 Greetings and welcome to the Introduction to Astronomy. In this video we're going to discuss some of the coordinate systems that are used in astronomy here on the earth and in the sky. So our primary thing we're looking at, in fact all we're going to look at in this lesson, is something about the coordinate systems. Now these are important because they allow us to determine directions. Where is something located in the sky? So how can we find out exactly where something is located? We need to give it a coordinate, some sort of positioning determining exactly where that object is located. So let's start off looking here at the earth first. So how do we determine positions on the earth? Well we use two different coordinates. We use the latitude here and we use the longitude. And specifying those two coordinates allows us to determine exactly where an object is on the surface of the earth. The latitude is an angular distance measured north or south of the earth's equator. So if we look at the first image here the earth's equator is zero degrees right there and you could go up to plus ninety degrees up to the north pole and you can go down to negative ninety degrees or ninety degrees south at the south pole but you could never have anything greater or less than ninety degrees because if you pass the pole you're on your way back down or back up and your latitude is then decreasing again. So it has a maximum latitude of ninety degrees and in fact the latitude will range from zero to ninety degrees or we could say zero to ninety degrees north and zero to ninety degrees south if we're looking at the south pole. So it'll always be between zero and ninety degrees. You will never have anything greater than that. The longitude again is an angular distance. Both of these are angle measures and this is measured east or west of what we call the prime meridian. A meridian itself is just a line, what we call a great circle on the earth that passes through the two poles and is therefore centered on the centered on the center of the earth. So any object that does that would be called a meridian. We select one to be the prime meridian because that is where we measure everything from. We need a reference point. Unlike the equator which gives us a very consistent reference point for everyone when we are looking at latitudes there is nothing for longitude. There is no specific reason why any one meridian is better than any other meridian. You need to predict, you need to select one of them as your reference point but any of them could be very easily used. And the consensus that has been come to is that the meridian that goes through Greenwich England is the one that will be used to determine longitudes. So you can, if you go east of that then you would have an east longitude. If you travel west of the prime meridian you would have a west longitude. So the longitude can then vary from zero to a hundred and eighty degrees east or zero to one hundred and eighty degrees west of the prime meridian. So areas in the Americas would be west longitudes. Areas in Europe and Africa and Asia would generally be eastern longitudes and it just matters what direction you're measuring relative to this prime meridian. Now it's there for convenience and for reference. Again it does not matter which one you select but everybody has to agree on the same one. And if the English are using the meridian that goes through London and the French are using the one that goes through Paris then that is going to give different values for the longitude and can cause issues with navigation. So one had to be agreed on and this is the one that was eventually accepted. Now an example of these coordinates and looking at how we define these would be here we're going to look at the Washington Monument. Where is that located? Well it's at a latitude of thirty eight point eight eight nine five degrees north and a longitude of seventy seven point zero three five three degrees west. So that tells us it's nearly forty degrees north of the equator and about seventy seven degrees west of the prime meridian. The more detail, the more decimal points we know, the more accurate we know those values. Now how can we apply this to the sky? We don't have latitude and longitude on the sky. What we use are two other coordinates and on the sky we use declination as which is comparable to the latitude and we use right ascension which is comparable to the longitude. So they work similarly on the sky to the way latitude and longitude worked here on earth. Note that they are both angular distances, they are all angular measures and the latitude is measured, the latitude or the declination is measured north or south of the celestial equator. So imagine the Earth's equator projected out to the sky that would be the celestial equator that divides the sky into two halves and north of it you have positive declinations going up to ninety degrees at the north celestial pole and negative ninety degrees at the south celestial pole. So just like latitude going from a negative from zero to ninety north or zero to ninety south declination goes from zero to uh... plus ninety degrees at the north pole and zero to minus ninety degrees at the south pole. So that gives us again a way to determine how far we are north or south of the equator but we need to find out how far we are along the equator that is also necessary and that is the right ascension. So much like longitude it is measured east in this case of the vernal equinox and much like the Greenwich Meridian the vernal equinox is simply a point that has been selected to determine where the right ascension is measured from. It is what astronomers have agreed on. The vernal equinox is the position of the sun on the first day of spring. Now the other thing that you note here is that the right ascension is measured in hours not in degrees. There are twenty four hours of right ascension and twenty four hours in a day it's a way to be able to work timing into this as well and it is convenient for astronomical uses and you can always convert it by knowing that one hour of right ascension is fifteen degrees. So you could use it in degrees as well you would just multiply the value by fifteen to convert the hours into degrees. So we measure we have our two ways of measuring them we measure the declination north and south of the equator we then measure the right ascension east or west or sorry just east in this case I apologize it is always east of the vernal equinox so in this case the right ascension goes from zero hours to twenty four hours it is always measured in the same direction always east of the vernal equinox so if we look at an example of Betelgeuse Betelgeuse is at a declination of seven point four zero seven zero six degrees and a right ascension of five point nine one nine five three hours and what that means is that it's a little bit north of the equator it's a positive declination not negative so it's a little bit north of the equator and is it about six hours around from the vernal equinox six hours to the east of the vernal equinox so what that means is if the vernal equinox is just setting is just on the western horizon then Betelgeuse will be six hours east of that or would set six hours later or in this case five point nine one nine five three hours later but we can use these coordinates to determine the exact location of Betelgeuse in the sky and be able to point a telescope accurately to it we would know exactly where it is just as the latitude and longitude told us exactly where the Washington monument is the declination and right ascension tell us exactly where Betelgeuse is in the sky so let's review a little bit about what we've covered in this section just to summarize we looked at two different things we looked at latitude and longitude which measured the distances on the surface of the earth we looked at declination and right ascension which measured the positions on the celestial sphere so two ways to measure here is one for the earth one for the celestial sphere are some very good similarities in them the declination and latitude are very very similar right ascension and longitude are also very similar but remember that right ascension is measured in hours whereas longitude was measured in degrees here on the earth so that concludes our discussion of the coordinate systems here on the earth and in the sky we'll be back again next time for the next lecture so until then have a great day everyone and I will see you in class