A Saturnian is not necessarily a Capricorn.
Here's a chart with the Sun and Moon in Taurus, the Ascendant in Aries, and only one planet -- Saturn -- in Capricorn.
But look how Saturn is separated from all the other planets. He "stands out" or "stands apart".
And Saturn is in the 10th House, the "natural house" of Saturn. He is high in the sky and "stands above all else."
And there are 5 planets (including Sun and Moon) in Earth signs.
And another 5 in Cardinal signs.
So there is a "preponderance" of Earth, and a "preponderance" of Cardinality.
The Cardinal Earth sign is Capricorn, so there is a Capricornian overtone to the chart, and Saturn rules Capricorn and inhabits that sign, which belongs to him.
Because of the
combination of many factors the chart is "Saturnian." Saturn dominates the horoscope (the personality, the life.)
There are other potent factors in the chart, such as the immolation of the Moon in the fires of the Sun, but the idea is that Saturn plays a predominant role in directing the life.
You might like to read Frank's description of how he teaches beginning astrology classes at
http://www.astrologyweekly.com/forum/showthread.php?t=18247
And welcome to our community and to astrology.