 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy, we are going to look at the concept of albedo and what that means in terms of studying our planets. So what do we mean by albedo? Well, the albedo of an object is, by definition, the fraction of the incident light that an object reflects. So some objects would be very bright and would have a very high albedo. The maximum an albedo can be would be one, and that means it's reflecting all of the light that's incident on it. Of course, that would be ideal and that situation is not going to occur. But some objects do reflect a lot of light, bright snow, for example. Shortly after a snowfall will reflect a large percentage of the light, maybe 80% of the light striking it. Now an albedo of zero would mean that no light is reflected. And of course, that again is an ideal situation. Everything is going to reflect a little bit of light. So everything else is going to be in between those two values. So your lowest albedo would be zero and your highest would be a value of one. Now let's look at one example here and we see in our image the planet Saturn. And in the foreground we see the moon Titan. Now, Saturn and Titan are the same distance from the Sun. So they're receiving the same amount of sunlight. So if they were the exactly the same, made of the same materials, they would reflect the same amount of light and would look the same brightness. We can see here that Titan is much dimmer than Saturn. So even though it's receiving the same amount of light, it is not reflecting as much. So based just on the image we would say that the albedo of Titan here is much less than the albedo of Saturn behind it. And if we look at the actual values, we see that this is correct. Titan has an albedo of about 0.22 and Saturn has an albedo twice that of 0.5. So Saturn reflects more than twice the amount of light that Titan does, making it appear brighter even though they receive the same amount of light. Now let's look at a few other objects here and we can look at a nice image showing a few objects in the sky. And in fact, this is an image showing three different objects. We see in the middle here is the planet Venus. And up above that is the planet Mercury. And then down below we see the moon. Now you'll note that the moon is not very bright and that's because it's a crescent moon. Only that little portion is actually illuminated. Now, when we look at their albedos, what do we see? Well, Venus has one of the highest albedos reflecting nearly 70% of the light that strikes it. And that's why we often see Venus very prominently either in the evening sky after sunset or in the morning sky before sunrise because it does reflect a lot of light. It's going to look bright and it also is relatively close to Earth. However, Mercury can get relatively close to Earth as well. And not quite as close as Venus, but still is relatively close compared to the more distant planets. However, it looks a lot fainter because it reflects much less light. And in fact, even if you put Mercury and Venus at the same distance from the Sun and looked at them, Mercury only reflects one-fifth the light that Venus does. So Mercury would look five times fainter in the sky than Venus just because of the amount of light that is reflected. Now, of course, they also take into account the distance. If one something's further away, it's also going to look fainter. And then we can also do for comparison our moon, the second brightest object in the sky, but it does not reflect a lot of light. The moon is made of very dark rocks and reflects only about 12% of the light that strikes it. So if the moon were made of icier materials such as some of the outer moons of the outer planets, it would be much more reflective and would be even brighter in the night sky than it is. So it only reflects a very small portion of the light incident on it. So that's what the albedo is, is how much of the light that strikes an object that is actually reflected off to be seen. So again, concluding, we talked about the definition of albedo as a measure of the amount of light, the percent of the light reflected from an object. Some objects, Venus, reflect a high percentage of light and our moon reflects a low percentage of the light. So it depends on what the composition of those is. Venus has a lot of bright clouds, meaning it reflects a lot of light. Things like our moon have a lot of dark rocks which absorb a lot of light and reflect very little. So that concludes this special topic on albedo. We'll be back again next time for another special topic in astronomy. So until then, have a great day, everyone, and I will see you in class.