 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy we are going to talk about the astronomical unit and try to better understand what it is and what it is used for. So what is an astronomical unit? Well, it is defined to be, or has been defined to be, the average distance between earth and sun. So how far are we away from our sun? Well, it's about 150 million kilometers or about 93 million miles. And in 2012 we made an exact definition. So how long is an astronomical unit? Well, it is now defined as 149,597,870.7 kilometers. So a very large distance as the distance between earth and sun is much larger than either the size of the sun or earth themselves. Now what we see here, why do we use an astronomical unit? Well, it's a convenient size for measuring distances within the solar system. Mercury here is about 0.4 astronomical units away. This about 0.7, Mars about one and a half. So we have all of those are very easy numbers to follow. That's a lot easier than dealing with big numbers in the hundreds of millions. We can get easily compare these numbers that we would not be able to do elsewhere. And it works even as we get further out into the solar system. So we see here the entire solar system stretched out and we can see that we have the sun, then we have Mercury and Venus and earth all about an astronomical unit or less. Mars, Jupiter, Saturn are between one and 10 astronomical units. Uranus and Neptune are more than 10 astronomical units away. Now even using much larger things, such as getting out into the Kuiper Belt, which is not labeled here, and out into some of the various great distances that we have traveled, including the Voyager 1 spacecraft, over 100 astronomical units away from earth. So it has traveled a great distance, but is still very close to the very edges of our solar system. The Oort cloud, which contains comets ranging from about 1,000 to 100,000 astronomical units and actually approaching distances to the nearest star. So how do we determine the astronomical unit? How are we going to figure out this size? How do we determine that it is so many kilometers? Well, there are a couple different ways that this is done. One is through radar measurements, so we can send a radar signal from earth. Now we can't send it directly to the sun because the nature of the sun will not reflect radar, but we could reflect it off Venus and back and measure the amount of time it takes that signal to return. Radar waves travel at the speed of light, so we know how fast it travels. We know how long it took to travel where we can find the distance and calibrate the astronomical unit. This is one way it's been done in modern times, but it has also been done using parallax measurements, measuring parallax, how the planets shift from different positions on the earth as they are measured generally against a transit of the sun. So as they pass across the face of the sun, that would be Mercury and Venus, we can use that shift to be able to determine their distances as well. This has been done very early times, being able to try to get our first estimates of the astronomical unit. So let's go ahead and finish up with the summary. So what we find with the astronomical unit is that it is defined as the average distance between earth and sun, and for the last decade, defined as a very specific number of kilometers. It is convenient for measurements within our solar system. Now it's not good for measuring to stars because then it starts to get to be large numbers again. But within our solar system, this is very convenient, and we have measured the size of the astronomical unit by both parallax and radar measurements. So that concludes this special topic in astronomy on the astronomical unit. We'll be back again next week for another special topic. And until then, have a great day everyone, and I will see you in class.