"I tried to use a virtual size of the Earth that is nearly correct, as viewed from the Moon.

"The ratio of the diameter of the Earth to its distance from the Moon is about 1 : 30. Thus, an astronaut on the Moon who holds his hand out at arm's length - a distance of around 60 cm from his eyes - will see the Earth looking like it's about 2 cm across.

"I know the Earth shown in the sketch is a little bit too large compared to the 2.4 cm width of the finger tip; but I needed the extra size to show that it really is the Earth. Thus, it is a compromise."