Capricorn
What is already familiar, instinctive, and ready to be carried forward.
Aquarius
What is arriving, stretching the old shape, and asking for a new response.
A cusp is best understood as a threshold image: Capricorn's earth cardinal quality meets Aquarius's air fixed quality. The Sun is still in one sign at a time, while the neighboring symbolism can describe the season around it.
Capricorn brings
Capricorn is ruled by Saturn and speaks through earth cardinal movement. Its familiar lesson is about the elder.
Aquarius arrives
Aquarius is ruled by Saturn and Uranus and changes the texture toward air fixed expression. Its invitation is to practice the visionary.
A boundary, not a loophole
If you were born near a sign boundary, use an accurate birth time and place when a precise chart matters. The cusp language can be a useful metaphor, but it should not replace the actual Sun sign.