What fixed means
The twelve signs divide into three modes of four. Cardinal signs begin the seasons and start at the equinoxes and solstices. Fixed signs occupy the middle of each season. Mutable signs close it and hand over to the next.
Taurus is the fixed sign of spring in the northern reckoning the tradition was written in, sitting between Aries and Gemini. The reading follows from the position: cardinal is read as initiating, fixed as sustaining, mutable as adapting. Whatever one thinks of the interpretation, the structure behind it is a plain division of the year into three parts per season.
What earth means
The four elements group the signs by threes, and the three of each element sit at one hundred and twenty degrees from one another around the circle. Earth takes Taurus, Virgo and Capricorn. The tradition reads earth as concerned with the material and the durable, and it uses the grouping practically, since planets in signs of the same element are read as being in easy relation.
Combine the two and fixed earth is the sustaining part of the material grouping. Add Venus as ruler and the standard Taurus description falls out of it: steady, attached to comfort, slow to change position. That is where the adjectives on every other birthday page come from, whether or not the page says so.
The two divisions cut across each other by design. Each element contains one cardinal, one fixed and one mutable sign, so naming both places a sign uniquely: fixed earth can only be Taurus. That neat structure is the skeleton the whole sign vocabulary hangs on, and it is a piece of geometry rather than a claim about people.
The 17 May position
Measured at noon in every year from 1930 to 2035, the Sun on 17 May has stood between about twenty five point seven and twenty seven degrees of Taurus. That is in the third decan, the band from twenty to thirty degrees, which the Chaldean order gives to Saturn and the triplicity scheme also gives to Saturn by way of Capricorn.
It is also about four days short of the crossing into Gemini, so the sign is settled without qualification. The leap year cycle keeps that position steady from year to year, which is why the range across more than a century is only about a degree and a half wide.
Within a sign the degree matters for one plainly technical reason. Aspects are angular relationships counted in degrees, so a Sun at twenty six of Taurus forms exact contacts to placements that a Sun at six would miss entirely. Two people with the same sign and different degrees have measurably different charts before any interpretation begins.
The rest of the chart uses the same vocabulary
Mode and element are not exclusive to the Sun. Every placement in a chart sits in a sign and so carries a mode and an element, and the tradition reads the balance across the whole chart rather than from one body. Someone with a fixed earth Sun and a great deal of mutable air elsewhere is not described the way a sun sign paragraph would describe them.
Working that balance out needs the hour, since the Moon can be in either of two signs on a given date and the rising sign can be any of the twelve. The date supplies one term in a longer sum.
Questions people ask about this
What does fixed earth mean for a 17 May birthday?
Fixed places Taurus in the middle of its season and earth groups it with Virgo and Capricorn. The tradition reads the combination as sustaining and concerned with the material, which is where the usual Taurus adjectives come from.
Is 17 May definitely Taurus?
Yes. The Sun sits near twenty six degrees of Taurus and does not reach Gemini until around the twenty first, so no year, hour or time zone changes the answer.
Which decan is a 17 May Sun in?
The third decan of Taurus, running from twenty to thirty degrees. Saturn rules that band in the Chaldean order and also in the triplicity scheme, which reaches it through Capricorn.
Do the modes mean anything measurable?
The division itself is exact, since it follows the position of each sign within its season. What the tradition reads into cardinal, fixed and mutable is interpretation laid on top of that division.
Does my whole chart share the fixed earth quality?
Not usually. Each placement has its own sign, mode and element, and most charts are mixed. The balance across the chart is what the tradition actually weighs, and it needs a birth time to assemble.
How close to a sign boundary was I born?
About four days from the Gemini crossing, which is a wide margin. The widget above gives the exact distance from your year, hour and birthplace.