☉Western astrology · 4 min
Astrology is the traditional practice of seeking correspondences between the positions of celestial bodies, such as the Sun, Moon and planets, and human character or life events. It grew out of the observation of omens in ancient Mesopotamia, the personalised birth chart took shape in Hellenistic Egypt (around the 2nd to 1st century BCE), and Ptolemy's Tetrabiblos fixed much of the Western tradition in the 2nd century CE. Today most people use it as a language of self-reflection and symbolism rather than as a predictive science.
What is a birth chart?
A birth chart (also called a natal chart or radix) is a snapshot of the sky as seen from the place and moment of someone's birth. It is a circular diagram showing three main layers and the relationships between them:
- Planets: the Sun and Moon, Mercury, Venus, Mars, Jupiter, Saturn and, in modern astrology, Uranus, Neptune and Pluto. Each stands for a symbolic "role"; the Moon, for instance, for emotional needs, Mercury for thinking and communication.
- Signs: twelve equal 30-degree slices of the ecliptic, the Sun's apparent yearly path. The sign describes the style in which a planet operates.
- Houses: twelve divisions of the sky anchored to the horizon of the birth place. The house indicates the area of life where the planet's "role" is played out.
- Aspects: angular relationships between planets (an opposition or a trine, for example), describing how the roles relate to one another.
Three pieces of data
An accurate chart can only be calculated from three facts: the date, the time and the place of birth. The date gives the Sun sign, which changes roughly monthly. The Moon changes sign about every two and a half days, while the Ascendant, the sign rising on the eastern horizon, moves on roughly every two hours. That is why twins or people born on the same day can still have different Ascendants and house layouts. If the birth time is unknown, Sun and planet signs remain reliable, but houses and the Ascendant do not.
Where do the signs come from?
Western astrology uses the tropical zodiac: 0 degrees is the vernal point, the moment of the March equinox, and 12 × 30 degrees are counted from there. This is not the same as the constellations visible in the sky, which differ in size and have drifted away from the signs over the millennia because of precession. The signs are therefore a division of the year according to Earth–Sun geometry, not the stars themselves.
What does science say?
It is worth saying plainly: astrology's predictive power has not been confirmed in controlled studies. The best-known experiment is Shawn Carlson's 1985 double-blind test in Nature, in which experienced astrologers could not match personality profiles to birth charts better than chance. Much of the "this fits me" feeling is explained by the Barnum or Forer effect: generally worded, flattering descriptions feel personal to almost everyone. Astrology nevertheless has cultural, historical and symbolic value, and many people use it as a tool for reflection.
How to use it sensibly
- Treat the chart as a generator of questions, not a verdict.
- A description is useful when you test how true it is for you, not when you simply believe it.
- Do not make life, health or financial decisions on the basis of a sign.
- Combine it with scientifically grounded tools, such as the Big Five personality model.
Astro Atlas is built in this spirit: tradition is presented in the voice of the tradition, while the state of the research is stated separately and honestly.