Human cultures have developed many views of the planet. The standard astronomical symbol of Earth consists of a cross circumscribed by a circle, , representing the four corners of the world. Earth is sometimes personified as a deity. In many cultures it is a mother goddess that is also the primary fertility deity. Creation myths in many religions involve the creation of Earth by a supernatural deity or deities. The Gaia Principle, developed mid-20th century, compared Earth's environments and life as a single self-regulating organism leading to broad stabilization of the conditions of habitability. Images of Earth taken from space, particularly during the Apollo program, have been credited with altering the way that people viewed the planet that they lived on, emphasising its beauty, uniqueness and apparent fragility.