The method used to divide the sky into twelve houses. SideraMea offers six.
Placidus is the most widespread in the West and is the default here. The others — Koch, Equal, Whole Sign, Campanus, Regiomontanus — divide the sky by different geometric criteria and produce different cusps. There is no «correct» system: it is a matter of school, and it is worth staying with one so charts remain comparable.
Quadrant houses and equal houses
House systems fall broadly into two families. Quadrant systems, such as Placidus, Koch, Campanus and Regiomontanus, start the first house at the Ascendant and the tenth at the Midheaven, then split each of the four quarters into three houses. The rule for that split differs: some divide the time a degree takes to rise, others divide space. So their houses vary in size.
The other family makes every house exactly 30°. In Equal houses the first house starts at the Ascendant; in Whole Sign each house is one complete sign, beginning with the sign of the Ascendant. Whole Sign is the oldest method, already used in Hellenistic astrology. In both, the Midheaven need not mark the start of the tenth house.
Choosing one for your chart
If you are new to this, Placidus is a sensible choice: most books and websites use it, so your chart will match what you read elsewhere. Students of traditional astrology often prefer Whole Sign for its simplicity and its long history.
One special case: for people born far north, near the polar circles, Placidus and Koch produce badly distorted houses, and beyond the circles they cannot be defined at all. An equal-house system is far more stable there. On SideraMea you can change system in the advanced options of a profile, and the chart is recalculated with the new cusps.
Frequently asked questions
Which house system is the most accurate?
None, in the usual sense. Every system is calculated just as precisely; they simply start from different ideas about dividing the sky. Which one suits you depends on the school of astrology you identify with. Rather than hunting for the "true" one, use one consistently and understand its logic.
Why did my planet change house when I switched systems?
Because the cusps have moved. A planet at the start of the fifth house in Placidus may land at the end of the fourth in another system. It happens mostly to planets close to a cusp. The planet's sign and aspects stay exactly the same: only the area of life where it is read changes.
Is Whole Sign better if I don't know my exact birth time?
It is somewhat more forgiving, because houses change only when the Ascendant moves into a new sign. Without a birth time, though, the Ascendant itself is uncertain, and on average it changes sign roughly every two hours. If your time is a few minutes out, Whole Sign holds up better; if it is entirely unknown, no system gives reliable houses.
See it in your own chart
Calculating your birth chart is free: wheel, planets, houses, aspects and a first reading. All you need is your date, time and place of birth.