Many national flags depict astronomical imagery. Unfortunately, many of these flags also depict common astronomical falsehoods. A selection of "corrected" national flags is shown below. (These are not intended as a criticism of the nations involved, but as a light-hearted attempt at astronomical education.)
The phases of the Moon correspond to variations in the portion (as seen from Earth) which is illuminated by the Sun. During a crescent moon, the majority of the Moon's disk may not always be visible, but it is still there, and will still block the view of anything behind it.
Many national flags depict a crescent Moon together with a star. Unfortunately, the star is often depicted as being within the cresent, in which case it should not be visible. The flag of Singapore takes this to extremes, as it has five stars, at least three of which should be occulted by the Moon. This problem can be corrected by moving the stars to the right.
The seven stars of the Big Dipper form one of the most prominent features in the northern sky. Two of these are also notable as the "Pointers", which indicate the position of Polaris, the pole star.
The flag of Alaska depicts the Big Dipper, together with Polaris. Unfortunately, it falls for the popular myth of Polaris being one of the brightest stars in the sky. In reality it is of a very similar brightness to the stars in the Big Dipper (being fainter than Alioth, Alkaid and Dubhe, but brighter than the other four).
The Sun is yellow, isn't it? Actually, no, it's white. If this seems surprising, think about when you've actually seen the Sun. When it is high in the sky, it is far to bright for the eye to cope with, and so any perceived colour is likely to be a form of temporary visual defect. When the Sun is low in the sky, it can truly appear yellow, but this is merely due to particles in the Earth's atmosphere (through which light at a shallow angle must pass through more of) scattering away light at the blue end of the spectrum. There may also be a cultural element to the myth, as the conventional yellow disk appears everywhere, from the illustrations in childrens' story books, to diagrams in scientific textbooks.
Unsurprisingly, the yellow sun myth has carried over into many national flags. In many of these it is somewhat justified, as a rising sun is depicted. In other flags, however, a daytime Sun is depicted. A representative example is the flag of Kazakhstan.
One of the most prominent constellations in the southern sky is the Southern Cross. Unfortunately, if you actually see it, it appears more like a southern triangle, with a fourth much fainter star (Delta Crucis) completing the cross shape. There is also a fifth star (Epsilon Crucis) which is fainter still.
The Australian flag correctly depicts Epsilon Crucis as being fainter, but it gives the false impression that Delta Crucis is of a similar brightness to the other three. To make matters worse, a sixth, much larger star is added, which shouldn't be there at all. (Technically speaking, this is the Commonwealth Star, and so is of political rather than astronomical significance.) Interestingly, the New Zealand flag does depict Delta Crucis as being fainter, although it under-corrects for the problem.
The Brazilian national flag associates the country's states with prominent stars. The state of Maranhão is associated with Beta Scorpii (also known as Graffias or Acrab), which is depicted on the state flag. Unfortunately, whilst Beta Scorpii is a double star, the flag only shows a single star.
