I miss you S5. Actually, I talked about this in another thread.
It's from the Chaldean Order. It goes Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon and that gets repeated around the chart.
The Chaldeans were the ones who created the 12 sign zodiac system (primitive version) and they identified all the planets from Sun to Saturn as important celestial influences and invented temporal hours, which predates the Ancient Greeks. Natal charts weren't used the same way we use natal charts today... we're more into psychology and personalities and what's going on inside the person; modern day, we're too wrapped up in the internal workings of a person. Whereas, before, traditionally, natal charts were used to predict what would happen to you because the main source of concern was what would happen to you in the external world- are you going to marry well? Are you going to die in war? etc. I think the Ancient Greeks used it because the bulk of Hellenistic astrology is predictive and I believe Ibn Ezra used the Chaldean order for natal charts too (he was from the 12th or 11th century).
I've always used the Chaldean Order because my teacher used it and taught it to me...that's why she stressed sticking with the natal chart and from that, not only can I "know" the person, I can also predict what happens to the native. I don't dabble with predictive stuff much since my teacher went into total world retirement, but I kept a lot of foundations she taught me and I apply it to natal/psychological readings.
Maybe this would help.
Page 4 is where it gets specifically addressed but it would be a good read from start to finish.
http://easyweb.easynet.co.uk/~sueward/articles/houses.pdf