A classic test for the curve of the earth is to see if things that are far away are hidden by the horizon. There's two basic ways of doing this (with a large number of variations.) You can A) look at a distant object like a ship or a distant shore, or B) look at a distant light. Both of these methods have been known since antiquity, with Pliny the Elder writing around 2,000 years ago: People who believe the Earth is flat (or sometimes just that local bodies of water are flat) are big fans of method B (distant light). Furthermore, they prefer to use a laser rather than a flashlight. They also like to do the test at night, rather than in the day. While there are some slight potential advantages to this method, I think their focus on the laser method is fundamentally flawed and results in a lot of wasted effort and unfounded conclusions. Let's divide the methods into three and look at the pros and cons of each one. The three methods are: Daylight observations of a distant shore Night observations of a powerful light Night observations of a laser 1. Daylight observations of a distant shore Pros: You can clearly see what is going on. Don't need anyone on the other side. Compelling photos and videos. Cons: Does not work at night. (source: https://www.metabunk.org/stand-up-to-detect-the-curve-of-the-earth.t8364/ ) The above image shows the Malibu Beach Inn viewed from Santa Monica. The two images are taken from different heights above sea level, but in both images, the beach and a lower part of the hotel are obscured by the horizon. You can quite easily see here that if someone had a powerful light and was standing on the beach, then that light would also be hidden by the horizon. The further you go, the more apparent this is. From the same location, I took a photo of Point Dume (a headland to the northwest of Malibu). I overlaid the photo with what Point Dume actually looks like from closer up, to show how much is hidden with the camera about two feet above the water: Point Dume is about 200 feet above sea level, 15 miles from Santa Monica, and more than half of it is hidden behind the curve. So again if there was someone standing on the beach waving a flashlight or a laser, then they would be about 100 feet below the horizon and totally invisible. So why, given these inarguable demonstrations in daylight, why would you want to use a flashlight or a laser? There are actually some benefits to using a light, but there's also a big negative - sometimes refraction will make the light visible. But let's look at the two other methods: 2. Night observations of a powerful light Pros: Works at night (and sometimes during daytime). Tells you if there's a line of sight from the camera to the light. Does not need magnification. Easy to aim. Relatively cheap. Cons: Night observations don't tell you if the line of sight is straight. Can't really tell what is going on if you can't find the light. Can't verify where the light actually is. In this experiment, you stand on one shore with a camera while a friend stands on the other with a powerful flashlight pointed in your general direction. You'd generally communicate by phone and they would flash the light on and off so you can distinguish it from other lights. If you can see the light then that means there's a line of sight from the light to the camera. This is good in that it allows you to identify the visibility of one particular spot, but it's terrible in that you've got no idea why that particular spot is visible. The problem, of course, is refraction. When looking at the horizon from a low elevation (a few feet above the water) the effects of the cool air directly over the cold water can often bend light downwards. This has the effect of visually flattening out the curve of the earth slightly. But quite often there a narrow band just grazing the water where a lot of the distant scene is compressed together. The beauty of doing a daylight observation is that you can see when this is happening. When there is significant additional refraction the view is distorted and highly compressed near the horizon. A good example of this is the view of Toronto from Fort Niagra by @jenna1789 Here we can see that the white arched building has been lifted up over the horizon, but has been terribly distorted and compressed (as have the other buildings, looking rather short and stubby). And really you can't see the base of it. However, we can see in this extreme example that may 100 feet is "visible" when it should not be. So it's quite possible that a light or a laser could also be made visible. But, and here's the key problem, you can't see if it's due to refraction, because it's at night! Think of it another way. With a flashlight, you are just getting data about one particular spot. In daylight, you are getting data about all the spots. You actually see what is going on around any particular point and if it has been lofted by refraction. With a flashlight at night, you can't tell. 3. Night observations of a laser Pros: Works at night and during dimmer parts of the day. Laser is more visible if aimed at the camera, can see where it hits if the beam does not spread out too much, can take measurements of beam height along the path, you can see a portion of the beam. Can "level" the beam. Cons: Powerful narrow beam lasers are expensive and fiddly. Risk of blindness. Difficult to aim. Night observations don't tell you if the line of sight is straight. Can't really tell what is going on if you can't find the light. Can't verify where the light actually is. A laser suffers from all the same problems as a bright flashlight, and if you are simply wanting to test if you can see something from one position it really not an improvement. It does have a theoretical advantage in that when it is aimed directly at the camera it is significantly brighter than a comparably powered flashlight. However power is not a huge problem - you can quite easily get a 3500 lumen flashlight that's visible from 10-20 miles away (at night). The only real advantage of a laser is that while the beam is still narrow, you can tell where the center is. You can then use this to take measurements along the route, and hence figure out the shape of the path of the laser relative to the water surface. In the above example, they are marking the position of the laser near the shore, and then a few miles out. By that point, the laser beam has spread out, but you can still see the approximate shape. This does not work beyond a few miles though, as it's impossible to keep a tight beam over long distances. It's also problematic if you only take two readings, as the beam might be tilted upwards very slightly. Ideally, you would take several readings and plot them on a graph to show the curve. So if all you are doing is trying to see if there's a line of sight at night, then use a powerful flashlight. It's a lot simpler. However you've really got to ask yourself if there's a benefit to performing the experiment in the dark. Why not use daylight observations, which gives you vastly more data to work with? One reason to prefer night observation is that it's cooler, so the air temperature is closer to the water temperature. However, if that is the reason, the simpler approach would be to start the observations at dawn, when the air was coolest, and continue to record visible observations of the distant shore as the day warmed up. Ideally, this would be a timelapse close-up video of the distant shore.