Cardinal, fixed and mutable
The twelve signs divide into three groups of four by mode. Cardinal signs open a season, fixed signs sit in the middle of one and mutable signs close it. Aries opens spring in the northern hemisphere, beginning at the March equinox, which is what makes it cardinal.
The reading built on that is structural rather than poetic. A cardinal sign is associated with initiation and with the change of state at a turning point, because that is literally where it sits in the year. Whether an individual behaves that way is a separate question the scheme does not answer.
Fire as one of four elements
The elements group the signs by four as well. Aries, Leo and Sagittarius are the fire signs, and share Mars, the Sun and Jupiter as their rulers. In the older material fire is read as heat and dryness, and by extension as activity and forward motion.
Element and mode together identify a sign uniquely, since only one sign is cardinal fire. That is why the pair is quoted so often. It is a compact way of saying Aries and it carries the reasoning behind the name.
Aries is the only cardinal fire sign, which is worth restating because element and mode are often quoted as if they were separate character traits rather than a coordinate pair identifying a position in the wheel.
Eighteen degrees, and a boundary ahead
At noon from 1930 to 2035 the Sun on 8 April has stood between roughly seventeen and three quarters and nineteen degrees of Aries. That is the upper part of the middle band of the sign, which runs from ten to twenty degrees and belongs to the Sun in the Chaldean order.
Late in a band is not the same as on the edge of it. Every year in the sample stays inside, so the decan is settled by the date, though a birth here sits nearer the Venus band that begins at twenty degrees than a birth at the start of the month does.
What element and mode cannot supply
They describe a sign, not a person and not a chart. The Moon on 8 April may be in either of two signs depending on the hour, since it covers between twelve and fifteen degrees a day. The rising sign can be any of the twelve, because the horizon turns through the whole zodiac while the Earth rotates.
A chart also has a distribution of elements across all its placements, and an Aries Sun tells you almost nothing about the balance of the rest. The widget above computes the exact solar degree for a given year and birthplace, works out the Moon and the Ascendant from a birth time, and reports the distance from the nearest sign boundary.
Modes and elements are classification, not description of an individual. They tell you how a sign sits in the structure of the zodiac, and that is the whole of their job.
Questions people ask about this
What does cardinal fire mean?
Cardinal is the mode, meaning the sign opens a season, and fire is the element. Only Aries is both, so the pair names the sign while showing the reasoning behind the label.
What degree is the Sun on 8 April?
Near eighteen degrees of Aries, inside a window of about a degree and a quarter across the last century. That is the upper part of the sign's middle band.
Am I close to the next decan?
Closer than an early April birth, but not on the line. The next band opens at twenty degrees and every year sampled at noon on this date stays below it.
Does the element say anything about my whole chart?
No. Every placement has an element of its own, and a chart's overall balance can be dominated by water or earth despite an Aries Sun. Only the full set of positions shows that.
Is 8 April ever Taurus?
No. Taurus does not begin until around the twentieth of April, so this date sits about a fortnight clear of the boundary in every year.
Which planet rules this position?
Mars as ruler of the sign, and the Sun as ruler of the ten degree band. The tradition reads the band as a qualification on the sign rather than as a competing claim.