Where the Sun actually stands
Measured at noon in each year from 1930 to 2035, the Sun on this date sat between about twenty six and a third and twenty seven and three quarters degrees of Gemini. Twenty seven is the working figure, and the spread across more than a century is under a degree and a half.
That tightness is not luck. The tropical year runs a little over three hundred and sixty five days, and the leap day added every four years absorbs almost exactly the surplus. The calendar and the orbit stay in step, so a given date keeps returning the Sun to nearly the same longitude.
The third face of Gemini
Each sign divides into three faces of ten degrees. A position near twenty seven falls in the last of them, running from twenty to thirty degrees. In the Chaldean order of faces, that band belongs to the Sun, which is an unusual pairing: a solar face sitting inside a sign whose domicile ruler is Mercury.
The older triplicity scheme divides it differently and gives the same band to Saturn, by way of Aquarius, the third air sign. Both schemes are traditional and both are still printed, so a source naming one ruler is not contradicting a source naming the other.
Faces are also called decans, and the two words are used interchangeably in most modern writing. The older Egyptian decans were star groups rising at fixed intervals through the night, and the ten degree divisions inherited the name once the scheme was mapped onto the zodiac. What survives in practice is the division and the rulers attached to it.
What the tradition reads into that band
Mercury as domicile ruler is read as quick, verbal and comparative, and mutable air is read as adaptable rather than fixed in method. The face ruler is treated as a shading on top of that, not a replacement for it. A solar face is traditionally read as the more visible and self directed end of a Mercury sign.
All of that is interpretation and should be marked as such. It describes what practitioners have attached to a position, which is a different kind of claim from the position itself, and it is not a forecast about anyone born under it.
How much of a chart this leaves open
The Moon moves roughly thirteen degrees a day, so within this single date it may hold one sign or cross into the next depending on the hour, and across different years it can be anywhere. The ascendant is looser still, since the whole zodiac passes the eastern horizon once a day and a fresh sign arrives about every two hours.
So the date pins a solar degree and a face and stops. The widget above takes your year, your hour and your birthplace and returns your own figure, along with how far your birth fell from the nearest sign boundary.
Worth naming separately are the slow planets. Saturn, Uranus, Neptune and Pluto sit in one sign for years at a stretch, so a birth date alone fixes them. They describe a cohort rather than an individual, which makes them useful background and poor material for a personal reading.
Questions people ask about this
Is 18 June always Gemini?
At noon it was Gemini in every year sampled from 1930 to 2035. The Sun sits about three days short of Cancer, which is a wide enough margin that no hour of the day changes the answer.
What degree of Gemini is this date?
Around twenty seven, with a century range of roughly twenty six and a third to twenty seven and three quarters. Your exact figure depends on your year, hour and birthplace.
Which decan does it fall in?
The third decan of Gemini, the band from twenty to thirty degrees. The Chaldean face order assigns it to the Sun, while the triplicity scheme assigns it to Saturn.
Why do sources disagree about the decan ruler?
Because they are using different schemes rather than making an error. The Chaldean order walks the planets in speed sequence around the whole zodiac, and the triplicity scheme steps through the signs of the same element.
Does a late degree mean the sign is fading?
No. A Sun at twenty seven degrees is as much in Gemini as one at three. The degree matters for aspects, for the face and for house placement, not for whether the sign holds.
Can I find my Moon sign from the date alone?
Usually only to within two candidates. The Moon changes sign every two and a bit days, so unless the change misses your day entirely the hour is what decides it.