The position and the remaining margin
Taken at noon in every year from 1930 to 2035, the Sun on this date has sat between roughly twenty seven and three quarters and twenty nine and a quarter degrees of Cancer. The sign ends at thirty, so the margin runs from about three quarters of a degree in the tightest years to a little over two degrees in the widest.
Three quarters of a degree is around eighteen hours. That is close enough to be worth stating precisely rather than waving at. Even so, a birth late on the evening of 21 July in one of those tight years still lands inside Cancer, because the shift from noon to midnight is under half a degree. The sign holds at every hour, in every year sampled. It is the last date in the run where that can be said without adding a condition.
What changes the very next day
On 22 July the same calculation comes out Cancer in most years and Leo in a handful, and on 23 July the split reverses. Those are dates where a birth in the morning and one in the evening can genuinely differ, and where a birth in the Americas can fall on the far side of the ingress from a European one at the same universal moment.
The mechanism is the same six hour annual drift that governs every ingress, corrected by the leap day. It moves the crossing across a window of roughly a day and a half, and 21 July sits just outside that window while the two dates after it sit inside. That is the entire difference, and it is why one date needs your birth hour and the other does not.
The face, which is not in doubt either
Twenty eight degrees is deep in the third face of Cancer, the band from twenty to thirty. In the Chaldean sequence that face belongs to the Moon, the ruler of the sign itself, and in the triplicity scheme it belongs to Jupiter by way of Pisces. Being near the end of the face changes neither attribution.
The tradition treats the closing degrees of a water sign as a winding down rather than a weakening, and describes this stretch as strongly lunar under the first scheme and broadly Piscean under the second. That is reported convention. It attaches to the degrees, not to the person born under them. The two schemes point in broadly compatible directions here, both landing in water, which is not true everywhere in the zodiac.
Confirming it for yourself
Because the margin here is smaller than on the dates before it, this is a page where running the numbers has some point. The widget above takes your year, your hour and your birthplace, returns the exact solar degree rather than a noon average, and reports how far that degree sits from the Leo boundary.
Expect an answer somewhere around a degree or two on the Cancer side. If you were told at some point that you were nearly a Leo, that figure is the reply. Nearly is not a category the zodiac recognises, and a number is more use than an adjective.
Questions people ask about this
Is 21 July Cancer or Leo?
Cancer, in every year from 1930 to 2035 and at every hour of the day. The margin is between about three quarters of a degree and two degrees, which no birth hour can close.
How many days of Cancer are left?
Between roughly one and two, depending on the year. The Sun covers a little under a degree a day and the sign ends at thirty degrees.
Which nearby dates are actually undecided?
22 and 23 July. On both, the answer depends on the year, the hour and the birthplace, because the ingress falls inside that window.
Does a birth close to midnight change anything?
It moves the Sun by less than half a degree from the noon figure, which is not enough to reach Leo on this date in any year sampled.
What decan is a 21 July Sun in?
The third of Cancer, near its end. The Chaldean order gives that face to the Moon and the triplicity scheme gives it to Jupiter.
Why does the boundary move between years?
Because the tropical year runs about six hours longer than the calendar year, and the leap day pulls the calendar back. The crossing drifts across a window of roughly a day and a half.