Simplify the following text for a Grade 7 native English-speaking person.
Original: The problems of creating reliable lunar calendars may explain why solar calendars , having months which no longer relate to the phase of the Moon , and being based only on the motion of the Sun against the sky , have generally replaced lunar calendars for civil use in most societies .
Simplified: Some calendars are still strongly based on the moon , or `` lunar calendars '' .