▣ Can I test whether an astrology prediction comes true? Seal one and find out

The fair complaint about a horoscope is that nothing in it can ever be wrong. Umbriel's free Sealed Envelope lets you make a prediction that can be: one sentence, a window you choose, a seal nobody can edit, and an opening on the day the window closes that asks what happened.

• What you'll needNothing: no chart, no birth details and no account
• What you getA sealed prediction that cannot be edited in the app and shows if it was changed outside it; an opening when your window closes; three equal answers, including "I can't tell"; the sky in your window, after you answer; and, from 20 answered, your rate with its 95% range
• CostFree, no account required

A prediction that cannot be wrong is not doing any work. The remedy is old and plain: write it down before, seal it, and look at it after. The Sealed Envelope is that, and nothing else.

You write one sentence, up to 200 characters, about something a stranger could check: "The parcel arrives before Friday" works better than "things will shift." You choose the day the window closes, from tonight to 366 days ahead, and seal it. From then on the app will not show you the words, and it has no way to edit them.

The seal is honest about what it is. Each envelope carries a SHA-256 fingerprint of its words, its two times and the envelope sealed before it, and the app checks them. If a sentence is changed outside the app, the fingerprint no longer matches and the envelope says so. That is a tamper-evident seal and not a lock: it lives on your own device, on your own clock, and it shows a change; it does not prevent one.

When the window closes, it opens and hands you your sentence back exactly as you wrote it, and asks what happened. There are three answers of the same weight: it happened, it did not, I can't tell. Nothing is pre-selected, nothing asks whether you are sure, and "I can't tell" is a complete answer that is counted on neither side. The first answer stands.

Then, and only then, it shows what the sky did in your window: any eclipse, the planets that turned retrograde or direct, the planets that were retrograde the whole time, the New and Full Moons. It is there as context, never as an explanation, and it is held back until you have answered so that it cannot steer the answer.

The envelopes are private and stay on your devices. There is no number about how often you were right until you have 20 predictions you could call. From there you get the rate with its honest 95% range as a drawn bar, and no luck line, because "the Sun will rise" and "I will find a coin" have very different odds and only you know which you wrote.

Nothing is looked up. You type one sentence, up to 200 characters and on one line, and pick the day the window closes, from today to 366 days ahead. The window opens the moment you seal and closes at local midnight at the end of the day you chose. The app then computes a SHA-256 fingerprint over your words, the two times and the fingerprint of the envelope sealed before it, and stores it with the envelope. Nothing in the app reads the words back while the window is open, and there is no way to edit them. Each time the room opens, the app recomputes every fingerprint; an envelope whose words no longer match is marked and left out of your tally. That detects an edit made outside the app and does not prevent one: the data is on your own device, the clock is your device's clock, and someone who could rewrite every envelope and recompute the whole chain could still do so. When the window closes, the envelope opens and asks what happened, with three answers of equal weight: it happened, it did not, I can't tell. The first answer stands. Only after you answer does the page show what the sky did in your window, from the same engine that draws every chart on this site: eclipses, planetary stations, planets retrograde for the whole window, and the New and Full Moons that fell in it. Under 20 predictions you could call one way or the other, the tally shows how many envelopes have opened and no percentage. From 20 it shows the 95% Wilson interval round your rate, drawn as a bar, with no luck line, because the odds of a sentence depend on the sentence.