 Greetings and welcome to the Introduction to Astronomy. One of the things that I like to do in each of my introductory astronomy classes is to begin the class with the astronomy picture of the day. From the NASA website that is apod.nasa.gov slash apod. At today's picture, for December 4th of 2023, it is titled Plane Crossing Crescent Moon. So what do we see here? Well, here we see a picture of the moon up in the sky and this is in the very early morning hours. This is a very thin, waning crescent moon approaching very close to the new moon phase. And while it may look like a bow and arrow here, that thin crescent is again the moon and the arrow going across it is an airplane that happened to cross through during the image when the photo was taken. So when we look at this, we see that both the moon and the trail left behind by the plane have a very reddish color. Now that is not how they actually would appear. The reason is because they are very low in the sky and when you look at things low in the sky, everything is going to appear a little bit redder than normal and that's because we're looking through a lot of atmosphere. The more atmosphere that we look through, the sunlight that's coming out will look very red. So the sunlight that is illuminating these, both of these would be very red because it's coming through a lot of atmosphere. Those very short wavelengths like the blues get scattered around and come from all directions in the sky, giving us a blue sky, leaving the remaining light to look much redder and the closer to the horizon you are, the redder that sunlight will look. Now here again we see the waning crescent moon and we know that it is waning crescent and taken in the morning because it is the left-hand side of the moon that is being illuminated and in another day or two, this would then reach new moon phase and be completely invisible. The cycle of moon phases going from one phase to the same phase again takes just about one month which is where our measurement of the month comes from. In fact, it's about 29 and a half days between, say, full moon and the next full moon. So it takes almost a month to go from one to another which is why we can sometimes get two full moons in the same month and have what we sometimes called a blue moon having nothing to do with the coloring but a blue moon is often called when we have two full moons within the same month. Now an image like this has to be taken by chance unless you have the airplane planning exactly where it's going to fly and you know exactly where to stand for it. There's a big difference in distance between the two and that means just a small change in position here on Earth will make the relative position of the plane as compared to the moon change. So if you moved a little in one direction or another the plane would have passed above or below the moon. But if you know where the flight paths are you can kind of plan when this might happen and be able to be on the lookout for images something like we see today. So that was our picture of the day for December 4th of 2023. It was titled Plane Crossing Crescent Moon. We'll be back again tomorrow for the next picture preview to be Powerful Ray. So we'll see what that is about tomorrow. And until then, have a great day everyone and I will see you in class.