The boundary is one longitude
In the tropical zodiac, the Capricorn sector ends and Aquarius begins at 300° longitude. At a calculated instant the Sun belongs to one 30° sector; a boundary birth is not assigned two simultaneous Sun signs. A typical birthday window is only a guide. Year, local time and timezone can change the local calendar day of the crossing.
An example with degrees
A Sun longitude of 299.999° lies in Capricorn; 300.001° lies in Aquarius. These are geometric examples rather than claimed historical dates. Near 29°59′ of one sign and 0°00′ of the next are nearby positions but different sectors. Displaying seconds is not a guarantee of absolute arcsecond accuracy.
Why might both themes feel familiar?
Capricorn is read through responsibility, building structures, accounting for time and turning effort into a sustainable result. Cardinal earth can translate an unformed intention into a workable plan. Seriousness does not require constant severity. The cost of a goal should include the rest of life as well as the measure of achievement. Aquarius brings shared systems, groups, principles, independent thought and evaluating an arrangement from a little distance. Fixed air can keep examining the consistency of an idea over time. Innovation means more than being different: it should show which need it meets better. Contributing to a community does not require every member to think alike. Finding both descriptions familiar does not prove that the Sun occupies two signs. The Moon, ascendant or other bodies may lie in either sign, and ordinary experience may also overlap with broad descriptions.
Check the birth information
First check the date, recorded local time and IANA timezone for that date. A city selection supplies location and timezone; do not calculate a historical birth using only the country or today's UTC offset. Do not treat noon as confirmed when time is unknown. If the uncertainty crosses the boundary, keep both possibilities visible and seek a more reliable birth record.
Certainty and interpretation are separate questions
The Sun degree and boundary note should explain which sector was calculated. A description feeling familiar does not change that calculation, and recognition is not the result of a personality test. Read the actual data first, then observe examples from the two guides separately in your experience. Recording what does not fit is useful too.
Sources and approach
These sources explain the page's traditional concepts or calculation context. Original Starmora examples and reflection prompts are not predictions attributed to the sources.