How Old is the Earth?

In the Christian tradition, the first thing you are told is that God created the earth in six days, and rested on the seventh. Before I was exposed to science, or philosophy, or, indeed, before I could understand what was right in front of my eyes, I was taught from the Bible. Yet, as I was exposed to more things both tangible and intangible, I started to wonder about the veracity of it all.

Einstein brought the world his theory of general relativity during the early parts of the 20th century, that is to say, how space and time interacts in the grand scheme of things. One of the first questions I remember asking was; is time truly constant? I reached a point in my life where I started to feel as though one day was very much shorter than what I remember it being as a child. Could it be that time was speeding up? The truth of it is that the answer is not a simple yes or no. One second in time remains one second. However, after one has experienced 10 years, 20 years, or in my case, 50 years of it, one second becomes an ever shrinking fraction of the whole. Thus, one would experience it both in its concrete form but also as the part of the whole that it represents. Therefore, to the observer, it may be that time, or the experience of it, speeds up.

The obvious next question, therefore was; has a day always been a 24-hour construct. Has the earth always taken this long to rotate once on its own axis? Again, the answer isn't a simple one. As it happens, the length of one day has varied over time due to the influences of forces, both within and without the earth, that have either lengthened or shortened the time that we experience a day to be in the time we live in. Regarding the long term, over millions of years, earth rotation has been slowing down due to friction effects associated with the tides driven by the moon. That process adds about 2.3 milliseconds to the length of each day every century. This means that just a few billion years ago an Earth Day was only about 19 hours.

The caveat to this has been that for the past 20,000 years, another process has been working in the opposite direction, speeding up its rotation. When the last ice age ended, melting polar ice sheets reduced surface pressure, and Earth's mantle started steadily moving toward the poles. Just as a ballet dancer spins faster as they bring their arms toward their body – the axis around which they spin – so our planet's spin rate increases when this mass of mantle moves closer to Ear