What is the difference between astrology and astronomy?
Astronomy is the name for the branch of physics that investigates things in the sky and on the large scale. Astrology is the name of the belief that trends in the future can be predicted from the relative positions of the planets, moon, and zodiacal constellations.
Didn't astronomy used to be astrology?
Yes, in ancient times. In fact, one of the most important astronomer/astrologers was Claudius Ptolemy of second century Alexandria, Egypt. He founded the first reasonably accurate system of predicting planetary positions and indirectly named most of the stars through his book called Syntaxis. This book was passed through the Arabs during the dark ages in Europe after which it returned with the name Almagest. By this time, most of Claudius Ptolemy's descriptive names had been converted to Arabic, which was their final version. Of course, by modern time these names have been so slurred by hard use that they are hardly recognizable even to Arabic speakers.
Is astrology valid?
As a science, no. There are a number of reasons for this. The first is that there is no physical force to explain the predictions. The second is that it has never been calibrated. The third is that it fails double-blind tests.
What are double-blind tests?
To test the value of the planetary positions as set in a horoscope, a set of precise birth dates and times (and locations) were given to real astrologers who actually know how to graph horoscopes (i.e., not the ones who write the columns in the paper!). They were asked to determine which were criminals and which were not. They were not allowed to know who they were investigating and even the person who handed them the names didn't know (this is the "double blind"). As it turned out, they could identify the criminals no more often than a control group of ordinary people.
But asking them to cast a horoscope of a person they don't know is unfair!
That's just it. When they sit down across the table from the person whose horoscope they are generating and engage them in conversation, they can quickly ascertain what kind of person he or she is. They can then make the prediction fit the person. But why cast a horoscope at all? What they have added by analyzing the horoscope contributes no value to the characteristics they have already determined through conversation. (After all, it is the same horoscope if it is plotted before the astrologer met the person.) If the horoscope alone can't identify what a person is, it certainly can't predict what he or she will be.
Above, you said there is no physical mechanism to explain the influence of the planets. What about the tides?
The tides are another name for differential gravitation, or the difference in gravitation from the far side of a body to the near side. It works only on big things, in strongly changing gravity fields, or where other forces are minimal. In the case of the tidal force on the earth, it is caused mostly by the moon, far less by the sun, and practically zero by the other planets. Over something the size of a human body at the surface of our planet, the tidal forces are practically nil.
What is calibration?
It is a fundamental attempt to determine what characteristic goes with which feature of the horoscope. It would require thousands of double-blind tests, as above. This is especially true as no physical mechanism has ever been suggested that would allow a theoretical calculation. There is no record of calibration ever having been done, or indeed, that astrologers ever thought it necessary. They certainly have never thought to redo it in modern times, or redo it to find out the effect of slow changes in the sky or the discovery of additional planets. Astrology, to the best of our knowledge, was developed by the ancient Zoroastrian Persian astrologer-priests as a revealed religion. It was carried to Greece and the Roman world, who developed their own variant. Who invented it is unknown, but it was passed down from priest to acolyte as inviolable knowledge from ancient sources.
My grandmother says astrology works because the three wise men used it to predict the birth of Jesus. Doesn't this prove it?
The Bible only mentions the wise men in the gospel of Matthew. The magi were said to follow a star, but the connection with astrology is not mentioned. Many astrological candidate events have been mentioned, but it is a well-known fact that lots of stuff happens all the time in the sky. Predicting what might have triggered the wise men is uncertain, because our astrology is closer to Greco-Roman astrology than Persian-Babylonian astrology. Even the dating is somewhat uncertain of that time and relies on accounts made at least 70 years later and could be tainted by second-hand memories.
Some of the possible astrological events are anachronisms. A lot has been made of the fact that Jupiter stopped in its retrograde loop about Dec 25, 2 BC, following an earlier close conjunction with Venus in June, but this neglects the fact that early Christians had no idea of when the wise men arrived in Judea and they didn't celebrate Jesus' birth. Dec 25 was chosen hundreds of years after the real event to match the festival of Sol Invictus, a Mithraist Roman holiday of the third century celebrating the lengthening days after the solstice. Furthermore, a planet stopping in its retrograde loop does not stop it in the sky, and could only have been inferred by the wise men a week or two later. It is a singularly uninspiring event. The close conjunction is inspiring enough, but there is no proof that it had anything to do with the birth.
There are also problems with the date of Herod's death, which seems to preclude all the likeliest astrological dates. Herod must have been around long enough to order the slaughter of the innocents, but astrology puts the birth too late for this to happen over time scales that make sense. Most efforts at obtaining an astrological explanation for the star of Bethlehem are indirect efforts to make Herod live two years longer. From what I've heard of Herod, this effort would have been extremely unpopular at the time!
Are you saying that the Bible doesn't prove astrology?
Exactly. We don't know what the Star was or if even it was predictable. The wise men are not mentioned in the Bible as being astrologers. Besides, if I were your grandmother, I would be reluctant to give credence to a rival religion, which is what astrology is.