👀 We see your comments about a zodiac story that re-emerges every few years. No, we did not change the zodiac.
— NASA (@NASA) July 17, 2020
When the Babylonians invented the constellations 3,000 years ago, they chose to leave out a 13th sign. So, we did the math: https://t.co/DQOs5VSjT7 pic.twitter.com/WlblguobGT
July 17, 2020
NASA wants you to know it did not change the Zodiac.
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11 comments:
It's 2020. Anything weird is possible.
While they're at it, they can tell us all the accomplishments of the Muslim Outreach Program.
So, we did the math
13-1 = 12.
"The Babylonians lived over 3,000 years ago. They divided the zodiac into 12 equal parts – like cutting a pizza into 12 equal slices."
Who knew the Babylonians had pizza? Thanks, NASA!
Babylonians had a base 60 number system. 12 is a factor of 60. 13 isn't.
Though it is true the ecliptic goes through more than 12 constellations, but if you can see a Ram in the stars of Aries I don't think a little thing like that is going to trouble you.
There are a little over 13 sidereal months in a year. A sidereal month is 27.3 days long. That's the time it takes for the moon to return to the same point on the celestial sphere with respect to the Earth.
Or another way to say that is that it takes 27.3 days for the moon to return to the same point in the zodiac.
Of course the Babylonians knew that and that makes it especially puzzling why they didn't have a thirteen month year that would go with their thirteen zodiacs.
But when you think about it the zodiacs are arbitrary. There is no necessity that there be thirteen. The fact that there are thirteen is a clue that the Babylonians originally had a thirteen month year calendar.
It wouldn't be surprising if they did since so many ancient civilizations had a calendar based on the sidereal month. China still has it today.
Dropping the 13th constellation is evidence that the Babylonians decided a twelve month year made more sense than a thirteen month year. And that's actually why we have a twelve month calendar. Because our calendar ultimately comes from the Babylonians.
So why did the Babylonians shift to a 12 month year?
The 12 month year is based on the synodic month, which is 29.5 days long. The synodic month is the time it takes for the moon to return to an imaginary straight line joining the earth and the sun. It's a very sophisticated observation that almost implies knowing the earth is moving in a circle around the sun. I say almost because you can calculate the same number of days if you assume the sun is moving around the earth which is how the Babylonians perceived it. Either way one has to be very aware of what is going in the sky overhead to even observe this.
The synodic month is thus a complicated concept compared to the sidereal month which anyone can observe simply by counting the number of days from one new moon to the next.
This is the major advantage of the sidereal month. If you are used to thinking in terms of it, you can just look up at the moon at night and basically see what day of the month it is. It's like there is a giant clock in the sky.
But the synodic month and it's twelve month has one major advantage over the sidereal months and that is that it is good at telling time with respect to the whole year.
Sidereal months shift quite a lot from year to year because 27.3 days don't divide very well into the 356 days it takes for the earth to go around the sun. But the synodic months divide almost perfectly.
Thus a farmer in a synodic month civilization will know that he is supposed to plant a specific crop on May 9th. But the farmer in the sidereal month civilization never really knows, unless someone who can do math tells him, when is the best time to plant or harvest.
So this whole business of the 13th Zodiac is evidence that the Babylonians shifted from a thirteen month to a twelve month year, and we can make a pretty good guess as to why.
Correction: 365 days.
Thank you, mandrewa!
We learned about this at Donaldson Observatory, Ft Davis, TX, last yr.
mandrewa-"The synodic month is the time it takes for the moon to return to an imaginary straight line joining the earth and the sun." True, which means the synodic month is the period of the Moon's phases (New Moon to New Moon, say), which is how it is determined. Lunar calendars are based on the synodic month
Thanks, Unknown. I made a mistake there.
Correcting, the synodic month is not difficult to calculate. It is just the time from one new moon to the next.
And that raises the question of why we don't line up our calendars with the Moon's phases?
And then I look at a calendar and realize hey, it is kind of lined up!
The sidereal moon is more difficult to calculate because it means observing the movements of the Moon with respect to the celestial hemisphere. Or in other words it takes a sidereal month for the Moon to return to its starting point as it moves across the zodiac.
So how does this change my narrative?
The synodic month, with slight annual corrections, still gives the farmer a consistent time to plant. And even without corrections, the time to plant is going to change only slightly during a person's lifetime.
The farmer living in a sidereal month civilization is always confused about the time to plant unless there is a priesthood to tell him when. And this is why Chinese calendars so often come with planting dates on them.
A person living in a sidereal month civilization can look at the sky at the same time in the evening and assuming that they know their zodiac, which most people today don't, can tell the day of the month.
A person living in a synodic month civilization will know that a month is approximately a lunar phase cycle long. Unfortunately it's far enough off because of the needed adjustments to stay on the year that the phases shift about a day every month, which means that practically speaking you can't use the sky to tell the day of the month.
So my basic narrative is correct, and it happened partly by accident. The sidereal calendar lets you read off the day of the month by looking at the sky. The synodic calendar gives you the correct time of the year so if you have a calendar you know where you are with respect to the year.
The Babylonians probably shifted from a thirteen month (sidereal) calendar to a twelve month (synodic) calendar.
Finally I'll note that the Chinese nominally have twelve month calendar even though it's sidereal. But that's not really true. They are just refusing to name the thirteenth month. Most years in the Chinese calendar there will be two duplicate months. For instance for some years there will be two Augusts. I'm not sure I understand the logic of this but it is probably something to do with trying to compromise between the synodic and sidereal calendar.
Ok, correction of correction. The irrationality of the traditional Chinese calendar not naming the thirteenth month weighed upon me and so I looked it up.
And guess what? The traditional Chinese calendar is based on the synodic month, not the sidereal month.
The difference between the Western and the traditional Chinese calendar is that the Chinese calendar takes the synodic month quite literally. The Western calendar is only approximately based on the synodic month, since days are added to the months so that every year is exactly twelve months long and so that May 9th for instance is always, within a day or two, the same time of the year with respect to the seasons.
In the Chinese calendar every month is lined up with the phases of the moon. If you look up at the moon and see it's phase you literally know what day of the month it is.
The Chinese calendar also adjusts but it's adjustment is pretty coarse since it consists of adding a duplicate month every three years or so.
My basic narrative is still, by some accident, correct. The advantage of the Chinese calendar is that you can tell the day of the month by seeing the Moon. The advantage of the Western calendar is that you know where you are in the year by knowing the date.
An advantage of a sidereal calendar would be that you could read off the day of the month if you could see the Moon at the same time of the day and you knew your constellations. Except no one today uses a sidereal calendar, although allegedly there were past civilizations that did, including possibly the Babylonians or the culture that preceded the Babylonians as evidenced by the 13th zodiac.
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