Why the minute matters
An astrocartography map is a picture of one moment. At the instant you were born, each planet stood over one meridian of the Earth, was rising along one curve and setting along another, and the map draws those places as your lines. But the Earth never stops turning: it makes a full turn against the stars in 23 hours, 56 minutes and 4 seconds, a degree of longitude every four minutes. Move the birth time and the whole pattern of lines slides east or west with it, by roughly that much. The planets move too. The Moon adds the largest drift of its own, about half a degree an hour, since it moves across the sky faster than any planet.
How far a line moves
A degree of longitude is 111 km at the equator and shrinks toward the poles, so the same error moves a line further in Singapore than in Oslo. At 45° latitude, the latitude of Milan and Minneapolis, four minutes of error moves every line about 80 km, a quarter of an hour about 300 km, and half an hour about 600 km. That matters because of the distances astrologers read lines within: this site counts a line within 250 km as strong, within 600 km as moderate and within 1,000 km as mild. An error of fifteen minutes is enough to carry a line from the strong band into the moderate one, or out of the strong band altogether. The first table below gives the distances for each latitude.
What a wrong time changes
To see what this does to real readings, the site measured it. In 200 seeded charts, each read at 30 big cities, about half of the readings (48%) had at least one line of the Sun to Pluto within 250 km. For those, a birth time four minutes later changed the list of strong lines in about three cases in ten (29%), and moved the nearest line out of the strong band in about one in seven (14%). A quarter of an hour changed the list in about four cases in five (81%). Half an hour changed it almost every time (99%). The second table below gives the full figures.
Checking your time
The birth certificate is the usual source, and many countries record the time to the minute, though it is often rounded to five or fifteen minutes. A time ending in :00, :15, :30 or :45 may have been rounded. Hospital records, a parent's diary or a baby book can help confirm it; a time remembered years later may be less reliable.
On the free map, open a city's Place reading and choose Check an approximate birth time. Select ±5, 15, 30 or 60 minutes. The comparison checks every minute in that range and shows each line's closest and farthest distances beside its distance at the entered time. The dot marks the entered time; the bar spans the checked distances. A line can appear here even if it is outside the ordinary reading's 1,000 km limit, because it comes closer at another possible time.
A line marked strong at every check stays within 250 km at all the sampled times. Strong to moderate means its strength depends on which time is correct. Use this to see which parts of a place reading depend on a precise time before comparing cities. It leaves your original chart and saved details alone. The comparison cannot tell you which birth time is right, and the sampled ranges do not give a probability of an astrological outcome.
If you don't know your birth time
Without a time the lines cannot be placed at all, since they depend on how the Earth was turned at your birth. Enter your birth date and tick that the time is unknown: the map then shows your parans, the latitude bands where two planets were angular together, which hold for the whole day, and the planets' signs, which rarely change within a day except the Moon's. Some astrologers offer rectification, estimating a time from the dates of major life events. It is interpretation rather than measurement, different astrologers can reach different times, and this site does not do it. The accuracy and sources page explains how the site converts your local time to Universal Time, including historical time zones and daylight saving.