The blue marble is a famous image that was taken of the earth as the lunar craft made its way back. Why does this matter? Whole earth photos are taken rarely as you have to get a great distance from earth to be able to take them (see calculation below). Yet in 2015 the first was taken for quite some time by the Deep Space Climate Observatory (DSCOVR).
Unfortunately, we are currently (at the same time) in a golden era of Science and an increasing era of doubt. Flat earthers have many problems. Vast areas of science (not to mention geography) have to be thrown out if they are to be correct. Gravity, for instance cannot work as we believe as it would mean as you approach the disk edge gravity would not pull you down but sideways towards the centre of mass of the disk.
Now, you might be wondering how anyone can believe this because we see whole world images all the time. Unfortunately these are made of composites. This is for a simple reason.
This is for an incredibly simple reason. If we are to take an image that has a distance across of roughly 120°, we can calculate the distance you have to take the picture from earth to give us the whole earth in its shot is distance = radius of the earth /Tan(60) which gives 6371km/1.732 which equals 3678 km or 2285 miles in order to see the who globe.
Why is this a problem? Well, when we are not visiting the moon (last trip was December 1972) we are not looking back at earth. There have been many probes sent away from the earth, but they have not turned around to take an image of earth, why would they?
It is rare that a craft needs to go this far from earth, so these shots are not going to be regular opportunity. Still for those of you who understand science, and do not think the earth is flat, this is an image to show your flat earther friends. It is unlikely to prove anything to them (they already have to have their fingers in their ears) but it is just one more point that shows it for good.
I suspect that flat earth will remain a theory that a sizable minority of the planet believes until we can all go to space for holidays – if that ever happens. For the rest of us though, we need to accept that some people will never be convinced and leave it at that.