Not astrology. A rhythm. Twenty-nine and a half days is a useful length for reviewing a life: long enough that something can actually change, short enough that you have not forgotten what you were doing at the start.
Why a rhythm at all
Days are too short to see anything. Years are too long to act on. Most people who feel their life is passing without shape are missing a middle unit: something that comes round often enough to be a habit and rarely enough to be an occasion.
Weeks nearly do it, but a week is work-shaped. It is about tasks and deadlines, and it is very hard to think about anything larger inside one. A month is the smallest unit that is not about your job.
A month you can see from the window keeps better time than a month on a screen.
Four phases, four questions
The full cycle divides into four parts, and each one asks something different. The new moon is dark: nothing to see, which makes it the natural place to write what you are beginning. The waxing half is for tending, when the thing you began needs attention rather than enthusiasm.
The full moon is the only phase you cannot ignore, and it is for seeing clearly: what is actually happening, as opposed to what you intended. The waning half is for release, which mostly means admitting what you are not going to do.
Why the sky helps
A calendar reminder is easy to dismiss because you set it. The moon is not negotiable, does not know you exist, and is visible from the street. That combination turns out to be surprisingly good at getting people to keep an appointment with themselves.
It also solves the problem of starting. Nobody has to decide when the next cycle begins. It begins when it begins, which is roughly four times a month that you cannot argue with.
Keeping it plain
None of this requires believing that the moon influences anything. The value is structural: a repeating four-part question, tied to something outside your control, that arrives whether or not you are in the mood.
If a page a month is all the reflection you ever do, it will still be twelve honest reviews a year, which is eleven more than most people manage.