 Greetings and welcome to the Introduction to Astronomy. In this video we are going to look at the Ph.E.T. simulation on gravity and orbits that we will be using for part of this week's lab. Now you once you click on the main link within the class it should bring you to this website and then in order to start the simulation you just need to click on the play button to go ahead and get it started. So if you do so it will load up and it will give you a screen something like this and you'll want to click on the model simulation. The other one would show the Earth and Moon. Earth would show it depending on how you want things to be shown. So here we look at you have various different simulations you can look at. On this side it has various different preset areas that you can use. So you can look at the Sun and the Earth, the Sun Earth and the Moon, the Earth and the Moon or the Earth and a satellite. You can depend on whether you turn gravity on and off. There are some selections here that we will look at to show the gravitational forces and you can make some adjustments. You can adjust the star mass or the planet mass and see what happens. So what we can do is take a look at some of these and in fact if we turn on the gravitational force we will now see the gravitational force between these two objects. If we turn on the velocity we will see in what direction the Earth is moving at that instant. Now even though it's not moving in the simulation it has to be moving because otherwise the gravitational force would pull the two objects together. So the Earth is moving in this direction of this velocity. And then we can go ahead and you can show the path if you like. So if we want to look at what the path is, see the orbit and we can turn on the grid to see a grid here. And then we can go ahead and hit play to run the simulation forward and as you do you can see the orbit here that the path that it is following and you can see the velocity how the velocity is changing in direction. You can also see if we pause this here that interestingly even though this looks pretty much like a circle but as we pause it we can see that it is not exactly a circle. So this is one of the examples of Kepler's laws that we'll look at but on this side it was right on this grid line whereas on this side it's not on the grid line and that is because it is actually an elliptical orbit. So it is two units here, two grid blocks here but a little less than two grid blocks this way. And that is because it's actually an elliptical orbit much like our own Earths. Sometimes we're a little bit closer to the sun and sometimes we are a little bit further away. For the Earth of course we're closest to the sun in January and furthest from it in July quite the opposite of what you might expect for our seasons. And as we go ahead and continue to let this run we can look at some of the other things that we can change. We can change the mass of the sun, for example. If we make the mass of the sun larger the gravitational force gets larger and now the Earth's orbit changes and it starts to make a much smaller orbit due to the additional gravitational force. You can also watch how the gravitational force lines change getting much much stronger when it gets close to the sun and then the gravitational force being larger when it is further away. Now if we go back, let's go ahead and reset things so we're actually back to the beginning and we have our Earth moving around again. And what happens if we change the mass of the Earth? So if we change the mass of the Earth the gravitational force increases but what is happening to the orbit? We're not seeing any changes. The mass of the Earth is really not important in terms of the orbit that would be followed and in fact you could replace the Earth by a speck of dust with the same orbital parameters and it would continue to orbit exactly the same. When we look at orbits it is the object at the center that is important, its mass. That will change things if we change the mass of our sun all of a sudden the orbit starts to change but if we change the mass of the Earth nothing happens. So those are just a few of the things that you can do and you can also hit a reset button here if you want to reset everything back to its defaults and we can look quickly at one more if we want to look at the path here shown by the Moon, Earth and Sun and if we run that simulation you can see a very interesting path for the Moon. The path that the Moon follows we see it as circling the Earth but it actually follows a very interesting path around the Sun. Now interestingly if you turn off the path and watch it it looks just like the Moon is moving right around the Earth which we know it is but if you watch its overall path it actually has this little kind of looping path as it orbits around the Earth and the Sun. So just a few things that you'll play with and you'll have to adjust some of these different things you'll use the little timer here to measure the period sometimes and you can use some of the other directions as given to within the program itself and within the lab. So that concludes this video on the Gravity and Orbit Simulator. We'll be back again next time to look at another one of these lab simulations. So until then have a great day everyone and I will see you in class.