For a very long time, there were very few photos of planet Earth showing a fully illuminated hemisphere. The reason is relatively simple: to get a photo of an entire hemisphere of the Earth, you have to be far enough away from Earth, and you have to be positioned between the Sun and the Earth.
If you are too close to Earth, like the astronauts on board the International Space Station, at an altitude of about 400 km, you do not see the entire globe, but only a portion: it is then impossible to take a picture of the planet Earth.
Earth photographed from the ISS, with a Soyuz spacecraft (foreground) and a Progress cargo ship (background). While the photo is magnificent and the Strait of Gibraltar is clearly visible, Earth is nowhere to be seen. Credits: NASA
Conversely, the majority of satellites located far from Earth are rarely situated exactly between the Sun and the Earth, which results in photos of crescent Earths: these photos are sublime too, but they do not show the Earth in its entirety.
Photograph of an Earthrise taken during the Apollo 8 mission. The Earth is not shown in its entirety, but rather a crescent shape. Credits: NASA
The consequence is simple: for a long time, the vast majority of images of the Earth were composites of different photos, in order to present an illuminated hemisphere of the Earth.
In practical terms, this means that several photographs of the Earth from space were retrieved, each being a crescent of the Earth for example, showing an illuminated area of the globe, and these photographs were cut out and pasted together, then the colors were reworked so that the seams would not be visible.
To have continuity in the clouds, I suppose the photographs came from satellites that photographed the Earth at relatively short intervals, but I'm not sure.
Note: This does not mean that all photos were composed this way. One of the most iconic photographs of the Apollo era is a photograph showing the Earth fully illuminated. This photo, titledBlue MarbleThe Blue Marble, or the 80-degree blue marble, is one of the most reproduced images in the world. And it was possible because the astronauts had the Sun behind them at that moment.
Blue Marble: the famous photo taken by the crew of Apollo 17 in 1972, the last mission to the Moon, is not a composite image but a single photograph of our planet Earth. Credits: NASA
Much more recently, NASA launched a climate observation satellite, simply calledClimate Observatory from Deep Space, in EnglishDeep Space Climate Obervatory, abbreviated as DSCOVR. It positioned itself approximately 1 million kilometers from Earth, at the L1 Lagrange point of the Earth-Sun system: in practical terms, this means, among other things, that the satellite is permanently located between the Sun and the Earth. And one of its instruments, the EPIC camera, regularly takes pictures of Earth, thus showing photographs of the fully illuminated Earth.
First image of North and Central America returned by the DSCOVR satellite on July 6, 2015. Credits: NASA/EPIC Team
There are several things to know about these photographs.
First, these are not actual photographs as they were taken. In fact, the EPIC camera has several filters, 10 to be exact. Among these filters, several are used for the photographs of Earth seen here: a red filter, a blue filter, and a green filter. Specifically, EPIC takes three photos with very little time between them.
These images are taken in black and white: with a red filter, for example, a white area will be an area where there is a lot of red light, and a black area is an area where there is no red light reflected by the Earth.
These 3 photos are then changed in color to match what they represent: thus, on a photo taken with a red filter, white areas become red, black areas remain black, and shades of grey become shades of red.
Then the three photographs—red, blue, and green—are stacked to create a color image. Incidentally, this type of process is often used for pixel colors in computers, with RGB components.
This can also be seen in some photos, particularly the incredible photographs of the Moon's transit. By zooming in on the Moon's outline, one can often see either a thin green band or a thin red band, because the Moon has moved slightly between the photos: the Moon was positioned in one place during the photo under the green filter and was slightly off-center in the following two, leaving a thin green band in the area that was only covered by one photo.
Photograph of the Moon's transit taken by the EPIC camera aboard the DSCOVR satellite. Zooming in on the Moon, a very thin blue band can be seen on the left, and on the right, two thin red and green bands, remnants of the three superimposed photographs. Credits: NASA/EPIC Team
And that's not all! The images are also processed to correct some distortions, and the colors are then reworked to achieve a rendering similar to what the human eye would perceive.
The EPIC team is also working on color images.augmentedto account for what would be visible without the attenuation of the atmosphere.