the orbit of the planet Mercury is a bit strange and doesn't act exactly as expected this was such a problem but we even invented another planet to account for its weird orbit in fact it's this strangeness that provided evidence that Einstein was right all along let's find out more this video was suggested in a comment so thank you Stephanie for the suggestion and if anyone else has any suggestions then leave a comment below please before we talk about Mercury um we'll get to that we need to talk about Einstein and relativity in fact let's go back even further to the time of Newton when Newton devised his laws of gravity he knew that gravity exists as a result of the force that massive bodies have on other bodies massive bodies feel an attractive force between each other and this can be used to explain why apples fall from trees and also why planets orbit the Sun and so on however Newton didn't understand what exactly causes the force that we call gravity but for the majority of observations and calculations Newtonian gravity works very well indeed and could be used to predict the movement of the planets and moons all except for mercury when Einstein came along he devised the general theory of relativity this States among other things that gravity rather than being a force felt between two objects is actually caused by the warping of space-time objects in the universe tend to try to move in straight lines however the warping of space-time will mean that a body that moves towards a massive body like a planet or a star will have its motion bent into a circular orbit by the warping of space-time the theory of general relativity also says that gravity affects time and that time Flows at different speeds the closer you get to a massive body the closer you are the slower the passage of time this effect is relatively small when considering a body the size of the Earth the core of our planet being the closest to the center of gravity is about two and a half years younger than the crust of the planet in other words time has passed more slowly for the core of our planet than the crust but that's two and a half years over the four and a half billion year life of our planet so that's not a great deal however the stronger the gravity the greater the time dilation near to black holes time will pass even more slowly but that's for another video so what has all this got to do with Mercury the planets orbit the Sun but their orbits aren't perfect circles they're ellipses as we can see here when the planet is closest to the Sun we call this the perihelion and the point at which the planet is furthest away from the Sun is called the aphelion in addition to this the planets don't travel around their orbit at the same speed the planet gets closer to the Sun it speeds up this is because and this is very much a simplification of what's really going on there are two kinds of energy controlling the orbit of the planet these are gravitational potential energy and kinetic energy the sum of these two energies must always be equal gravitational potential energy is related to the distance between the two bodies when the two bodies are the closest the gravitational potential energy is at its minimum when they are furthest away the gravitational potential energy is at its maximum a bit like if I have a ball and I hold it one meter from the earth it will have some potential energy if I move the ball so now it's 10 meters from the earth it's got greater potential energy kinetic energy is the energy of movement a higher kinetic energy is related to a higher speed because kinetic energy and potential energy must always add up to the same amount the higher the gravitational potential energy the lower the kinetic energy so the lower the speed conversely the lower the gravitational potential energy the higher the kinetic energy and so the higher the speed in fact Kepler's second law states that if I was to cut the orbit up into sectors of equal area the planet would spend equal times in each sector I've drawn this in a very exaggerated way but if the planet was taking equal times going through each sector it would have to speed up to make it round these sectors here in the same time that it would take it to move around these sectors here all planets then have an elliptical orbit but Mercury's orbit is very elliptical and it's only because of the elliptical orbits that we get to the next part of our story and this is called precession precession is the change to the elliptical orbits that happens as the planets orbit the Sun each orbit follows a slightly different path to the previous one the perihelion of each orbit is moved slightly from the previous and this happens as the elliptical orbits rotate slowly as I've shown here again in an exaggerated way precession is caused largely by the effect the other planets have on each other as they orbit the other planets in the solar system exert a gravitational pull on each other and this is what causes the orbit of each of the planets to process and here's where we get tile problem the precession of mercury isn't what it's expected to be using calculations based on the mathematics of Newton it was calculated that the orbit of mercury should process by 531 seconds of Arc per century one second of Arc is one three thousand six hundredth of a degree so 531 seconds of Arc is about a sixth of a degree per Century these calculations take into account the effect that the other planets have on each other also the slight defamation of the sun due to its Spin and the fact that the Earth is rotating and orbiting and is therefore not an inertial frame of reference however measurements of the procession of mercury give a value of 574 seconds of Arc per century that's a discrepancy of 43 Arc seconds per century now that might not seem like a lot but the discrepancies should be very close to zero a number of different fixes were suggested for this discrepancy including dust lying between the Sun and mercury and even an extra Planet close to the Sun affecting the orbit of Mercury this planet was even given a name Vulcan all of these fixes however were abandoned eventually it was found that Einstein's theory of general relativity accounted for the additional precession this means that even if there were no other planets or any other factors causing the procession of mercury the orbit of mercury would still process because of the curvature of space-time caused by the Sun the extra procession that happens in Mercury and Venus and Earth to a smaller extent those extra 43 seconds of Arc are caused by the curvature of space-time and that's because of the huge gravitational presence of the Sun this is because as Mercury orbits a move towards its perihelion it moves further and further into the Sun's gravity well where space-time is more and more curved and this makes the position of the perihelion move with each orbit so what does this all mean well general relativity says that inner gravity well time runs slower than clocks would do further away from that gravity well and the Earth whilst not as strong as the sun has considerable gravity of its own this means it have already mentioned that clocks on Earth would run slower than clocks in orbit and whilst this discrepancy might be minuscule it's important GPS systems use satellites to very accurately determine the location of the device that's using it these days pretty much every smartphone uses this technology and this time discrepancy needs to be built into the software that governs these systems so that we can accurately find our way to the nearest coffee shop I've decided to come to Mercury and have a little look around if you enjoy my Journeys around the universe then please consider subscribing and hitting that little bell icon thingy and for now and until next time thank you for watching