The seven-star giant coffin and seven copper coins are all related to the Big Dipper. The reason for putting seven copper coins on the seven-star pad is that the Big Dipper consists of seven points.
The copper coins on the seven-star pad are placed in the same position as the Big Dipper. In ancient times, people attached great importance to the burial of tombs, and they were feudal superstitious. The seven-star giant coffin was the product of their thinking.
At present, the Big Dipper is just a natural phenomenon for people, and it has its own unique scientific principles. But ancient people thought it was some kind of hidden secret formation.
They put seven copper coins on the seven-star giant coffin according to the arrangement and combination of the Big Dipper, because they think this array can ward off evil spirits and avoid harm.
This is a bit like geomantic omen. Now some people still believe in geomantic omen. They will take a special geographical position when building a house or burying a coffin.
Because they believe that this special geographical location may affect the lives of the living, the ancients used seven copper coins in the same way.
However, some people say that the seven-star giant coffin is actually seven coffins arranged according to the arrangement and combination of the Big Dipper, instead of putting seven copper coins on the seven-star pad. There are many kinds of statements, which are just people's different guesses.