now what about revolution revolution is the earth traveling in almost a circle around the Sun like I say it's almost a circle in fact it is what's called an ellipse which is a kind of oval I'll draw it rather really exaggerated there would be the Sun there would be the orbit of the earth we always say that the Sun is 93 million miles away at this point here the earth is only ninety one and a half million miles from the Sun at that point there I told you this is seriously exaggerated it's ninety four the average of those is the 93 million that we usually talk about these points on the Earth's orbits have special names this point here when we're closest to the Sun is called Harry Elia which is Greek for near to the Sun and the opposite side is called aphelion well six months later we are farthest from the Sun now of course the further the earth is away from the Sun the less he's received from the Sun so do you think that this is what causes seasons somehow a retro sort of the Sun winter when we're further away it's an appealing idea but no it is this should give you a clue to why perihelion is long the third of January aphelion is July the fourth so we are closest to the Sun in the middle of winter and furthest from the Sun the middle of summer the difference here 91 and a half to 94 1/2 is pretty small that difference only leads to a 2 or 3 percent difference in the amount of heat coming from the Sun and we are about to see factors that make a much much much bigger difference than that which are what actually caused the seasons so don't confuse this in any way with seasons how long does it take the earth to make one complete trip around the Sun or if you like how long is a year exactly the length of the year the time taken for the earth to go around the Sun is 365 and a quarter days 365 days and 6 hours so theoretically each year starts 365 days and 6 hours after the year before so if one year starts at midnight the next year should start at 6 a.m. the one after that at 12 noon the one after that 6 p.m. and so inconvenient we want a year this is a whole number of days ok so how does that work out we start here after one trip around the Sun the earth hasn't quite got back 365 days later to where it should it's it's 6 hours short but that's the 365 day a year after a second 365 day yeah yes is now 12 hours short after a third 365 day the earth is 18 hours sure and after a fourth yeah we're 24 hours short so what do we do in that fourth year we add another day we give that out year 366 days and now we're back where we started so every fourth year has a one extra day and that's what we call a leap year where do we put the extra day we put it at the end of the year huh it's in February that's not the end of the year well think about the calendar historically our calendar was basically invented by the Romans and they started out naming months after their gods so they started with March named ro Mars then they got to June label named for Juno then they quit on gods are sorry naming their months after their most famous emperors July is named for Julius Caesar August is for Augustus Caesar and after that they just went to simple numbering seven eight nine ten think about it September September October M is nine Dec M is 10 December is the tenth month in that calendar January is the 11th of February the last one there is the 12th so they did tack on the extra day at the end of the year the end of the Roman yeah by the way why are the months different lengths well a month is based on the moon orbiting around the earth the moon orbits around the earth in roughly 28 days if you divide 28 into 365 or 306 alive in a quarter you come out with about 13 so the Rayleigh should be 13 months in the earth trouble with 13 is it's a prime number you can't divide it by anything that you can't have half a year a quarter year so what do we do for practical purposes we stretched out on submit so we could have 12 of them 12 is a wonderful number you can divide it by 2 by 3 by 4 by 6 it's one of the most divisible numbers there are so let's say we added a few more days to the month to come up with 12 when you divide 12 into 365 you come up with about 30 and 1/2 so what do we do we alternate we have a 31-day month for 30 day and 31 at the 30 or 31 but at the end of the year we're not quite right we're a little short and so once again in the last month February god only 28 days that was all that was left over and since February was kind of shortchanged another good reason to give it the extra day when the fourth year rolls around and we have an extra day that we need to include the path of the earth in its orbit around the Sun lies in a plane same plane as all the other planets remember that that plane has a special name it's called the plane of the ecliptic remember that name you need to know that name plane of the ecliptic plane in which the Sun and all the planets lie so the earth travels along the ecliptic now what about the Earth's axis is yes axis up right as it travels along the ecliptic no it's not the Earth's axis is tilted as an angle like so as it travels along its alt I'll show you the picture of that here it is alright there are some numbers here you need to know first of all the purple plane here is the plane of the ecliptic from the earth after the towards the Sun the orange plane here is the Equator perpendicular to that the Earth's axis going from the North Pole to the South Pole yes axis the angle between these two planes the tilt of the Earth's axis is 23 and 1/2 degrees appear there showing the angle between a perpendicular line to the plane of the ecliptic and the Earth's axis again that angle is 23 and 1/2 degrees that's a more rigorous geometric proof 23 and 1/2 degrees between two lines than an angle between two planes because depending on the direction you look in you can have different values but still 23 and 1/2 degrees this is why all globes have the equator at an angle of 23 and 1/2 degrees please notice the other angle here nine the compliment ninety minus twenty three and a half degrees is sixty six and a half degrees and that's going to become important a little bit later on now this is very important as the earth travels along its orbit around the Sun its axis always stays pointing in the same direction these are always points to the same spot well using that it points to Polaris that far far far distant star that is always appear above the North Pole this feature where the Earth's axis always points in the same direction wherever it is around its orbit has a special name you'll see in your textbook it's called the parallelism of the Earth's axis what that means is that on one side of the Sun over here the sun's rays fall in the southern hemisphere the other side of the Sun over here the sun's rays fall on the northern hemisphere southern hemisphere specifically on the right the sun's vertical raise for at 23 and 1/2 degrees south a special line of latitude called the Tropic of Capricorn and on the left the sun's vertical raise for latitude of 23 and 1/2 degrees north a special line of latitude called the Tropic of Cancer this has a profound effect on the amount of heat received in different parts of the Earth from the Sun in this picture here look at these two equal amounts of the sun's rays where the sun's rays fall vertically that portion of sun's rays hits an area this big down here they are spread out over three times the area so the difference between the amount of heat here and the amount of heat here per unit area this is 300% more heat than in this region here that 300% is a huge difference much much bigger than the two or three percent difference in heat resulting from exactly how far the earth is from the Sun at different times during the year this then is the major reason for seasons notice that when the sun's vertical rays are in one hemisphere yeah the more than sphere the oblique rays are in the other hemisphere the southern hemisphere so while the northern hemisphere experiences summer the southern hemisphere experiences winter and vice versa