The sun and moon originated together.
In the centre of the sun resides a shadow place,
Which belongs to the true essence of the moon,
Thus the moon cavern is not on the moon but within the sun.
This is therefore called the ‘moon cavern’,
To call it moon alone is not sufficient.
The centre of the moon resides a light place,
Which belongs to the true essence of the sun,
The light of the sun is contrary to the moon centre,
This is therefore called the ‘root of heaven’,
To call it heaven alone is not sufficient.
When sun and moon are separated, they are not a unified whole.
[~ Translation of Secret of the Golden Flower (part 8) the alchemical union between yin and yang]