Solstices

[Lothia]

Ave Civitas,

When was Winter Solstice observed in ancient Rome?

In my novels, this one set in AD 395, I have run across a problem.

Christmas was established as 25 December in AD 336. [Note: In an old list of Roman bishops, compiled in A. D. 354 these words appear for A.D. 336: "25 Dec.: natus Christus in Betleem Judeae." December 25th, Christ born in Bethlehem, Judea. This day, December 25, 336, is the first recorded celebration of Christmas.] So there is no argument with me on when Christmas was.

However, the Romans at that time were using the Julian Calendar, and there is a thirteen day difference between the two days celebrating Christmas.

Today the Orthodox Church celebrates Christmas on the Julian Calendar's December 25, which is January 7th on the Gregorian Calendar that the Western world uses today.

I am fine with that. If everyone back then was using the same calendar, the Julian, then the 25th is the 25th.

Howsoever, neither Julian Caesar nor Pope Gregory had any control over the celestrial movement of planets.

On the Gregorian Calendar the Winter Solstice happens in December 20th or 21st. I am fine with that too.

But, when was the Solstice marked on the Julian Calendar? Would it have been December 12th or 13th?

And... would that have made the Summer Solstice June 13th?

Hmmm.

[John W Davison]

Hi Tom,

The difference between the two is continually moving (albeit slowly).  The actual length of the solar year is c365.24 days.  The Julian calendar has 365.25 days, so there is a gradual drift out of alignment with the solar year (i.e. every fourth year is a leap year so the Julian year is too long).

The Gregorian calendar corrects this by the omission of a leap year when the year is divisible by 100 EXCEPT when the year is divisible by 400.  This sounds complex but the average year length can be easily computed by considering a 400 year period.  In such a period (say 1601 to 2000) there will be three years which are non-leap years when comparing to the Julian system (i.e. 1700, 1800 and 1900 are NOT leap years).  Therefore there are only 97 leap years and not 100 as in the Julian in this 400 year period.

So, average day length becomes (365 x 400 + 97 x 1 leap days)/400 = 365.2425.

For the Julian calendar, average year length is 365.25 days.  In other words, the two drift further out of alignment by 3 days every 400 years.

When Caesar reformed the calendar, extra months were inserted to realign the calendar with the seasons so at that point it was astronomically "correct" (if late December is mid winter and late June is mid summer in the northern hemisphere, i.e. where Italy is located).

Therefore, 2000 years ago, both the Julian and Gregorian were just about in alignment.

The equinoxes (equinoces?) and solstices are not really days as such but are the day on which a specific momentary event occurs.  The equinox this week was actually at 20.20 BST 22nd Sept, the moment when the axis of the earth (the north-south polar axis of rotation) tipped so that the north pole started to be aligned away from the sun and the south pole started to be aligned towards the sun, so that the northern hemisphere receives less daylight than the southern from 22 September until some time in late March 2022 (21st or thereabouts).  I guess the ancients had darker skies and brains just as big as ours so could observe these moments so I think the answer is that the winter solstice Julian calendar is very close to 21st December.

(I knew that orbital mechanics module at university would come in useful some day.)

Regards, John

[Lothia]

Thanks for the reply. Very educational. Glad you took that course.

[Hanny]

Romans celebration were to Mark Sol Invictus, this was a sun cult the Emperor was associated with and gave a public holiday for as it being part of the Imperial cult, this was the same day Mithras followers had it’s protection of Empire day, it also being Mithras birth day ,( as opposed to the brumalia shortest day) but the pre Julian Callander was out of sequence over time, so Caesar moved it to the 25th in his reforms, but it was still out of step over time.It would not be the 25th in modern Callander as there are different numbers of days in each.You can calculate how many days out it will in your book by starting at Julian Callander adoption date and your books date, and how many days out it will be by the.


Second Cent AD a a Roman Africanus, worked out Christ was born on the 25th, there being nothing in scripture to give anyone that date, so early Christians just went along with the idea, calling it the birth of the son of justice.St Augustine was anti using this Roman date for the birth of Christ.

[Lothia]

Thank you. I will see how I can work that in.

[Hanny]

Try a net search for sol invictus and Christmas at university of Chicago, you might find some things for 386 you can use.

[Lothia]

Again, thanks.