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I was reading on NASA's website about how the Babylonians came up with dividing the sky into different slices and picked a pattern of stars in each slice to represent it. This ties in to astrological signs, and that being born "under" a sign means that that particular constellation is aligned with the continuation of an imaginary line from earth to sun (and onwards out into space).

But here's what I don't get: when "the sun is in Scorpio", isn't the actual constellation "Scorpius" impossible to see? It would be "behind" the sun, right?

This seems backwards to me, but then again I'm not a Babylonian from 3000 years ago. Am I misunderstanding something, or is this really how it works? You're born "under Aries", yet Aries isn't even visible on the sky at that time?

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It is true that, for an observer standing on Earth, the daytime sky blocks out our ability to see the stars (with the exception that stars are visible during a total eclipse).

It is possible to get pretty close. In a different post, it was asked if the the center of the galaxy is only visible for 1/2 of the days of the year. That post is here: Is it true that we see the center of the milky-way for only half of the year? The center of the galaxy happens to be close to the ecliptic and located roughly between Scorpius and Sagittarius. In that post I worked out that it is possible to see that spot in the sky (which happens to be near the ecliptic) roughly 10 months out of the year.

The point of the above post is that if we pick a specific location (for example... that spot in the sky between Scorpius and Sagittarius) you cannot see directly at that spot during the daytime. But choose to observe merely a month earlier or a month later and you can see that spot either just prior to sunrise or just after sunset. Since ancient observers would certainly have been able to observe these constellation side-by-side during months when they are visible at night, it would have been easy to derive (based on the first visible constellation after sunset or the last visible constellation prior to sunrise) where the Sun is located on the current day -- and to do this without needing to see the stars that beyond the Sun's current location.

Also consider that there are roughly 365 days per year and 360° in a circle. Each day the Sun advances about 1° along the ecliptic. The ancients would have understood this as well... and would have been able to pretty accurately plot the current location of the Sun without actually being able to directly observe those stars located in the sky beyond the Sun's current location.

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Star constellations are not visible during the day, but they are at night. The sun entering a constellation means that it rises in a constellation, i.e. if you look at the night sky just before dawn at the place where the Sun will rise, you will see that constellation. Also cultures differ in what they consider the start of a new day. In Judaism, for instance, a new day begins at sundown. I'm not sure how the Babylonians reckoned the start of a new day, but it looks like they considered the constellation that dominates the eastern night sky before dawn to be leading for the whole day.

So, if you go to Babylon in November (it's 80 miles from Baghdad in Irak), will you see Scorpio precede the Sun at dawn? You would if the Earth spun perfectly straight, but instead the tops of the axis describe a little circle: this is called precession. So these days, in November, the sun is preceded by Virgo.

Compare these two screenshots from the app Stellarium, for the city of Baghdad in Iraq. The first one is from November 7, 1020 BC, the second one november 7, 2020 (yes, Stellarium allows time-travel).

Stellarium: sunrise at Baghdad November 7, -1020

Stellarium: after sunrise, Baghdad November 7, 2020

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