Birth-time uncertainty calculator
Enter a plausible birth-time interval to see which Sun, Moon, rising and Midheaven signs stay the same in minute samples and which change. This comparison cannot determine your true birth time.
Compare your plausible birth times
Date and time stay in this browser, without storage or URL sharing. Only your place search is sent to OpenStreetMap’s Photon service. Select the actual birth town or city.
Start with an interval you can justify
Use a birth record, a family recollection or another source to choose the earliest and latest plausible local times. Enter a complete date at each end. For example, a recollection of “around midnight” might justify 23:30 on one date through 00:30 on the following date. Those are two calendar dates, even though the interval is only one hour. The tool does not assign probabilities to the times you enter.
Select your birth town or city from the suggestions rather than leaving an unconfirmed place name. The selected coordinates determine an IANA timezone locally. Check that you chose the right country and region, especially for cities with common names. A nearby city is not an exact substitute when an angle is close to a boundary.
Read stability and changes separately
The comparison includes the Sun, Moon, rising sign and Midheaven sign in the tropical zodiac. It samples both endpoints and every minute between them. A whole day can begin and end with the same rising sign while passing through other signs in between; an endpoint-only comparison would conceal those changes. Here the intermediate samples are included, and each detected sign change is listed with its surrounding one-minute bracket.
Stable in samples means that the same sign appeared at every sampled instant. It does not mean that the degree stayed fixed, that your entire chart is stable, or that your recorded birth time is correct. A detected change means that more than one sign is possible within your proposed interval. Neither outcome identifies which instant was your birth.
Precision and boundary cautions
The one-minute sampling interval is a display and search convention, not a promise of one-minute astronomical accuracy. A change is bracketed between adjacent samples; its exact second is not calculated. A placement within 0.05 degrees of a sign boundary at any sample receives an additional warning, even when every sampled sign agrees. Treat that placement cautiously instead of using the stable label as certainty.
Very brief transitions lasting less than a minute can escape a sampled comparison. The tool does not prove continuous stability, interpolate a definitive crossing time, or establish confidence percentages. Coordinates, historical timezone data and the underlying astronomical model introduce further limits. Narrowing your interval with reliable records is more useful than choosing whichever sign description feels most appealing.
Clock changes and supported dates
Intervals must be longer than zero and no longer than 24 hours. Dates before 1900 and future instants are rejected. Locations outside 65 degrees north or south are unsupported because rising-angle behavior near polar regions requires additional care. The comparison does not include other planets, aspects, house placements, chart-ruler changes, sidereal zodiacs or a birth-time rectification service.
Daylight-saving changes can skip a local time or make it occur twice. Historical civil-time changes can do the same. This tool checks local times by converting candidate instants back to the selected timezone, and rejects intervals containing ambiguous or nonexistent minutes or a clock-offset discontinuity. It does not silently select the earlier or later occurrence. Choose a supported interval only if your evidence actually permits it; otherwise consult the original record and an appropriate historical timezone reference.
What is calculated and what is interpreted
The astronomical positions use the same Astronomy Engine functions and angle formulas as the site’s natal-chart engine. Timezone rules come from your browser’s timezone database; coordinate-to-zone lookup is an approximation, particularly near borders, and historical rules may differ between browser versions. A sign comparison is a calculation within these conventions, not scientific evidence for personality descriptions or predictions.
Use the result as a boundary on interpretation: which sign descriptions remain relevant across your plausible times, and which depend on information you do not yet have? For a personal preview, keep birth time unknown when it is unknown. Do not substitute the midpoint as a verified time. Your birth date and interval are calculated in this browser, without being saved, added to a link or included in calculator analytics. Only the place-search text goes to the existing location-search service.
Try it with your birth date
Calculated in your browser. Your date is not sent anywhere or added to the link.