 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy, we are going to talk about right ascension and declination, a coordinate system used in astronomy. So what are right ascension and declination? Well, they are coordinates used to measure positions in the sky. And the right ascension, or RA, is similar to longitude here on Earth, whereas the declination is similar to latitude. So we use them as ways of measuring things on the surface of a sphere. Let's take a look at this on our chart. The declination is measured north and south of the celestial equator. So here this dotted line is our celestial equator. And that means that if we want to measure the declination, we could go either north of that, and that would give us positive values up until you get to the pole. The pole would be 90 degrees. And then you could also measure south of the celestial equator, which would give you negative values and you would get negative 90 degrees for the south celestial pole. Now just like latitude here on Earth, the declination cannot be greater than 90 degrees. That's the maximum that you've reached the pole. You cannot go any higher than that. If you go past the pole, you start to decrease the declination again. Now the right ascension instead is measured a little differently. It's measured in hours, and it's measured from a point called the vernal equinox. So what we have is the vernal equinox. What is that? Well that is a point on the celestial sphere where the ecliptic, which is the pink line here or path of the sun, intersects the celestial equator. Now those intersect twice, but we're looking here at the one where it intersects as the sun is moving above the celestial equator. So here's the sun on the ecliptic going from March into June. It is getting higher and higher. So it's crossing the celestial equator, moving toward the north, moving up higher and higher in the sky. So that is the right ascension, and it's always measured eastward of the vernal equinox. So you don't measure it both directions, which we do with longitude here on earth. That's a little bit different. And it's measured in hours, which tie in pretty well to the way things are measured in the sky. Now let's look at an example of this, and let's look at a star chart here. And let's look at a position of the star Rigel. Now Rigel, one of the bright stars in the constellation of Orion, is at a right ascension of 5 hours 14 minutes and a declination of negative 8 degrees and 12 minutes. Let's just round that off to find the rough location. Its right ascension is about 5 hours, and its declination is about negative 8 degrees. Let's look at that on the chart. Now note that the right ascension of zero is off here to the right, and the right ascension increases moving to the left. And it's because the sky charts are always going to be backwards, because you are inside the sphere looking up at it. So it would be like looking from the interior of a crystal earth that you would see all of the countries would be backwards from what you are used to seeing. If you could look from the interior of earth, you would see that it would look like that California and New York were reversed in positions relative to that. The whole country, the United States, would be flipped backwards, the same for every other country. So let's look at what we are looking for here. We are looking for about 5 hours of right ascension. So we would want to move up this line. And then we want to find a negative 8 degrees. This is about negative 10 degrees. Let's look at that. And the bright star that is close to where those two intersect, and we can see that here. Here is the bright star here in Orion. That is the bright star, Rigel. So we use these to measure the coordinates and the position of objects in the sky. Now one thing that happens is that the coordinate system is shifting. And that is because of what we call procession. Procession occurs because the earth is spinning like a top. It spins on its axis and it does that spinning quickly here once per day. So that's one day that it takes to spin on its axis. However, it also can spin. It also spins in a slow wobble around the pole here. Meaning that the position of the pole star changes. So the pole star, which is close to Polaris is close to right now, is close to 90 degrees. The exact pole would be exactly 90 degrees. But that is changing. So the position of the pole changes means our entire coordinate system is changing. So when we specify the coordinates of a star, we have to specify them for a specific year. That this is the location of the star that year. Do they change quickly? No, it's a very slow change. But when you need to precisely point a telescope, it is very important to take that into account so that you know the exact location of those of that object for your current day for which you're observing. So let's go ahead and finish up with our summary. And what we looked at was that right ascension and declination are coordinates that are used to measure position in the sky. Right ascension was similar to latitude, declination similar to longitude. And we looked at how these coordinates slowly change due to the procession of Earth's axis. So that concludes this lecture on right ascension and declination. We'll be back again next week for another special topic in astronomy. So until then, have a great day, everyone, and I will see you in class.