How long is a minute and an hour really?

So I learned that a day isn’t exactly 24 hours. It’s 23hrs 56mins and 4secs. So if this is true then doesn’t that mean a minute isn’t exactly 1min? And doesn’t a minute not being exactly 1 min mean an hour isn’t exactly 60mins? How long are minutes and hours exactly?

3 Answers

  • ?
    Lv 7
    3 years ago
    Favorite Answer

    The SI unit of time is the second. All our time measurements are based on the definition of the second. It is the 'base unit' of time.

    So the question is how do you work out how long one second is? And the answer is that you can work it out by doing an experiment. In short, what you do is take atoms of Caesium-133 and contain them in a tube. You hit them with microwaves and the energy causes the electrons jump to between energy levels in the atom. There is a particular pair of energy levels in Caesium that are really close together (these the 'hyperfine levels of the ground state'). So when the microwaves are turned on, and the electrons are excited to higher energy, they can eventually drop from the higher of these hyperfine levels to the lower level, thereby producing a photon. Here's the clever bit - that photon has an energy and frequency directly related to the difference in the energy of those two levels but you also need the microwaves you're using to have the same energy and frequency. If your microwaves have the wrong frequency no photon is emitted. So what you do is tune your microwaves until they have the right frequency to cause a transition between the two hyperfine levels! This piece of equipment is called an 'atomic clock'. And we use it to define what a second is - it is the time it takes for exactly 9,192,631,770 cycles of the microwave energy you need to excite these hyperfine levels in Caesium-133.

    So you can see that we no longer define the second (and we haven't done since 1967!) in terms of the Earth's spin. Time, on Earth, is determined by a network of these atomic clocks all over the planet.

    A minute is defined as exactly 60 seconds. An hour is defined as exactly 60 minutes, or exactly 3600 seconds.

    So the question then becomes 'how long is a day'? Well, it takes the Earth 23 hours, 56 minutes, 4.0905 seconds to spin once on its axis. This time period is known as a sidereal day.

    Let's suppose you wait until the sun is at its highest point in the sky. And then you look again 23 hrs, 56 mins, 4.0905 secs later. The sun won't be at its highest point in the sky but the Earth will have turned once on its axis. However, in that time period the Earth will also have moved slightly along its orbit around the sun so you're now pointing slightly away from the sun. The Earth needs to rotate for another 3 mins 56 seconds before the sun appears at the highest point again (slightly more than one full rotation on its axis). This is the 'solar day' which is 24 hours.

    So, in short, the 24 hour day has to do with the time it takes for the sun to appear at the highest point in the sky and is the result of the Earth having to spin slightly more than 1 full revolution to bring it to that point due to the motion of the Earth around the sun itself. So all of those times you heard that 'it takes the Earth 24 hours to rotate once' were a simplification!

    A good diagram showing this effect is:

  • Anonymous
    3 years ago


  • 3 years ago

    Let's do away with time and see everything run smooth and clean right back to reading the sun in the sky to know you have to eat dinner or ride a bus.

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