The constellations of the Zodiac are not all the same size, nor are they evenly spaced. Some of the constellations even overlap one another. It's somewhat arbitrary where to draw the line between one constellation and another. Part of this problem is alleviated by simply declaring that the divisions between the twelve constellations are evenly spaced.
The zodiac is divided into twelve equal areas, with each area containing one zodiac constellation. Each area is known as a "house". So when the sun is in the constellation Capricorn, it may be said, "the sun is in the house of Capricorn."
If we divide a circle into twelve equal parts, and place the twelve constellations of the zodiac in order around the circle, one in each part, we see that each constellation occupies 30 degrees of the circle.
|Dec. 22 - Jan. 19||Capricorn (the goat, in the wedge marked 30°)|
|Jan. 20 - Feb. 18||Aquarius (man pouring water from pitcher)|
|Feb. 19 - Mar. 20||Pisces (the two fish)|
|Mar. 21 - Apr. 19||Aries (a sheep, ram, or lamb)|
|Apr. 20 - May 20||Taurus (the bull)|
|(The Milky Way lies between Taurus and Gemini)|
|May 21 - June 20||Gemini (the twins Castor & Pollux [see Acts 28:11])|
|June 21 - July 22||Cancer (the crab)|
|July 23 - Aug. 22||Leo (the lion)|
|Aug. 23 - Sept. 22||Virgo (the virgin, holding a sheaf of wheat, or an ear of corn, or a fruit)|
|Sept. 23 - Oct. 22||Libra (the scales)|
|Oct. 23 - Nov. 21||Scorpio (the scorpion)|
|Nov. 22||Sagittarius (the archer or hunter, depicted as a centaur, half man half horse)|
The tradition of having 360 degrees in a circle probably comes from it being a nice convenient number very near the number of days in a year. 360 is a nice round number that can be evenly divided into 2 parts, or 4 parts, or 10 parts, or 12 parts, or 36 parts, or 72 parts—whatever you feel like, whereas the actual solar year of 365.2421934 days is not as easy a number to deal with.
The sun travels about one degree each day. It's not exact, since there are actually 365.2421934 days in a solar year instead of 360. Also the earth's orbit isn't a perfect circle, it's slightly elliptical. The earth travels slightly faster during the winter months when it's slightly closer to the sun, and it travels slightly slower during the summer months when it's slightly farther from the sun. In general though, one degree equals about one day. (The seasons are primarily caused by the earth's tilt, not by it's varying distance from the sun. See Cause of the Seasons [offsite])
It's still ambiguous exactly when the sun leaves one house and enters another. The standard dates used by astrologers for the past two millennium (the ones given in the table above) correspond to a time somewhere in the range of around 600 B.C.E. to 1 C.E. Today, some 2000 years later, due to the Earth's precession the sun is in Sagittarius on Dec. 22, and you have to wait until around Jan. 18 before you can say the sun is in Capricorn. However, the tradition of having Capricorn cover the days from December 22 to January 19 still remains today. Have a look at any astrology or horoscope section of a newspaper (if your newspaper has such a section. No one today really believes in astrology, but some newspapers still have one just for the fun of it.) You'll see there listed the twelve constellations of the zodiac, covering the full year, with Capricorn beginning on December 22, the Winter Solstice. Though the winter solstice is a natural place to begin the year, you'll notice the astrology section always begins with Aries, March 21, which is the vernal equinox, another common place to begin the year.
For our purposes it doesn't matter exactly what day the sun leaves one house and enters the next. It's the overall story line we are concerned about. We want to know the general sequence of events that occurs during the solar year.
If we accept the old tradition of having the sun enter Capricorn on the Winter Solstice, a natural place to start the yearly cycle, then when the sun has traveled 30 degrees the sun exits Capricorn and enters Aquarius. Thus an allegorical story of the sun's annual travels could have the sun character visiting the Aquarius character at "Age 30" That's actually 30 degrees, or about 30 days, but an allegory could use 30 years instead.
2. On the Winter Solstice the sun enters Capricorn.
The Sun then passes through Capricorn, traveling 30 degrees in 30 days,
to reach Aquarius at "age 30". Entering Aquarius is the first big event of the journey.
Jesus is said to have begun his ministry at age 30 (Luke 3:23). Jesus begins his ministry by visiting John the Baptist (Aquarius), who baptizes with water.
3. The Sun's annual journey around the zodiac takes one year.
Jesus' ministry is said to have lasted one year. [At least according to the Synoptic Gospels (Matthew, Mark, Luke). The Gospel of John implies Jesus' ministry lasted 3 years. The Gospel of John contradicts the Synoptic Gospels many times, which, by definition, is why it's not one of the "Synoptic" Gospels. Here we are concentrating on Jesus' mission as described in the Synoptic Gospels.]