Why does my chart look different on every site?

The houses split the sky into twelve sectors, and there is more than one way to draw the lines. Placidus, the usual default on Western sites, divides the sky by time, so its houses come out uneven. Whole sign, the oldest method and the default in several apps, makes the rising sign the whole first house, the next sign the second, and so on.

The planets keep their signs and degrees. What changes is the house they land in, and every reading built on houses changes with it.

In the sample, 5 of the ten planets change house between the two systems.

Western astrology uses the tropical zodiac, which is tied to the seasons: 0° Aries is the point where the Sun crosses the equator in March. Vedic astrology uses a sidereal zodiac, which is tied to the stars. The equinox drifts slowly against the stars, so today the two zodiacs are about 24° apart.

A site that defaults to sidereal, or an app built on Vedic astrology, moves every point back by that angle. A planet in the first 24° of a sign lands in the sign before it.

Lahiri ayanamsa at the sample moment: 23.59°.

An aspect is an angle between two planets: 0° for a conjunction, 60° for a sextile, 90° for a square, 120° for a trine and 180° for an opposition. Planets are rarely exact, so every calculator allows a margin, the orb. There is no standard. Some sites allow 10° or more, others 5° or less.

A wider orb finds more aspects, and it can complete a pattern such as a grand trine or a T-square that a tighter orb leaves open. When two sites list different aspects for the same chart, compare their orbs first.

Aspects found in the sample: 12 with tight orbs, 14 with ours, 20 with wide orbs. The table lists only the aspects that at least one of the three sets misses.

Tight: conjunction 6°, sextile 3°, square 5°, trine 5°, opposition 6°. Ours: conjunction 8°, sextile 4°, square 7°, trine 7°, opposition 8°. Wide: conjunction 10°, sextile 6°, square 10°, trine 10°, opposition 10°.

The rising sign runs through the whole zodiac in a day, about one degree every four minutes, and the Midheaven and the house cusps move with it. The Moon moves about half a degree an hour, the planets far less.

Birth records are often rounded to the quarter or half hour, and family memory is looser still. If your rising degree sits near the edge of a sign, a small change in the time can change the sign itself.

A chart is cast for Universal Time, so a calculator has to know what the clocks in your birthplace said on that exact date. The rules changed often, and a site that applies today's rules to the past can be off by an hour.

Turkey is a clear case. According to the IANA time zone database, Turkish clocks moved to UTC+3 in April 1978 and stayed there, winters included, until the end of October 1984. In the summer of 1983 they went one hour further, to UTC+4. Turkey then used UTC+2 in winter again, and it has stayed on UTC+3 all year since 2016.

Bulgaria had no summer time from 1945 until 1979. Germany had none from 1950 until 1980, and Berlin used double summer time, UTC+3, for part of 1945 and 1947. France moved its clocks to German time, UTC+2, in June 1940 and had no summer time from the autumn of 1945 until 1976.

The database's own documentation warns that many entries before 1970 rest on thin sources. For an older birth, check the time basis on the birth certificate or with a local archive.

When a site applies today's rules

• Istanbul, January 15, 1980, 14:30: the clocks said UTC+03:00. A calculator that assumes UTC+02:00 puts the rising sign at Gemini 23°09′ instead of Gemini 7°26′.
• Sofia, July 15, 1978, 14:30: the clocks said UTC+02:00. A calculator that assumes UTC+03:00 puts the rising sign at Scorpio 0°03′ instead of Scorpio 11°37′.

00:00 is the first minute of a day, and 24:00 is the end of the same day, the same instant as 00:00 of the next one. A birth written down as midnight can land on either date, depending on how the record was kept, and the two charts are a whole day apart.

Many forms also fill in 00:00 when the time is unknown. The rising sign and the houses are then a guess, and the Moon can be off by up to 13°. If you do not know your time, say so: our tools then use noon, which keeps the Moon within about 7° either way, and mark the chart as approximate.

The rising sign depends on longitude as well as time: one degree of longitude is worth four minutes of sidereal time. A site that takes the capital or the nearest big city instead of your real birthplace shifts the angles, even inside one time zone.

The table casts the same clock time, 14:30 on the sample date, in three Turkish cities. Latitude changes the house sizes too, which matters most for Placidus at high latitudes.

Under every chart, the birth chart tool prints how it read your time: the local time you entered, the UTC offset in force on that date with summer time flagged, and the resulting Universal Time. It also names the house system and lists the orbs it uses. The offset comes from the IANA time zone database built into your browser.

Placidus and whole sign are one toggle apart, and so are tropical and sidereal. A birth at exactly 00:00 gets a note, and an unknown time falls back to noon with a warning.