (http://www.merlyn.demon.co.uk/).  If you want to know what the current MJD is, the US Naval Observatory has a web page that displays it to five decimal places; and the German standards laboratory Physikalisch-Technische Bundesanstalt has an excellent page on atomic clocks

For reference, Caesium clocks run at (by definition) 9,192,631,770 Hz, Rubidium at 6,834,682,610.904,324 Hz, and Hydrogen masers at 1,420,405,751.7662 Hz.  With a single Ytterbium ion trapped in an ion-trap, and a femtosecond comb generator, PTB has made a great step towards the realisation of an optical atomic clock.  They found the frequency of an optical Yb+ transition at 688,358,979,309,312 Hz.  For more information on atomic frequency references, see:

MSF and Loran have moved to Anthorn in Cumbria

The British Time and Frequency transmission service on 60 KHz (known by it’s call-sign "MSF") has moved from its traditional home near Rugby in central England to a new site - Anthorn in Cumbria.  This is in the north-west of England, but much closer to the centre of the British Isles, so reception in Scotland will be dramatically improved.  See the NPL’s web page at http://www.npl.co.uk/time/msf/.  British Telecom ("BT"), who own the Rugby site, have lost the contract to Vosper Thornycroft ("VT" !) who already have a large aerial farm at the Anthorn site

The new transmitter site at Anthorn is at latitude 54° 54.7' North, and longitude 3° 17.3' West.  The old site near Rugby was at latitude 52° 22' North, and longitude 1° 11' West, and has not transmitted the signal since the 1st of April 2007.

The trial transmission of Loran from the Rugby site ceased on the 4th of July 2007, and has also been restarted at it's permanent home, again at Anthorn, from the 1st of October 2007.  See the announcement from Trinity House.

Sundial “Errors”

The exact length of a solar day varies throughout the year due to both the Earth's elliptical orbit around the Sun, and the inclination of its polar axis with respect to the plane of our orbit.  The time difference from the mean only amounts a few seconds at most, but accumulates to plus and minus about fifteen minutes, and this is known as the Equation of Time.  If the position of the sun was plotted (or photographed!) at the same time every day, a figure-of-eight shaped path would be drawn, which is known as the analemma.  Three websites which have excellent descriptions of the effect, and hence why a sundial can often apparently be "wrong", are http://www.perseus.gr/Astro-Solar-Analemma.htm, http://www.analemma.com/ and http://www.sundials.co.uk/equation.htm

E.B.U. / S.M.P.T.E.   Time Code

A table with the bit-allocation of television time code (as distinct from the IRIG codes used by NASA and the United States military) is on the next page