Friday 23 April 1999
Yes. Because of how we measure time, we sometimes need to add leap seconds to match up our clocks with the actual position of the Earth around the Sun. The rotation of the Earth about its axis isn't exactly even, which makes the days vary in length. These slight changes didn't matter until 1967, when atomic clocks were invented and the second was defined as a certain number of oscillations of a caesium atom. This definition is what gives us International Atomic Time (TAI).
But the TAI isn't based on the earth's rotation, so a calender based on TAI gradually becomes out of step with one based on GMT. We could use TAI as our official time (and in fact we do, with slight alterations of leap seconds) but then the time would get out of step with day and night and the seasons. So for convenience we use GMT and adjust TAI accordingly. In 1972 a new Coordinated Universal Time scale (UTC) was adopted for international use. It combines all the regularity of atomic time with most of the convenience of GMT. The seconds of UTC are of the same length as those of TAI, and then UTC is kept within one second of GMT by the insertion of extra leap seconds.
Q Which way do the Earth's magnetic field lines go?
On a bar magnet, the field lines go from north to south, but the direction of the Earth's magnetic field goes from south to north.
The problem arose because maps originally used the top of the Earth as north, and the bottom as south. Lodestones (naturally occurring magnets) were used as direction pointers, and it was a long time before physicists came up with bar magnets and theories for magnetism. Unfortunately this mistake was not realised until it was too late to rectify, so the problem has stuck with us.
You can visit the Technoquest World Wide Web site at http://www. sciencenet.org.uk
Arts & Ents blogs
- 1 Al Pacino on suffering from depression: 'It can last and it's terrifying'
- 2 Half of young women unable to ‘locate vagina’ and 65% find it difficult to say the word
- 3 Saudis risk new Muslim division with proposal to move Mohamed’s tomb
- 4 A teacher speaks out: 'I'm effectively being forced out of a career that I wanted to love'
- 5 Mexican woman becomes world’s 'oldest person' at 127
Jessica Chastain demands Scarlett Johansson-fronted Marvel superhero movie
Downton Abbey series 5 start date revealed: ITV drama to return in late September
Nicki Minaj suffers wardrobe malfunction during MTV VMAs performance with Ariana Grande and Jessie J
How to read Will Self: Unlike 'talented mediocrity' George Orwell, you must approach this writer dictionary in hand
New Netflix releases: Films and TV shows coming in September 2014
Rotherham child sex abuse scandal: Labour Home Office to be probed over what Tony Blair's government knew - and when
What do immigrants really think of Britain? Polish immigrant's Reddit post goes viral
Ashya King: Parents of five-year-old boy refused permission to visit him in hospital and denied bail at Spanish court
With Douglas Carswell joining Ukip, my party has taken another giant step forward
When elitism grips the top of British society to this extent, there is only one answer: abolish private schools
Ashya King: 'Cruel NHS has not given us the treatment we need', says father of five-year-old with brain tumour who fled to Spain