 So this video is specifically for students who may have missed class on the day when we first started introducing time. Now in that lecture we did an introduction to Stellarium and if you haven't seen those videos you want to go watch those videos. But then we used Stellarium to understand a specific time description for dusk and for dawn, which are both collectively called twilight. So I have things set up for that particular class day and it was at 9.30 in the morning so the sun was well up in the sky. And we're going to come over here to our time and date window and we're going to turn that on. And we're just going to back things up an hour at a time until it's clearly nighttime. Now when we do that, I'm just going to move this down here over in the ground. You see it's very hard to actually see the ground from the sky. And that's part of what defines that it's nighttime. So we're going to move time forward but rather than moving it forward at the normal rate we're going to move it forward a minute at a time here. And you see the stars start to move as I'm clicking time forward. And after a little bit, and it may be hard to see it on the video, we start to see a little bit of a glow here. The first thing you'll start to see is that you can define the horizon compared to the sky. And that first little bit of light in the sky is what we call astronomical twilight. As it continues to grow here, it becomes more and more dominant till even in the video, we can now see the horizon. That's called nautical twilight and that's when the sailors on the sea would be able to start to tell the difference strongly between the sky and the horizon. Now as it continues to get brighter, the sun's not quite up yet, but we're starting to be able to see a little bit of maybe the details of the grass. Now the word mercury is coming up over the horizon, but that's not the sunrise yet, but the day is starting to glow a little bit more. Once the sun is up high enough in the sky to start to be able to see some details on the ground, that's considered civil twilight. And that continues until we actually get to the point where the edge of the sun comes up over the horizon. So it can actually get fairly bright and you can definitely see some features on the grass before we see the top of the sun start to poke out here. And it's a little hard to see on here, but we've got the label for the sun is now showing up over here. This entire time of the transition between night to day is what we call dawn. And then day actually starts once the sun is up in the sky. And here we can start to see the nice yellow golden ball of that's the sun that's risen. Now on the other side, so we're going to take the sun up in the sky and move it around and we'll pull over here towards the western sky. In the evening, as the sun is starting to go down, we will again advance minute by minute until the sun actually sets below the horizon. But notice you can still see some features. So again, this is part of civil twilight. The sun's not up, but you can still see some. But this is now dusk rather than dawn. And we go through the same process of where you go through civil twilight and then into nautical twilight. And then finally into astronomical twilight as we start to lose that definition. And then finally moving into night. So that gives you an introduction to the concept of twilight, both dawn and dusk, and how we simulated that in Stellarium.