The measured position
Sampling noon in every year from 1930 to 2035, the Sun on this date lay between roughly seven and four fifths and nine and a fifth degrees of Cancer. Eight and a half is the working figure, and even the highest of those values stays below the ten degree mark.
The Sun advances a little under a degree a day, so the line at ten degrees is about a day and a half ahead. That is a comfortable margin, and no hour of birth on this date carries the position across it.
Why an internal line matters at all
Each sign splits into three faces of ten degrees, giving thirty six across the zodiac. The face is a real layer of traditional practice, with its own ruler, and it narrows a description from a whole sign to a third of one. Almost no birthday page mentions it, which is part of why they all read alike.
Crossing from the first face of Cancer to the second changes the face ruler from Venus to Mercury in the Chaldean order, and from the Moon to Mars in the triplicity scheme. The sign, its element and its domicile ruler stay exactly as they were.
Face lines fall every ten degrees, so the Sun crosses one roughly every ten days and crosses a sign boundary only monthly. Anyone paying attention to the finer division therefore has three times as many transitions to keep track of, and almost none of them are marked in popular writing.
What applies on this side of it
The first face of Cancer runs from zero to ten degrees. The Chaldean order gives it to Venus, and the triplicity scheme gives it to the Moon, which also rules the whole sign, so under that scheme the ruler doubles here rather than shifting.
The tradition reads Cancer as cardinal water: opening a season, and concerned with attachment and memory rather than abstraction. The face ruler is treated as a shading on that reading. All of it is interpretive convention, distinct from the measurable position it is attached to.
Faces are usually called decans now. The name descends from Egyptian star groups that rose at regular intervals through the night, later mapped onto ten degree segments of the signs. Thirty six of them cover the zodiac, and the two rulership schemes represent two different ways of assigning planets to them.
Everything the date leaves out
The Moon travels about thirteen degrees a day, so within this date it may hold one sign or cross into the next, and across different years it can be anywhere. The ascendant changes roughly every two hours, so without a birth time all twelve possibilities remain live.
Those are the placements that distinguish two people born on the same date, and a date alone cannot reach either. The widget above uses your year, hour and birthplace to give your exact degree, which settles which side of the ten degree line you fell on.
The slow planets are the one further thing a date supplies. Saturn, Uranus, Neptune and Pluto hold a sign for years at a time, so the birth year is enough to place them, and they are shared with an entire cohort rather than belonging to any one chart.
Questions people ask about this
Is 30 June Cancer?
Yes, at noon in every year checked between 1930 and 2035. The Sun stands around eight and a half degrees along, well inside the sign at any hour.
Is this date in the first or second decan?
The first, in every sampled year. The highest noon value across the century stays below the ten degree line that opens the second decan.
How close is the decan line?
Roughly a degree and a half ahead, which the Sun covers in about a day and a half. Being near a line does not blend the two bands, since a longitude falls in one of them.
Which planet rules this decan?
Venus in the Chaldean face order and the Moon in the triplicity scheme. The Moon also rules the sign as a whole, so the two attributions coincide under the second scheme.
What changes at ten degrees?
The face ruler, from Venus to Mercury in the Chaldean order and from the Moon to Mars in the triplicity scheme. The sign, element and domicile ruler are unaffected.
Does the birth hour change the decan on this date?
No. The Sun moves less than a degree in a day and the nearest value sits over a degree from the line, so the whole date falls in the first decan.