Using astronomy to date the nativity …….


The Bible itself does not give an exact date for the birth of Jesus, or even enough indirect information to pin it down with certainty.

At the request of Pope John 1st in the early 6th century, the birth of Jesus was to be used as a dividing line for dating historical events into BC and AD. However, it's been known for quite sometime that the chosen point of division must be in error.

The birth of Jesus was marked by two astronomical events. The relatively short lived Star of Bethlehem is known to most people. Less well known is that fact that an eclipse of the Moon also helps to fix the date of the nativity.

Jesus was born shortly before the death of Herod and Herod is recorded as dying shortly after an eclipse of the Moon. So there was an eclipse of the moon close to the time when Jesus was born.

Dating eclipses is easy – by applying Newton's Laws of Motion and his Universal Law of Gravitational Attraction you just have to work out the orbits of the Moon, Earth and Sun to see when they were in lined up in this order – as viewed from Jerusalem. There are just two relevant possibilities – in 4 and 1 BC.

The Star of Bethlehem ought to tie the date down with precision it we can identify exactly what sort of astronomical event it was. It must have been pretty impressive as it attracted widespread attention.

There are several possibilities but only those that persisted for several months need to be considered. This reduces the number of candidates to just two.

A comet was noted in the meticulous astronomical records of both the Chinese and the Babylonians. They describe a comet with a bright tail that appeared in the spring of 5BC and was visible for 70 days. The description of the 'star' in Matthew's gospel may mean that the comet's tail was seen from afar as being vertically above over Bethlehem. If this comet was the 'star of Bethlehem' then Jesus was born in the mid April of 5BC, which makes it the lunar eclipse of 4BC that occurred just after Herod's death.

The other possibility for the Star of Bethlehem is the apparent coming together in the sky – or conjunction – of the paths of Jupiter and Saturn in 7BC. Their combined brightness would have made them quite obvious in the night sky. In fact, due to the peculiarities of the motions of the planets as seen from Earth, Jupiter and Saturn would have moved back and forth close to each other in the night sky three times over a period of several months, before moving away to quite distinct regions of the sky.

To be guided by either these events – a comet, or the conjunction – the three wise men would have to travel typically 1000km in two to three months – say 1000/75 days or about 14km/day – which seems to be quite feasible.

I favour the comet explanation not least because it would have been quite a bit brighter than the conjunction. Furthermore, although the multiple conjunction of Saturn and Jupiter is a rare event – it does repeat every 800 years or so. The comet of 5BC only visited our solar system just once. Unlike Halley's comet for example, it doesn't keep returning.

So mid April of 5BC looks the best bet for actual date of the nativity.

In a previous article I explained how the crucifixion can be accurately dated using astronomical events. The date referred to our present day calendar was Friday 3rd April AD33

So for how long did Jesus live on Earth? It was from either 7 or 5BC until AD33. However, simply adding 7 or 5 plus 33 to get wither 40 or 38 years gives the wrong answer! Remember that 1AD and 1BC start at the same instant. There is no zero AD - or zero BC for that matter. Counting back 1 year from 1AD gets you to 1BC. So to work out any time interval that spans the BC - AD divide, you must subtract 1 from the addition of the BC and AD years.

So Jesus lived for either 39 years if the Star of Bethlehem was a conjunction of Jupiter and Saturn, or, more likely, 37 years if it was the comet of 5BC that was the 'star' of Bethlehem.

Rick Marshall


The Christmas Star

By Rick Marshall