Fifteen degrees is the centre of a sign
Every sign is thirty degrees wide, so fifteen is its exact midpoint. Taken at noon across 1930 to 2035, a 6 June Sun has stood between about fourteen point nine and sixteen point two degrees of Gemini, which brackets that midpoint almost perfectly.
The midpoint is not a mystical spot, but it is a useful one. It is the furthest a Sun can be from both of the sign's boundaries at once, which is why this date is never in any doubt and why arguments about cusps have no purchase here at all.
It also makes the date a good illustration of scale. The whole variation in the Sun's place on 6 June across the last hundred years would fit inside a fiftieth of the sign, and the nearest boundary is fourteen degrees away, so no amount of leap year wobble could ever reach it.
The band the Chaldean order gives to Mars
Inside each sign there are three faces of ten degrees. Fifteen degrees falls in the second of them, running from ten to twenty, and in the Chaldean sequence that face belongs to Mars. Mercury still rules the whole sign, so tradition reads this position as Mercury's material with a Mars overlay.
The older triplicity count disagrees, tying the same band to Libra and therefore to Venus. Neither reading is a measurement. They are two filing systems built on the same ten degree division, and a source that does not say which it uses is telling you less than it seems to.
What both agree on is the division itself. Ten degree faces are among the oldest subdivisions in the practice, older than the houses in some accounts, and they exist precisely so that a sign spanning a month can be discussed in finer pieces than a single label allows.
Why the number barely moves
A calendar year runs a little short of the orbit, so the Sun arrives at each date about six hours further along than the year before. February then gains a day and pulls the whole thing back by roughly eighteen hours. Over four years those two effects almost cancel.
The residue is small. Across more than a century the 6 June position wanders barely more than a single degree, which is why a figure computed for one birth year is a good guide for any other, and why this date has never once been anything other than Gemini.
What the date leaves for the hour to decide
A birth date fixes the Sun and very little else. The Moon covers roughly thirteen degrees in a day, so on 6 June it can end up in either of two signs depending on when you were born, and occasionally in a third if the crossing falls inside that window.
The rising sign moves faster still, changing about every two hours, so any of the twelve is possible and the birthplace matters as well as the clock. The widget above takes your year, hour and location, returns the exact degree, and shows how far you stood from the nearest sign boundary.
Even the Sun's house depends on the hour, since houses are anchored to the horizon. A Sun at fifteen degrees of Gemini can sit at the top of the chart or beneath the ground, and the two arrangements are read very differently, on a difference the calendar date never records.
Questions people ask about this
Is 6 June always Gemini?
Yes. At noon in every year from 1930 to 2035 the Sun on this date stood near the middle of Gemini, far from either boundary. Neither the birth hour nor the time zone can move it.
What degree is the Sun on 6 June?
Between about fourteen point nine and sixteen point two degrees of Gemini, depending on the year. The widget above narrows that to a single figure once it has your year, hour and birthplace.
Which decan does this birthday fall in?
The second, which runs from ten to twenty degrees. The Chaldean order assigns that face to Mars, while the triplicity scheme ties it to Libra and so to Venus.
Does the midpoint of a sign mean anything?
Chiefly that the position is stable and unambiguous. Some practitioners treat the middle degrees of a sign as its most characteristic stretch, but that is interpretation rather than measurement.
Can a 6 June birth be Taurus or Cancer?
No. The Sun leaves Taurus in the third week of May and does not reach Cancer until around the solstice, so this date sits a fortnight clear of both.
What does mutable air describe?
Air is the element Gemini shares with Libra and Aquarius, and mutable is the mode it shares with Virgo, Sagittarius and Pisces. Tradition reads the pairing as adaptable and communicative, which is a description of the sign, not a claim about you.