The face in question
Each sign divides into three faces of ten degrees, thirty six in the zodiac altogether. Seventeen degrees falls in the second face of Gemini, the stretch from ten to twenty, and the Sun on this date has been inside it in every year checked at noon since 1930.
Two systems for naming its ruler have come down to us. Both are old, both are still in print, and they do not agree, which is why an ordinary birthday page can quote a decan ruler with confidence and be contradicted by the next page you open.
They are not rival descriptions of an object, since a face is a portion of a circle rather than something in the sky. They are two ways of labelling the same portion, which is why the disagreement can persist indefinitely without either side being refuted.
Mars by the Chaldean order
The Chaldean sequence runs the seven visible planets in order of apparent speed and walks that order through the zodiac, ten degrees at a time, starting with Mars at the beginning of Aries. Counted from there, the second face of Gemini falls to Mars.
Tradition reads that combination as Mercury's quickness with an edge of force behind it: argument rather than conversation, according to the older texts. The description belongs to the scheme rather than to anyone born on the date, and it is worth keeping that distinction in view.
Venus by the triplicity count
The other method treats each face as a borrowing from the next sign of the same element. Gemini is air, so its faces run Gemini, Libra, Aquarius, and the middle one takes on Libra. Libra's ruler is Venus, which is about as far from a Mars reading as the two schemes get.
Neither table can be checked against the sky, since faces are divisions of a circle rather than objects in it. The honest position is that they are competing conventions, and the useful habit is to note which one a given source is using before taking its adjectives seriously.
The triplicity method has the advantage of restarting at each sign, so its pattern is easy to reconstruct from memory. The Chaldean method runs continuously around the whole circle, which makes it harder to recall and gives it an internal regularity the other lacks.
Everything the date leaves open
The Sun is one placement. The Moon shifts about thirteen degrees a day, so an 8 June birth can find it in either of two signs depending on the hour, and the difference in the resulting reading is far larger than anything the decan contributes.
The rising sign is fixed by the clock and the location together, and any of the twelve can come up. The widget above takes those details, computes the exact solar degree for your own year, and reports how far that degree stood from the nearest boundary.
Houses depend on the same details. The Sun's degree may be settled on 8 June, but whether it falls near the midheaven or close to the horizon is decided by the hour, and that placement carries more interpretive weight in most traditions than the face does.
Questions people ask about this
Is 8 June Gemini or Cancer?
Gemini. The crossing into Cancer is the June solstice, close to a fortnight later, so no hour or time zone brings this date near it.
Which decan ruler applies to me?
Both are traditional. Mars comes from the Chaldean order and Venus from the triplicity count by way of Libra. Practitioners pick a scheme and stay with it rather than merging the two.
What degree does the Sun reach on 8 June?
About seventeen degrees of Gemini, with a noon range of roughly sixteen point eight to eighteen point one across the years sampled.
Are decans part of mainstream astrology?
They are old and widely documented, and they appear in both Hellenistic and later European sources. How much weight to give them is a matter of practice rather than of evidence.
Does the decan change my sun sign?
No. It subdivides the sign without replacing it. A Sun at seventeen degrees of Gemini is fully in Gemini whichever face it occupies.
What if I do not know my birth time?
The sun sign is unaffected on this date. The degree shifts by less than a degree across a full day, so a noon default is close enough for that purpose, though not for the Moon or the ascendant.