The division is simple; its interpretation is not universal
The geometric division produces 36 decans around the zodiac. You need the degree within the sign, not just a calendar birth date: a date-only decan chart is an approximation for the Sun and cannot locate the Moon or another planet.
Use the unrounded position at a boundary. A planet at 9°59′ is in the first decan; at exactly 10°00′ it enters the second. A label rounded to 10° can hide which side of the boundary the underlying coordinate occupies.
| Decan | Degree interval |
|---|---|
| First | 0°00′ ≤ degree < 10°00′ |
| Second | 10°00′ ≤ degree < 20°00′ |
| Third | 20°00′ ≤ degree < 30°00′ |
Ruler tables need a named system
Traditional face rulers and modern sign-based decan schemes assign planets differently. A claim such as ‘second decan ruler’ is ambiguous until the scheme is named. Do not combine a ruler from one table with the interpretation of a different system without saying so.
The chart supplies positions; it does not currently label decan rulers. You can use the measured degree with a separately identified reference table. A more detailed symbolic label is not evidence of more accurate prediction.
Keep the surrounding chart visible
A decan is a subdivision of a sign, not another house. Changing house systems can move a planet between houses while leaving its zodiac degree and decan unchanged. Changing the zodiac can change its decan because the reference origin changes.
Start with the planet, sign, house and major aspects. Add a decan only when it answers a specific interpretive question and you can state which method you used.
A CONCRETE EXAMPLE
Worked example: 74.25° longitude
74.25° is 14°15′ Gemini: subtract Gemini’s starting longitude of 60°. 14°15′ belongs to the second decan. You cannot infer its decan ruler from the number alone without selecting a ruler scheme.
By comparison, 69.999° is just under 10° Gemini and remains in the first decan even if a short display rounds it to 10°. Both numbers are teaching examples, not measured placements.
Check the coordinate.
Use the longitude converter to check signs and intervals, or calculate your chart to find actual placements.
Open the degree converterCommon questions
Are decans and houses the same?
No. Decans divide zodiac signs into three equal parts. Houses divide the local chart by a selected house system.
Can every planet have a decan?
Any zodiacal longitude can be located in a decan. Interpretation and assigned ruler require a named convention.
Calculation & interpretation
New personal calculations use Skyfield with JPL DE440 data and the site’s documented house and zodiac rules. The preserved Greenwich teaching chart has its own historical source label. Worked arithmetic and geometry examples are labelled separately. Astrological meanings are symbolic conventions, not established scientific predictions.
Birth time and place affect angles and houses. Unsupported or ambiguous inputs return an error rather than an invented chart. Personal results stay in browser state and do not create public result URLs.
Read the calculation settings guide