Some Notes on the
Visibility of the 5BC Chinese Star
The 5
BC Chinese star is often associated with the Star of
Chronology
The evidence that King Herod the Great died in late March or early
April 4BC is generally regarded as conclusive. Thus it is thus generally
supposed that the birth of Jesus took place between one and three years
beforehand, between 7 and 5BC. This date is also coherent with the known date
of a census that was ordered by Caesar Augustus in 8BC. We also know from an
inscription called the “Lapis Venetus” (below left) that Publius Sulpicius Quirinius,
legate of the emperor in
We know though
that Augustus ordered censuses only in 28BC, 8BC and 14AD, none of which
coincide with the dates of Quirinus’s period as governor of
Although it cannot be proved beyond all doubt, it seems reasonable that the census described in Lapis Venetus was the same one that was ordered by Augustus in 8BC.
It is also a safe assumption that Augustus Caesar ordered the census long before it was to be executed, perhaps several years, to allow the Imperial burocracy time to promulgate, plan and execute the census successfully in all corners of the Roman Empire, however distant.
Finally, we know that the chronology of Dionisius Exiguus has a built-in error of 5 years and that his date for the Nativity of December 25th 1AD, when corrected, is December 25th 5BC, just three years after Caesar Augustus’s census decree.
However, Luke’s
comment that shepherds were in the mountains around
As we can see from the temperature and rainfall graphs for Jerusalem (above), December, January and February are cold, wet months, totally unsuited to sleeping outdoors on a hillside. The range of dates when Spanish shepherds may sleep outdoors with the flocks is from March through to September. Any shepherd who attempted to sleep outside with his flock in December would have risked pneumonia. However, multiple contextual clues in Luke suggest that the Nativity occurred during lambing time, in spring and, probably, around the date of Passover, which is given below for the years from 7BC to 4BC.
|
Jewish
year |
Date
of Passover |
Gregorian
year |
Notes |
|
3754 |
April 12th |
7BC |
Earliest plausible date of
Nativity. |
|
3755 |
April 1st |
6BC |
|
|
3756 |
March 21st |
5BC |
Most probable date of
Nativity. |
|
3757 |
April 10th |
4BC |
Herod’s death occurred
during the previous lunar month. |
The
Chinese description of the Star
If we
accept mid to late March 5BC as the most likely date of the Nativity, we
discover that there is an interesting coincidence in date with one of the very
few Chinese records that exist from the period between 1BC and 20BC.
The
records dated 10BC and 12BC are both known to be of Comet Halley. The former is
a so-called “ghost record” with a date error. However, the Chinese records
allow us to get an excellent idea of the movement and brightness of Comet
Halley in 12BC
We know
from the extremely early sighting in the morning sky that Halley must have been
unusually bright in 12BC to have become visible to the naked eye at this time
without prior knowledge of its position. The Chinese observations allow us to
track the comet’s movement from the first sighting in Gemini to its final
disappearance in twilight in Scorpio. However, it is evident that this was not
a particularly good return of the comet for naked-eye observations, as its
greatest brightness would have come well before perihelion when the comet would
have had only a faint tail and even that would have been pointed almost
directly at the Earth, making the comet appear like a large, dim, nebulous
patch of light in the sky.
The fact
that the Chinese give such a good description of what was not a spectacular
return of Comet Halley acquires additional significance given the paucity of
detail in the other Chinese records around the time of the Nativity.
The chronicle, the
“Ch'ien-han-shu” states that:
"In the second year of the period of Ch'ien-p'ing, second month, a hui-hsing appeared in Ch'ien-niu for more than 70 days"
This record can be translated as:
“During the interval between March 10th and April 7th of 5 BC, a comet
that was visible for more than 70 days appeared close to Alpha and Beta
Capricornii”
Despite the use of the term
“hui-hsing”, or “tailed comet”, the chronicle contains several elements that
are inconsistent with this object being a comet.
The chronicle gives a fixed
position over two and a half months, not reasonable if the object really was a
comet. Remember that the dim and relatively unspectacular appearance of Halley’s
Comet in 12BC was described in some detail, with the comet’s path in the sky
clearly described. The same chroniclers are, seven years later,
give almost no detail at all, despite the fact that this 5BC object was
supposedly a bright, tailed comet. Bright comets were usually described in some
detail in the oriental chronicles with the comet’s movement, tail length, form
and even sometimes even the comet’s colour described; here though we have none
of this information.
However, although the Chinese had
a special term, “ko-hsing”, or Guest stars, for such events their use of this
term was often inconsistent. Sometimes, “hui-hsing”
was used to describe bright novae such as Tycho’s Star. Such confusion was not
limited to
We must also take into account
that May marks the start of the monsoon season in China, although there are
wide regional variations across the country with the monsoon in general
starting earlier in the south. In
If we measure a 10 week interval
from the date of the first sighting the object would have been observed until
May 19th (assuming it appeared on March 10th) or June 16th
and well into the rainy season assuming that it appeared on April 7th. The
comment that the object was visible for “more than 70 days” hints strongly that
it was still clearly visible when lost to bad weather.
A second, but more controversial
record is found in the Korean “History of Three Kingdoms - the Chronicle of
Silla (Samguk Sagi)”
“Year 54 of Hyokkose Wang, second month, (day) Chi-yu, a po-hsing appeared in Ho-Ku”
Korean records from that epoch
are less detailed and far less reliable, a fact clearly betrayed by this
chronicle, as the date that is given was inexistent in the Chinese calendar. It
is as if the chronicler had unknowingly written February 30th. It is now believed
that "Chi-yu" really should be "I-yu", a character written
in an almost identical fashion in Chinese and easily confused with it. If so, this can be
translated as:
“On March 31st of 4 BC a bushy star appeared
close to Altair”
We thus have a Chinese object in the north of Capricorn in
March 5BC and a Korean object in the constellation of
However, this interpretation has several serious problems.
Why did the Chinese chroniclers not observe the Korean star of 4BC? Given that
Chinese chronicles are far more detailed and complete than their contemporaries
in
It also seems to be a quite remarkable coincidence for two objects to
appear in consecutive years in adjoining constellations in the same month and
for one to be seen only from
There is also though a relatively simple explanation for difference in
position. The Chinese constellation of Ho-Ku includes Altair, by far the
brightest star in this region of the sky and various stars from the south of
the constellation of
We thus have a probable position of the star in a circle of radius
approximately 5 degrees in southern
R.A. 18h30m, Dec. –12º (J2000)
This is between Theta Aquilae and Alpha Capricornii.
The
visibility of the Nova
It is now
widely assumed that the Magi were not Babylonian (as had been widely believed
previously, although not based on any solid evidence), but instead Zoroastrian
priests from northern
First we
will calculate its visibility from
·
Astronomical,
when the upper limb of the Sun is 18º below the horizon. This is the moment
when the horizon first begins to brighten although the zenith is dark.
·
Nautical,
when the upper limb of the Sun is 12º below the horizon. This is used by the
navy as the moment for dawn action stations and in the army for the dawn
stand-to. At this point the eastern horizon is bright and light is extending up
towards the zenith.
·
Civil,
when the upper limb of the Sun is 6º below the horizon. Better known to drivers
as “lighting up time”, this is when effectively the transition from day to
night occurs.
The local
time of twilight in
|
|
Nautical twilight |
Astronomical twilight |
|
March 1st
5BC |
|
|
|
March
15th 5BC |
|
|
|
April 1st
5BC |
|
|
The (very
approximate) hypothetical position of the nova at
The nova
would have appeared low in the east or southeast in the pre-dawn sky. The
earlier the date that the nova appeared and the further to the south that it
was located, the later it would rise and the lower it would be in the sky at
dawn.
If it
appeared close to the star SAO 144144, dimly visible near the centre of the
circle as a magnitude 6.5 star, its circumstances would have been:
|
Date |
Astronomical twilight |
Altitude |
Azimuth |
Time of rising |
|
March 1st 5BC |
|
28º |
133º |
|
|
March 15th 5BC |
|
33º |
142º |
|
|
April 1st 5BC |
|
37º |
152º |
|
In
contrast, had it appeared alongside Alpha Capricornii, it would have been
significantly lower in the sky and would have risen a quarter of an hour later.
|
Date |
Astronomical twilight |
Altitude |
Azimuth |
Time of rising |
|
March 1st 5BC |
|
24º |
135º |
|
|
March 15th 5BC |
|
29º |
143º |
|
|
April 1st 5BC |
|
33º |
153º |
|
From
Medea, in the north of
However,
this cannot explain a much-commented problem with the nova: unless the nova
appeared at least a month earlier than the Ch'ien-han-shu
suggests, it is hard to square its position in the sky
as a heliacal rising according to the modern translation of Matthew. The nova
would have been seen in the east in the pre-dawn sky, but would have reached a
significant altitude by that time.
One possibility is that Matthew is being over-interpreted and that
he intended to say that the Magi had first seen the Star in the first light of
dawn. However, there is another possibility. The months from December to March
are rainy season in
Visibility
from
We assume
that the Magi took anything from
A star
rises 2 hours earlier each month as a result of the change of perspective
caused by the Earth’s orbital motion. Nova
At
astronomical twilight, as seen from
|
Date |
Local time |
Altitude |
Azimuth |
|
30/04/5BC |
|
46º |
176º |
|
15/05/5BC |
|
45º |
190º |
So,
effectively, had the Magi headed for Bethlehem as the first light of dawn broke
on the eastern horizon at any time in the first half of May, the nova would
have been almost exactly due south and ahead of them on the road as we see in
the following figure.
Other
factors affecting visibility
The
factors that affect the visibility of a nova such as Nova Bethlehem are
various:
·
The
nova’s peak brightness
·
Its
rate of decline
·
The
phase of the Moon
·
Lunar
conjunctions
Attempts
have made to calculate the peak brightness of the nova based on its period of
visibility. These have no scientific validity as the rate of decline of novae
is highly variable from object to object. Astronomers generally define a fast
nova as any that declines from maximum by 3 magnitudes or more in 50 days,
whereas a slow nova takes longer than 50 days to decline by 3 magnitudes from
maximum.
Even
within the fast novae the range of behaviour is enormous. Nova Herculis 1991
(V838 Herculis) had the fastest rate of decline of any known nova falling 3
magnitudes in just 2.8 days. Nova Persei 1901 (GK Persei) was magnitude +0.2 at
maximum, but declined beyond naked eye visibility in just 2 weeks. Nova Aquilae
1918 (V603 Aquilae) was the brightest recorded nova, with its peak magnitude of
–1.8; it was a fast nova, dropping six magnitudes in three weeks, but it then
took a further 8 months to drop below naked-eye visibility.
In other
words, apart from guessing that the widespread visibility of the nova means
that it must have been quite bright, we can make no other statement about it,
other than it was almost certainly a fast nova. There is a relationship between
the luminosity of a nova and the time taken to decline the first two magnitudes
from maximum, but we need to know the brightness and distance of the nova to
use it, which we most obviously do not, so this relationship is no help
whatsoever.
Much has
been made of the fact that King Herod was unaware of the Star. In fact, there
is no mystery in this either. Herod was an old man of 68 in an age when the
life expectancy was far less than this; he was infirm and soon to die and was
facing the growing threat of the split of his kingdom on his death (Judea was
duly divided between his three sons on his death and they duly rules it so
badly that Caesar Augustus was obliged to take over direct rule of the
province, in part at the petition of its disgruntled citizens). Had Herod taken
to observing the night sky before dawn in winter it would only have hastened
further his soon to arrive death.
Similarly,
we know that despite their high level of culture, there are very few Roman
records of comets, unless exceptionally brilliant, and few Roman astronomical
records of any kind, so it is not particularly usual that in a region subject
to such strong Roman influence a moderately bright, or even a bright nova
should go almost unnoticed and uncommented.
However,
even for assiduous sky watchers, other factors could also severely affect the
nova’s visibility. It appeared at a low ecliptic
latitude; this means that apart from the days around Full Moon when moonlight
drowns out all but the brightest stars and planets in the sky, once a month the
Moon would also have passed close to the nova.
The dates
of Full Moon (poor visibility) and New Moon (excellent visibility) would have
been:
|
New Moon |
Full Moon |
|
March 8th 5BC |
March 23rd 5BC |
|
April 6th 5BC |
April 22nd 5BC |
|
May 6th 5BC |
May 21st 5BC |
Note that
after Full Moon an object only visible in the morning sky would be even more
seriously affected as the waning Moon would come closer to it night by night.
Conditions
for discovery of the nova would thus have been best either at the start or at
the end of the interval given in the Ch'ien-han-shu, that is, early March
or early April. Interestingly, the Korean chronicle gives a specific date for
its first observation of March 31st 5BC, but this date coincides
with the nova’s conjunction with the waning quarter Moon, which would have been
no more than 15º away in the sky at the time. Unless the nova was still
relatively bright when the conjunction occurred, moonlight would have been a
severe impediment to its observation. This Korean date seems unlikely to have
been a coincidence; rather than being the true date of the first observation it
seems more likely to have been given as a significant date of observation due
to the conjunction. It also explains why the Koreans gave Altair as the
reference for the position, as the presence of the bright Moon in Capricorn
would have made the rather faint nearby stars of Capricorn and Aquarius totally
invisible on this date; only Altair would have been visible through the Moon’s
glare.
On dates
when the Moon passed through Capricorn and thus passed close to the nova its
visibility could be seriously affected according to the Moon’s phase and thus
brightness. When we look at these dates we notice an important effect:
|
Date of lunar
conjunction |
Moon’s phase |
Moon’s age |
|
March 2nd-3rd 5BC |
25% |
24 days |
|
March 30th-31st 5BC |
50% |
22 days |
|
April 26th-27th 5BC |
69% |
20 days |
|
May 23rd-24th 5BC |
90% |
18 days |
Over the
period of visibility of the nova the Moon’s phase would have got progressively
larger at each monthly conjunction and thus Moon interference would have been
correspondingly more severe. In late May, with the nova fading severely, the
Moon would have been just past full at conjunction, thus the nova might well
have been invisible due to moonlight for a week and a half in total. Even in late
April the waxing gibbous Moon would have been a severe impediment to observe
the nova if, by then, it was third magnitude or fainter. Between Full Moon,
lunar conjunction and possibly the odd cloudy night it is again quite plausible
that the nova could have been invisible visually for 8-10 days. However, by May
1st, the Moon would have waned sufficiently and moved sufficiently
far away make the nova easy to see again with the naked eye. This provides a
simple, natural explanation why the Magi may have lost sight of the Star for
what was, for them, a worryingly long time before recovering it as they left
Conclusions
All the known
aspects of the observations of the Star of Bethlehem can be understood in a
simple fashion if it was a simple bright nova observed in northern Capricorn or
southern