Imagine if our world had a diary and on the interesting days, someone pulled out a big book and wrote about it.
It was about 4,475 million years ago and it was a warm day, well, hot actually as the surface was mostly molten rock tipping the scales at around 1000 degrees centigrade. Our freshly minted sun was just a pup, only 150 million years old and had gobbled up most of the material in this particular part on the outer rim of this particular arm of this particular spiral galaxy in this particular cluster of galaxies, well, you get the idea.
The few scraps (0.14%) that were left over included the rocky bits, Mercury, Venus, Earth, Mars, Theia, ….. Theia? Yep, there was a planet called Theia zooming around the new sun, just as we were.
Among the others, all gas and no substance, poor old Jupiter couldn’t find enough hydrogen and helium to really get going as a star and other gassy ones, Saturn, Uranus and Neptune were way out of contention for ‘star’ material.
We not sure about Theia’s orbit but we do know it was rocky and on a collision course with us because one fine morning, it arrived, tootling along at 5 times the speed of a bullet and smacked into the northern hemisphere of earth.
We know that because the collision tipped the earth over 24 degrees to the side, which turned out to be a very good thing for people who enjoy spring. Without that collision, we would have no seasons and I’d miss that.
If you were around on that day, you would have wanted to be standing well back behind the barrier because it made quite a bang with a large chunk of rock being blasted off into space. Unfortunately, lots of bits didn’t quite make it that far and a rain storm of disconsolate rocks began pelting down at an alarming rate. If ever there was a day to remember your hard hat, this was the one.
Over the next little while, most of the rocks that stayed ‘up there’ (a relative term as there is no up or down in space) became very attracted to each other and after mere 100 laps around the sun, a century as we know it, we had a moon.
We also had a fairly brisk day too and if you had been around to see it, you would have been able to watch the sun strolling across the sky because the day was only 3 hours from sun up, to sun down, that is, a 6 hour day.
This is because when Theia paid her visit, she came from our south west, hitting the top half a glancing blow that caused us to spin like a merry-go-round.
In the time that has elapsed since that fateful day, we have slowed down quite a bit to our familiar 24 hour day. Our orbit around the sun has slowed quite a bit since then too but that’s another story.
At this point, the moon was ‘right there’, in front of you, a mere 22,000 kilometres away. This is so close, it covers the sky so you would not be able to see any stars and the tide, well, if we had water, which we didn’t, suffice to say, the tides would have been memorable.
As it happened, when our water was finally delivered, the moon was still incredibly close, by our standards today and yes, the tides were something to behold, 1,000 foot high and that’s after they died down a bit.
At the beginning the moon was sneaking off at a fair cracking pace but has slowed its escape now to just 38mm a year giving it an average getaway speed of 85mm a year, which is not much I hear you say, but it’s fast enough to have drifted 360,000 kilometres.
What future then for our pet rock? Well, that depends on the Sun which is planning some serious expansion in another half a billion years or so and it will be reaching out once a month trying to catch the moon as it orbits the earth.
By then, the orbit will be much wider and the ‘month’ will be longer too, probably about 42 days, so it’s all a bit of guess work at this stage. Unfortunately, the most likely scenario it will end up much like one of Saturn’s rings only rocks around the earth, not pretty ice crystals around Saturn.
(You can see ALL the interesting ‘days’ in the magnificent science poster which you can download and print. See what it looks like here.)