The Zodia Scorpio is actually composed of TWO constellations, Scorpius and Ophiuchus.

In order to understand the above statement, you need to understand the definition of and difference between three things:

1. "Sun sign"

2. Constellation

3. Zodiac

Let's unpack each of these terms, so that we can understand what they really mean - both astronomically and astrologically.

"sun sign" - this refers to the sign or zodia against which the sun appears in the chart; it takes about a month for the sun to traverse a particular sign or zodia. This concept is based upon our view from here on Earth and is often refered to as "geocentric" because it appears as if the sun, moon, asteroids, and planets are moving around us.
NOTE: If one uses the Tropical zodiac system, the "sun sign" of the person isn't even related to the actual constellation (see below) of the same name - but rather where the constellations were hundreds or thousands of years ago!

constellation - in modern astronomy there are 88 constellations; these constellations are "official" in that the IAU declared them to be the official way to divide up the sky in 1930.  Different cultures throughout history have had different constellations - and myths associated with them - but the ones used by the IAU were predominantly from the Greek and Roman cultures. If you want to learn more about the different constellations here is a good list of online star charts.

Zodiac - the imaginary belt of the heavens, which extends approximately 8° on either side of the ecliptic, within which are the apparent paths of the sun, moon, astroids and principal planets. It contains thirteen actual constellations which are divided into twelve divisions called signs of the zodiac or zodia. Each division, however, because of the precession of the equinoxes, now contains the constellation west of the one from which it took its name.

Now let's look at the constellations that compose the zodia of Scorpius.

First, here what Scorpius looks like:

See that blue line? That's the ecliptic - now look at how narrow the white area is at that point... that means that it takes only about six or seven days for the sun to move through this region of the ecliptic.  This means that if you are born during the first six days of the zodia Scorpio, the constellation that the sun was in was Scorpius.

This corresponds closely to the system of Decantes which the ancient astronomer/astrologer Manilius proposed in his work Astronomica. What are decantes you ask? They are a way of dividing the zodia into 3 decans or decantes of 10° each; each of the decans is associated with a zodia sign, ruler, and particular feeling or mood.

The decan of the zodia Scorpio that the constellation Scoripus is associated with has the following qualities according to Manilius:

First Decan of Scorpio = Decan of Scorpius
constellation Scorpius
decante degrees 0°-10°
decante ruler Saturn/Kronos
decante zodia Capricorn
decante association labor, cruelty, or suffering

Now let's look at the constellation of Ophiuchus:

See how much more white area there is in Ophiuchus? Ophiuchus' section of the ecliptic is approximately twice as large as that of Scorpius.  This corresponds neatly with Manilius' second and third decans of the zodia Scorpio:

Second Decan of Scorpio = First Decan of Ophiuchus
constellation Ophiuchus
decante degrees 11°-20°
decante ruler Uranus/Ouranous
decante zodia Aquarius
decante association transformation and rebirth

Third Decan of Scorpio = Second Decan of Ophiuchus
constellation Ophiuchus
decante degrees 21°-30°
decante ruler Neptune/Poseidon
decante zodia Pisces
decante association the supernatural or Numinous

In my next installment, I will discuss the myths associated with each of these signs, how those myths relate to the attributes of each constellation in astrology, and catasterism.