 Have you ever really thought about just how big a nebula is? Here's some perspective. The longest man-made structure in the world is the Great Wall of China, and it's just over 21,000 kilometers long. Walking every day, it would take you about a year and a half to walk from one end to the other. The Lagoon Nebula is 312 trillion kilometers across. Walking every day, it would take you over 22 billion years to walk from one end to the other. But knowing the lagoon is 312 trillion kilometers across will give you no sense of how large it will actually appear in the night sky from Earth. The reason being is astronomical objects are at all different distances away. Some like the lagoon are actually much closer to us in the Milky Way than others. For this reason, as an astrophotographer, I talk about deep sky objects almost exclusively in terms of their apparent size, meaning how large they appear in the night sky. The apparent size of objects can be quite non-intuitive. For example, the Lagoon Nebula is three times the width of the moon. And you might be thinking, how could that be right? I see the moon all the time, I've never seen the Lagoon Nebula with my naked eye. Doesn't that mean that it is smaller? Well, no, actually. The moon does appear smaller than the lagoon. But the moon is very, very bright, which is why we can see it all the time. It's bright because it's lit by our sun. Well, the Lagoon Nebula is very dim, not only because it's many light years away, but because it's a diffuse nebula, which by their nature are very dim, being diffuse clouds of gas. So the way, um, anyways, the way we measure apparent size is actually pretty simple. We measure things in something called angular diameter. Let me explain. Imagine yourself out in the desert, no trees, perfect horizons in every direction. You'll be seeing the entire night sky, which can be thought of as a dome, or one half of a celestial sphere, the full sphere being 360 degrees around. So half a sphere is 180 degrees from horizon to horizon. With this in mind, we can define anything that we see in the night sky in terms of angular diameter. For example, the whole big dipper is about 25 degrees across, while the moon is just half a degree across. And remember from earlier that the Lagoon Nebula was three times the width of the moon? So yes, it's about one and a half degrees across. Now a handy way to get to know these measurements and how they actually appear with our eyes is to hold up your hand at arm's length in front of the night sky. And if you make the hang loose sign, that's about 25 degrees. The rock-on sign is about 15 degrees, a closed fist is 10 degrees, the three middle fingers is five degrees, and a single index finger is one degree. So how does all of this help us photographically? Well the first thing you have to understand is the field of view of your system, of your camera and telescope or your camera and lens. Once you know the width and the height of your field of view in degrees or arc minutes, we then know how different objects will fit. And before I show a tool here, let me just explain arc minutes, because I've said that term, but I haven't yet explained it. One arc minute is one sixtieth of one degree, so half a degree is 30 arc minutes. We can break that down further into arc seconds, and arc second is one sixtieth of an arc minute. Typically when we're describing deep sky objects, we use arc minutes because so many of them are under a degree wide. The Lagoon Nebula, which I talked about earlier, is fairly large for a deep sky object at 90 arc minutes across one and a half degrees. Just to give you an idea for some other popular objects, the Ryan Nebula is one degree or sixty arc minutes. Then Dramata Galaxy is a whopping three degrees across at its widest point, or six full moons. Back to field of view though, I'm going to jump into one of my favorite tools for showing this, which is Blackwater Sky's Imaging Toolbox. It's a free web app. I'll put the link in the description. To use it, you're going to go over here and use these icons. I'm going to start with this icon in the top left corner. From here, you can pick your camera and telescope and click Calculate, and then it will update over here, and this green box is your field of view. If we zoom in here on the left hand corner, it can show you that this field of view is 2.6 degrees wide by 1.78 degrees high. Let's say you went into this here and you couldn't find your combination of camera or telescope, or maybe you were using a camera lens, what you can do is just click on this second box if you don't see them represented in that list, and enter these things manually. These would be specs about the camera, and then this is specs about the telescope or lens. Let's say I was using a 200 millimeter lens instead, I just click Calculate, and just like that it updates. This has been another installment of Five Minute Fridays. This week, I'm going to start counting down some of the best Nebulae in the night sky for astrophotographers. Till then, this has been Nico Carver from Nebula Photos, Clear Skies.