What's [the 30-day trap]?
The moon used to have an exact orbital period around the earth of 28 days. Because the earth is rotating as well, the time it takes to get from one new moon to the next used to be exactly 30 days. Therefore, the lunar calendar was an accurate measurement where every month had 30 days, the start of the month would be the new moon and the 15th day would be the full moon.
A Lunarian sage (obviously posited to be Eirin) slowed down the rotational period of the moon to be 27 and a third days, which also means that a full moon does not actually occur on the fifteenth day, as it used to, because the "month" is now actually about 29 and a half days. Slowing down the moon's rotational period naturally caused the orbital period to also slow down to 27 and a third days due to gravity. Because of this, one part of the moon is always facing the earth, so we have the "near" and "far" sides of the moon.
In order to get to the moon, one would wait until the full moon so that the moon's whole surface can be seen in the sun's reflection and a connection between the reflection and the real moon can be made. Rather, they would wait for the 15th day on the calendar. Once they arrive, they will find the lunar seas. Now because the moon always faces the earth in the same way, the earth will be reflected back in the seas, which is the first part of the trap. You would see the earth's reflection and know that you can come back the same way you came. You spend some time on the moon, but when it's time to go back, the path is already closing because the moon is no longer full exactly on the 15th day and instead starts to wane earlier.
