Method & limits

What this is, and what it isn't

What the name means

A week is seven days. A pentad is five. It is an ordinary word in meteorology — climate records are still published in pentads, and there are exactly seventy-two of them in a year. That number is not a coincidence: the Chinese calendar divides each of the twenty-four solar terms into three five-day periods, which also makes seventy-two. The two traditions counted the same thing from opposite ends, one from the weather and one from the sky.

So the name is the unit the product is built on. One card per day, a new one every five days when the season turns, seventy-two times a year.

The astronomy is real

A solar term begins at the instant the sun's apparent geocentric longitude passes a multiple of 15°. The March equinox is 0°, the June solstice 90°, and so on around the year. These instants are geocentric, so they happen at the same moment everywhere on Earth — a term that falls on 20 March in London falls on 21 March in Tokyo only because of the date line.

Pentad works this out as you use it, to within about fifteen minutes — more precision than a calendar needs and less than an almanac. Sunrise, sunset and day length come from the same working, with the standard correction for how the atmosphere bends light at the horizon. Nothing is looked up from a table and nothing is fetched over the network: ask it about 1970 or about 2050 and it arrives at the same answer by the same route.

The seasons are measurable, and they matter

The reason a seasonal product is worth building is not tradition, it is that the effect is large and repeatable. A 2025 meta-analysis pooling 24 studies and 32,866 participants found a pooled prevalence of 5.01% for seasonal affective disorder and 9.37% for its milder form, with prevalence rising about 0.2 percentage points per degree of latitude. In the United States, roughly 5% of adults meet criteria for SAD and a further 10–20% report milder winter symptoms.

The behavioural changes are just as consistent: pooled data show people are about 30 minutes more sedentary in winter than in spring, and sleep 15 to 25 minutes longer. None of this is a reason to buy an app. It is a reason for an app to stop pretending every day is the same.

How the five elements are used here

Your birth date determines which solar term was running, and every term carries one of five elements through the classical month-branch mapping. The current term carries another. The relation between the two — the same element, generating, being generated, controlling, being controlled — produces one of five themes for the term.

That is the entire model. It is a framing device, and it is deliberately shallow: it does not know your chart, your hour of birth, or anything about your life. It gives you a sentence about the shape of the next two weeks and a practice to go with it. If that sounds thin, it is — the alternative is a horoscope that pretends to be deeper than it is.

What Pentad does not do

• It does not diagnose, treat, cure or prevent any condition, and it is not a medical device.
• It does not predict your future, your compatibility, or your fate.
• It does not replace a therapist, a doctor, or a friend.
• It does not send your data anywhere. There is no analytics script, no pixel, no account.

Privacy, in one paragraph

Your birth date, city and reflections are stored in your browser's local storage and are never transmitted. There is no account, no database of readers and no third-party script. Whether a key is genuine is decided on your own device, not by asking us. Clearing your browser data deletes everything.