very curious, there were dogs around in Rome, right?
broccoli!
There were dogs in Rome, and Greece, and even Babylon, and, as a matter of fact, there are two constellations in the sky called “the big dog” and “the little dog”. Neither of those constellations are in the arc of the Zodiac, though…there are 13 constellations that are.
so, there’s no Dog sign because it wasn’t on come Astrological arc in the sky?
broccoli!
I is joking think Captain Amazing is joking. He’s quite Sirius.
There are three constellations representing dogs in the night sky as seen by the Greeks and Romans. The first two are Canis Major and Canis Minor, the Great and Little Dog. They are both situated to the south of Orion (since they are said to represent his hunting dogs) and Canis Major includes Sirius (The Dog Star), the brightest star in the night sky.
The third is Canes Venatici, the Hunting Dogs, situated near Ursa Major and Boötes (whose dogs they supposedly represent).
The Zodiac, to turn to the next part of your question, is the band of constellations that lie on the ecliptic, which is the apparent path of the sun through the sky, and on which astrology has its basis. None of these constellations lie on the ecliptic so therefore they’re not in the Zodiac. Just an accident of perspective.
Time preview post me next quack.
Well, Canis Major was KIND OF close to the ecliptic, and it was actually part of the original thirteen (or “baker’s dozen”) signs of the Zodiac.
But too many people complained about being born under the Sign of the Pooch…they wanted to be COOL stuff, like their friends – Goats, Crabs, Water-Carriers, etc. – so the newspaper feature writers of Ancient Greece took it out.
And that’s why September (Latin for “Seventh Month”) is now the ninth month of the year.
Ike, I’m gonna sue you for damages, you give me eyestrain from causing involuntary eyerolls like that. 
There are actually 14 constellations in the arc of the Zodiac, the other two being Ophuichus the Snake-holder and Cetus the Whale. That is, the planets can pass through these constellations, as defined by modern astronomers, as well as the usual 12. Astrologers usually ignore them, though.
so, if the 12 ‘everyday’ signs we know about are sun signs, is there a moon Zodiac 12 as well?
broccoli!
The moon pretty much follows the ecliptic too and more or less is within the same zodiacal band.
Astrologers pretty much ignore all logic.
I once read that the ecliptic crosses 13 constellations and that the other eight planets can be in up to 20-something constellations. Unfortunately, I can’t find the reference at the moment - - sorry.
Ancient astrologers looked at the position of the moon and the planets in the sky but the overarching determinate was the sun’s position. That was the basis for everything else. Not surprising, since it is the single brightest object in the sky and it wasn’t hard to guess it helped life along, what with plants turning towards it and the connection between longer days and the seasons of growth and plenty that are summer and autumn. So position of the sun determined one’s personality and the position of the other celestial bodies determined other, lesser aspects.
Of course, astrology was around long before modern astronomy and the division of the night sky into ‘constellations’, or the marking of property boundaries. So it’s not entirely just to say that astrology ignores all logic since the logic we’re talking about didn’t exist at the time.
This is not to say I believe astrology makes sense or has any scientific grounding. 
I can’t recall where I read about the 14, but I believe ther was some reform-minded astrologer who wrote a book doing some “analyses” based on these 14 constellations 20-25 years ago. If you include Pluto, which goes way off the plane of the ecliptic, I don’t doubt you might get 20+ constellations.