- The moon revolving around / orbiting the earth causes the phases of the moon
- The moon orbiting the earth occurs once per month
- Therefore, each of the moon 's phases of earth's moon occurs once per month

So, each of the moon 's phases of earth's moon occurs once per month
Q: Scientists know that Earth is constantly changing. They are familiar with the processes that are repeated daily, monthly, and yearly. Which of these is a natural event that can be predicted to occur monthly?
Context:
- The moon revolving around / orbiting the earth causes the phases of the moon 
- The moon orbiting the earth occurs once per month