Little by little, the length of Earth's day is increasing by 1.7 miliseconds every century over the past 2700 years. However, you don't need to reset your watch anytime soon. But just how much longer will the day become eventually and how soon?
What is The Length of Earth's Day ?
Earth's Day is measured from the time each day when the most upper portion of the Sun's disk first appears on the horizon (i.e. sunrise) to the time when that same upper portion vanishes below the horizon (i.e. sunset). Presently, the length of Earth's day is twenty three hours, fifty six minutes, and approximately four seconds. However, the solar day (sunrise to sunrise) is a bit longer at twenty four hours with a one millisecond range.
The History of Earth's Day Length
The length of Earth's day has increased over the eons. Initially, around 4.5 billion years ago (when the Earth was still brand new), the length of one Earth day was approximately six hours according to today's computer simulations. 620 million years ago, it was 21.9 hours according to alternating sandstone layers known as rhythmites. Due to the way in which a single second is defined, the mean length is presently 86,400.002 seconds.
You might be asking yourself what is a 'second' anyway? Seconds are units of time that are measured by a clock, based on fundamental properties of nature. Before the nineteenth century, seconds were defined by the motions of the Earth (i.e. twenty four hours, one day, was equal to 1/86 400 of the time necessary for our planet to conduct a single rotation on its axis). Every century, the length of Earth's day is increasing by 1.7 milliseconds on average (based on the past 2700 years).
The Factors of Earth's Day Length
The Moon: Tides generated by the Moon cause Earth's rotation to slow.
Why is the Earth's Day Length increasing?
The reason the day is getting longer is the result of the Earth's tidal interactions with the Moon. The Moon is slowing us down, gaining energy and moving away from us. Approximately 4.5 billion years ago, our Moon was much closer to Earth (hence the day length was six hours at the time). Every year, the rotation of the Earth is becoming slower and slower because the Moon is creeping further away from us at a rate of about two inches per year.
Don't let this alarm you though, we are not going to lose the Moon and the Earth is not going to stop spinning. At the rate that our day length is increasing, we will never notice the difference in our lifetimes. In about 5000 years, the Earth's day will be longer by a second, so you can forget about resetting your watch right now. And just in case you're wondering what life on Earth would be like if it were to stop spinning, well, let's just say that the force keeps your feet on the groudn will cease to exist, so let's be glad that we have good old gravity.
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