The solstice as a boundary
Ninety degrees along the ecliptic from the March equinox, the Sun reaches its highest northern declination and stops climbing. That instant is the June solstice and, in the tropical zodiac, it is by definition the start of Cancer. This is the clearest illustration of what the tropical system actually measures: not the stars, but the Sun's position relative to Earth's tilt and therefore to the seasons.
It also explains why the boundary refuses to sit on a fixed date. The gap between one solstice and the next is about 365.2422 days. The calendar year is 365, with a leap day inserted every fourth February to make up the difference, so the solstice drifts about six hours later through each three year run and jumps back roughly eighteen when the leap day arrives. Over a century that wobble carries it across the 20th, 21st and, in the past, the 22nd of June.
The longest month in the zodiac
Earth reaches aphelion, the point in its orbit furthest from the Sun, in the first week of July. That is inside Cancer. At aphelion the planet is moving as slowly as it ever does, so the Sun's apparent motion along the ecliptic is at its minimum and the thirty degrees of Cancer take the longest to cross of any sign, close to 31.5 days.
Set that against Capricorn, which contains January's perihelion and takes about 29.5, and the two extremes of the year are nearly two days apart. This is a real and measurable asymmetry that no published date range acknowledges, since a range rounded to whole days cannot express a difference of hours. The days column in the table above carries the figure for the year you have chosen.
The Moon as ruler
Cancer is a cardinal water sign and the only sign ruled by the Moon. Cardinal signs open a season, which Cancer does literally in the northern hemisphere by beginning summer. Water signs are read through feeling and attachment rather than argument or action, and lunar rulership adds the specific quality of changeability, since the Moon is the fastest moving body in the chart and completes its own cycle of signs in under a month.
Practically, this means any Cancer placement is read alongside the Moon's own position, in the same way an Aries placement is read alongside Mars. The traditional themes of the month, home, family, memory and shelter, come from that combination rather than from anything about July.
July birthdays and the hemisphere question
Birthdays on 20 to 22 June and 22 to 23 July fall close enough to a boundary that the printed range is unreliable. The Sun is never between two signs. It has one ecliptic longitude at any instant and that longitude is inside exactly one thirty degree arc, so the answer is definite once you supply a birth time and the time zone genuinely in force at the place of birth on that date.
The dates hold everywhere on Earth without adjustment, because a solar ingress is a single global moment and only the local clock differs. What inverts is the meaning. In Melbourne or Cape Town, Cancer begins at the winter solstice, the shortest day, so the seasonal imagery of high summer that the sign carries in European writing does not describe the sky the reader is standing under.
Questions people ask about this
Does Cancer season always start on the solstice?
Yes, by definition. The June solstice is the moment the Sun reaches ninety degrees of ecliptic longitude, and that point is the start of Cancer in the tropical zodiac. The calendar date of that moment moves between the 20th and 21st of June.
Why is Cancer the longest sign?
Because Earth passes aphelion, its greatest distance from the Sun, during it. Orbital speed is lowest there, so the Sun takes about 31.5 days to travel Cancer's thirty degrees, roughly two days longer than the signs around January's perihelion.
When does Cancer season end?
Around 22 July, when the Sun reaches one hundred and twenty degrees and enters Leo. The precise moment drifts across the 22nd and 23rd depending on the year, and the table above gives it in UTC.
Is Cancer season winter in Australia?
It is. The dates are identical worldwide since the ingress happens at one instant, but in the southern hemisphere Cancer opens at the winter solstice rather than the summer one and the season it covers is the coldest of the year.
What does it mean that the Moon rules Cancer?
It means Cancer placements are interpreted together with the Moon's own sign and house in the chart. The Moon moves faster than any other body, changing sign every two and a half days, and that speed is where the sign's reputation for shifting moods originates.