Q: Dr. Gill, would you please explain the many calendars of past and how they determined AD and BC?
A: About the calendars
Recently I've been asked, does BC stand for "Before Christ" and A.D. stand for "After Death". Obviously they couldn't have made the calendar BC before the birth of the man they call Jesus. But in today's world BC is universally accepted to mean as before Christ. Actually, AD stands for the Latin phrase Anno Domini, which means in the year of our Lord. So the BC & AD system is not taught in the Bible. And it was not fully implemented and accepted until several centuries after the death and resurrection of the man they call Jesus.
Obviously the purpose of the BC / AD dating system was to make the birth of Jesus Christ the dividing point of world history. However, when the B.C. A.D. system was being calculated, they actually made a mistake in pinpointing the year of Jesus' birth. Scholars later discovered that Jesus was actually born in around 4 to 6 BC. Not in one AD. This isn't really a crucial issue. The birth, life, Ministry death and resurrection of Jesus the Christ are the turning points in world history. So I guess it's fitting that we separate the old and new by B.C. & A.D.
It would take more time that I would like to list all the different kinds of calendars so the ones I'm going to mention to begin with are those that use roughly the 28 day moon phase.
We will start with a sidereal month in the spirit of the moons orbit as defined with respect to the celestial sphere known as the esters. That's the time it takes the moon to return to a given position among the stars. This type of month has been observed among cultures in the Middle East, India, and China. They divided the sky into approximately 28 lunar mansions identified by the prominent stars in them.
Next we have the tropical month. It's customary to specify positions of celestial bodies with respect to the vernal equinox because of the procession. This point moves back slowly along the ecliptic, therefore it takes less time to return to zero then to the same point among the fixed stars. It is slightly shorter, about 27 1/2 days, and is known as a tropical month.
Next we come to the anomalous month; the moon's orbit is in the elliptical or egg shape, rather than the circle. However, the orientation as well as the shape of this orbit isn't fixed in particular. The position of the extreme points makes a full circle in about nine years. It takes a longer-term to return to the same point because it moves ahead during one revolution. This longer period is called the anomalous month and its average length is a little over 28 days usually the diameter of the Moon varies with this, and therefore this type of the calendar has some relevance for the prediction of eclipses.
Now we come to the draconic month, also called the Nodic month. The orbit of the moon lies in a plane that's tilted with respect to the plane of the ecliptic. It defines two points on the celestial sphere as the Moon moves into the northern hemisphere and the descending node. When the Moon's path crosses as it moves into the southern hemisphere the draconic or Nodic month is the average animal between two successive transits of the moon through its ascending node.
Because of the sun's gravitational pull of the moon, the moon's orbit gradually rotates westward on its axis, which means as it rotates around the earth, the time it takes the moon to return to the same node is shorter than the Sidereal month. It lasts a little under 28 days maybe 27 1/4 days, this is because the moon's orbit is inclined with respect to the ecliptic. The sun, the moon, and the earth are in line only when the moon is at one of the solar or lunar eclipse.
The title draconic refers to a mythical dragon. The Moon's appearance depends on the position of the moon with respect to the Sun, as seen from the Earth; while the moon's orbiting the earth the earth, is progressing in its orbit around the sun. This means that after completing a Sidereal month it goes a little farther to reach the new position of the earth with respect to the Sun. Because of the orbits of the moon, the actual time may range anywhere from 28 to 29 days and it has 13 months in a year.
The solar calendar or what we know as the Julian or the Gregorian calendar is in use throughout most of the world. It was sponsored by Pope Gregory the 13th in 1582 as a corrected version of the Julian calendar. Originally the Julian calendar had a month called Mercedonius which was an occasional month after February used to realign the calendar and then there was Quintilis, renamed to July in honor of Julius Caesar and Sextilis renamed to August in honor of the Augustus Caesar. Both of these months took days from Feb.
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