Well, that is simple enough; it is 24 hours long, right? Well not exactly, and depending if you are talking about a solar day or a stellar day. The stellar day is the time it takes the earth to rotate one time in respect to the celestial background and a solar day is what we are most familiar with, when the sun reaches the same spot in the sky for each earth rotation. 45 years ago, the second was defined in terms of the wave length of light and up until recently when some neutrinos seemed to break the speed of light, that was the most accurate (and still is) constant to base time on. So, I guess that means that the average length of a day is 86400 seconds.
OK, everything seems to be running smoothly but wait, there is always a catch somewhere. As far as the Earth is concerned it is the Moon that is putting on the brakes. Because of the Moon’s gravity, it has slowed the rotation of earth down a bit. Did you know awhile back the length of a day was more than two hours less than it is now. Let’s see, 2 hours times 365 days equals 730 hours divided by 24 equals 30.42 days. That means we have to live about a month longer each year to be the same age as you would have been if you lived back 620 million years ago!! Since I will be 70 my next birthday, if I lived 620 million years ago I would be almost 76 years old instead of 70.
Wow, now I feel a little bit younger (grin), how about you? All you young people out there have a great day now, you hear? You have an extra two hours to enjoy it that I bet you didn’t know you had.