The Zodiac was first put together thousands of years ago, when ancient astronomers looked to the skies and saw which constellations passed around the plane of the ecliptic (the plane on which the planets orbit the sun, i.e. the plane on which eclipses occur). The sign a person is born under is the sign on the opposite side of the sun from the Earth on the person's birthday. However, what most people don't know is that over long periods of time, the constellations shift around in the sky, meaning that the dates of the classical, astrological Zodiac are no longer relevant. Astrologers haven't accounted for this, and are still using the dates from when the Zodiac was originally set up.
Problem 1: The Earth's Precession
The reason the Zodiac moves around is due to the axial precession of the Earth. The Earth is not spinning perfectly on its North-South axis. Like a top or a gyroscope that has started to wobble, the Earth's poles move around in a small circle with a period of about 25,800 years. This also means that the North Star (currently Polaris) changes with the Earth's precession.
Problem 2: Constellation Size
Aside from the Earth's precession, the classical Zodiac has two other problems. First, the original Zodiac was set up more or less evenly distributed across the calendar year. This doesn't match up with the actual position of the sun in the constellations in the sky, because the constellations are not of uniform size. Pisces, for example, is very large compared to the others, and the part of Scorpius that actually intersects the ecliptic is very small (only 7 days!).
Problem 3: The 13th Zodiac Sign
A somewhat bigger problem, however, is that there are actually thirteen constellations in the ecliptic, not twelve. The little-known constellation Ophiuchus (the serpent bearer) falls between Scorpius and Sagittarius.
Below is a chart comparing the modern, astronomical Zodiac with the classical, astrological Zodiac (each line is approximately 1 week). Only about 14% of the dates match up, most people will find that they were born under a different sun sign than they thought they were.
Astro- Astro-
nomical logical
1
1
1/19 ♑ 1 ♒ 1/21
1
2/6 ♒ 2
2
2 ♓ 2/20
2
3
3/12 ♓ 3
3 ♈ 3/21
3
4
4
4/19 ♈ 4 ♉ 4/21
4
5
5/14 ♉ 5
5
5 ♊ 5/22
6
6
6/20 ♊ 6
6 ♋ 6/22
7
7
7/21 ♋ 7
7 ♌ 7/23
8
8/12 ♌ 8
8
8 ♍ 8/24
9
9
9/16 ♍ 9
9 ♎ 9/23
O
O
O
10/31 ♎ O ♏ 10/23
N
N
11/23 ♏ N
11/30 ? N ♐ 11/22
D
D
12/18 ♐ D
D ♑ 12/22