In August there were two full moons. In September there was a full moon today, Sept 29. Since the moon goes around the earth in 28 days, why weren't there two full moons in September. The answer is that the number of days between full moons is 30, even though the moon orbits in 28 days. To understand this detail, takes some time.
Astrologers have known this for 6000 years, but I never understood how long it takes the moon to go around the sun.
I remembered from school that the moon goes around the sun in 28 days. It is actually 27.3 days, but close enough. On the other hand, the new moons are 30 (29.5) days apart. What is going on?
The time for the moon to go around the earth is called a sidereal month, and the time between new moons (or full moons) is called a synodic month.
Here is a good illustration from AstroGirlWest on YouTube. Notice that the earth's movement around the sun causes the relative position of the sun to change.
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