Technical Notes 

Astronomy and Chronology 

    Astronomy provides useful information in the study of Biblical chronology. With modern day computer 

programs, scholars can determine the past dates and times of various astronomical events, including solar 

eclipses, lunar eclipses, equinoxes, and lunar phases. We can also determine where each eclipse was visible, 

how long it lasted, and its general appearance. 

    The Jewish calendar is based on the phases of the moon and, to some extent, the Spring Equinox. In 

reconstructing the Jewish calendar of ancient times, we need information on the date and time of new moons 

and of the Spring Equinox. With such information, we can determine when the important feasts, such as 

Passover, the Feast of Weeks, and the Feast of Tabernacles, occurred. In some cases, we can be fairly certain 

about such determinations. For example, some scholars think that Christ was crucified in 

A.D.

 30, and some 

think 

A.D.

 33, but no scholar thinks Christ died in 

A.D.

 31 or 32. A reconstruction of the Jewish calendar for 

A.D.

 31 and 32 shows, with a high degree of certainty, that Passover did not begin on a Friday in those years. 

    On the other hand, for certain years in the ancient Jewish calendar, a human decision was made by the 

leaders of the Jewish faith whether or not to make a particular year a leap year. The ancient Jewish calendar 

did not follow the modern pattern of leap years, called the Metonic Cycle. Rather, the religious leaders decided 

when to have a leap year, adding a 13th month to the calendar just before the month of Nisan, based on a 

number of factors. Those factors included whether or not the grain crop was ready for harvest, whether or not 

the fruit trees were ready for harvest, and whether the start of Passover would fall before or after the Spring 

Equinox.

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 If the crops were ready for harvest and the Passover would fall after the Spring Equinox, a leap 

year was not needed. Since we have no way of knowing what the state of the harvest was in particular years in 

ancient times, whenever the date for the start of Passover is close to that of the Spring Equinox we end up with 

some uncertainty about the leap year. 

    An example of uncertainty in reconstructing the Jewish calendar is seen in the year 5 

B.C.

 If a 13th month, 

AdarII, was added to the calendar in spring of 5 

B.C.

, then the dates of Passover and the Feast of Tabernacles 

would be a month later than their dates without the addition of a leap month. Because the Passover would fall 

close to the Spring Equinox without a leap year, we cannot determine with certainty whether or not the Jewish 

leaders decided to add the leap month of AdarII. This results in two possibilities for dates in a reconstructed 

calendar (see chapter 12). 

    The ancient Roman historians often recorded astronomical events, such as eclipses and comet sightings, 

because they associated these with historical events. If a comet was sighted, or if an eclipse occurred, the 

Roman people generally believed that this had some meaning for the Roman empire. Comet sightings were 

often interpreted as foreshadowing the death of the Roman emperor. See chapter 13 of this book for numerous 

examples. 

    Lunar eclipses are visible from anywhere on earth where it will be dark at some point during the eclipse. 

Lunar eclipses can last for several hours and can also be visible at dawn and dusk. There are anywhere from 2 

to 4 lunar eclipses each year. However, some of these lunar eclipses are very brief, partial eclipses where the 

shadow of the earth passes over only a small portion of the visible disc of the moon. Such eclipses (called 

partial penumbral lunar eclipses) are not generally observable without a telescope. 

    Solar eclipses are generally more helpful than lunar eclipses in determining an historical date. Any particular 

solar eclipse can be seen from only a small portion of the earth's surface, whereas any lunar eclipse can be seen 

from any place on earth where it is dark during the eclipse, and from places where is it dusk or dawn. As a 

result, there are more lunar eclipses visible from the Roman empire than solar eclipses. Since a solar eclipse is 

less likely to have been seen, its mention in relation to an historical event is more specific and more likely to 

help us determine the date of the historical event. 

    Comet sightings were also mentioned by the ancient Roman historians. Computer astronomy programs are 

of limited value in determining when ancient comet sightings occurred. Many comets are seen only once; only 

a few comets return century after century, as Halley's comet does. However,  ancient Chinese and Korean 

astronomers observed the stars closely and kept careful records of the date, location, and appearance of 

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