Cusp, in full
In technical astrology the word usually refers to house cusps, the twelve dividing lines that carve a chart into houses, calculated from the birth time and the latitude of the birthplace. A sign cusp is the simpler case: 0 degrees of each sign, twelve fixed points on the ecliptic. Either way a cusp is a line rather than a band. The Sun holds exactly one ecliptic longitude at any instant, so it is in exactly one sign.
The popular idea of being born on the cusp, and so belonging to two signs at once, is folk usage rather than something a chart produces. What is genuinely true is that births near a boundary are easy to get wrong, because the changeover happens at a clock time that moves from year to year and is published in universal time. Astrologers deal with those cases by casting the chart rather than by splitting the difference.
The dates move because the tropical year, the interval from one March equinox to the next, runs about 365.2422 days. A calendar year of 365 days is short by roughly a quarter of a day, so each equinox arrives about six hours later than the last until a leap day pulls it back. Every sign boundary inherits that drift, which is why published date ranges disagree by a day between sources and between years.
- Astrology
- Sign cusp, House cusp
A worked example
A birth on 22 August is normally called Leo and one on 23 August Virgo, but the Sun's entry into Virgo happens at a specific moment that shifts by several hours from year to year. A birth late on 22 August in Los Angeles is already 23 August in London. The chart settles it: the Sun holds one longitude, and 149.9 degrees is Leo while 150.1 degrees is Virgo.
Sources
The cycle of the seasons, the tropical year, runs about 365.2422 days, which is why leap days are inserted to keep the calendar in step.
US Naval Observatory — Leap Years
See also
Where it comes up on this site
Cusp: frequently asked
No. At any instant the Sun holds one ecliptic longitude, and one longitude falls in one sign. A birth close to a boundary is a reason to have the chart cast properly, not a reason to claim both.
Because the boundary is a moment rather than a day. The tropical year runs about 365.2422 days, so each sign change arrives roughly six hours later than it did the year before, until a leap day pulls the whole sequence back. Any published date range is a rounding of that moving target, and sources round in different directions and quote different time zones.