# Why is ancient astronomy right?

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mfb
Mentor
You won't see that much with the naked eye however.

davenn and 1oldman2
sophiecentaur
Gold Member
Technically, though, the ancients must have been really on the ball. Distances had to be done with trigonometry - as with a rangefinder and with a maximum baseline of hundreds of km. As far as I can estimate, this would need measurement of the difference in angle of two views of, say, the Sun against the 'fixed' stars from two terrestrial sites. A baseline of 150km and the Sun's distance being 150 X106 km would give a difference in angle of 10-6 radians, which is less than 1" of arc. Good stuff, that you would find difficult to do with stuff out of your garage. (No Scientific Equipment supply shops in those days.)
EDIT: The Sun was a daft example, of course. You can't see any fixed stars when the Sun's out. But you could do it with Mars or Venus.

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mfb
Mentor
The distance of Moon got estimated via triangulation with a somewhat reasonable accuracy. The sun is too far away for that. In principle you can use the Earth/Moon baseline if you estimate the fraction of the visible part of the moon that gets sunshine (it is a bit more than 50% if the sun is just at the horizon for you and moon is directly above you), but that method needs telescopes and ideally cameras to be reliable. The speed of light plus precise timing of Jupiter moons and later previously Venus transits (for triangulation) gave more reliable estimates around ~1650-1750.

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sophiecentaur
sophiecentaur
Gold Member
Interesting and it resolves my problem with such tiny angles - but they didn't have telescopes and the early instruments were not too good.
They must have used many different measurements of as many different astronomical objects as they could. Their advantage was that they had a lot of time to play with.
Once they had the Sun's distance, they could invent the Astronomical Unit and could use parallax for finding some of the bigger distances from different places in the Earths orbit. An impressive network of information and measurements which pulled itself by its bootstraps from relatively small distances to larger ones.
I was gobsmacked when I learned how the oscillation rates of the Cepheid variables would tell you the absolute magnitude and that would give you the distance of amy variable you saw from its relative magnitude. But that was all relatively recent. It wasn't until the 1920s (iirc) that other galaxies were recognised as being outside of our own. That's nearly in my lifetime - at least my Dad's lifetime.

mfb
Mentor
Once they had the Sun's distance
They did not have that. Here is a summary table. All the Greek estimates were not better than guesses. Wendelin (1635, Earth/Moon baseline triangulation) and Horrocks (1639, size of Venus during transit) were the first estimates that were somewhat reasonable - both got the distance wrong by a factor 2.

Huygens got a good estimate in 1659, but probably more based on luck than on actual precision.

Cassini and Richter used the Mars parallax in 1672, their value was just 10% off. The venus transits in 1761 and 1769 then allowed to narrow down the distance to sun to ~2% uncertainty, and the next set of transits 1874 and 1882 together with abberation and the known speed of light brought it down to 0.1% uncertainty.

Modern radio astronomy allows to measure the distance with a precision of meters.

QuantumQuest
jim mcnamara
Mentor
There are cell phone apps for this. If you live in a big city there will be only a few nights every year that will allow you to see much. And you may have to get out of town. These apps point you in the correct direction and help you locate planets and other objects like the Andromeda nebula.

Sky Free is one for iPhone, for example. Here is a list of free apps:
http://heavy.com/tech/2015/09/top-5-best-free-astronomy-space-stars-apps-for-iphone-android/

1oldman2
Mars and Betelgeuse were visibly red, not so much now.

Lucivaldo said: How come i haven't been capable of pointing out any planets on my own? and the only one planet that i was pointed to looked allot like a normal star in the same white seeming color. What colors do you see the planets you pointed out in?

To the naked eye all the planets look just like bright stars. The ancients noticed they moved in relation to the "fixed" stars. The word planet comes from the greek word for "wanderer." They move slowly enough that you don't notice it unless you are paying close attention. The color difference can be rather subtle too. Venus is definitely whiter than Saturn and Jupiter and Mars does have a reddish cast.

JCMacaw and russ_watters
Lucivaldo: Perhaps you think the planets look like, well, like the planets we are shown on photos from telescopes, ie; Jupiter with a red spot, Saturn with rings, etc, etc., but, no, the planets all look basically the same as the stars. There are a few slight differences, however. Mars is noticeably redder, Venus is quite bright and so is Jupiter (depending on their distance from us, of course) and they move very slowly across the sky. You need to watch them and track them for several weeks in order to notice any apparent movement. Also, don't forget, the ancients watched and kept very close track of the stars daily for thousands of years. Their movement was extremely important to their religions and their religions were often passed down to other empires. The stars were first observed by the Sumerians and their charts and observations were later seamlessly adopted by the Akkadians, Egyptians and Babylonians among many others. The movements of the stars, planets, the moon and the sun was so important to these peoples' religions that their kings would be executed according to what phase and position the moon was in for example (until the kings finally smartened up). The rising of the star Sirius usually coincided with the annual flooding of the Nile River and when, because of the precession of the equinoxes (look it up), it failed to rise on time eventually, it caused complete havoc and revolution among the Egyptians because their whole religion needed to be rewritten. So, like the man says, "Keep on looking up."

Okay then, this is a pretty good example.

PeroK
Here is mercury, a good example of a "star" wandering.

PeroK and Drakkith
sophiecentaur
Gold Member
Here is mercury, a good example of a "star" wandering.
View attachment 103930
Nice picture but it is not 'as seen'. The clouds would have moved much more than Mercury during the time for the multiple exposures.
I am still having to come to terms with the difference between the astrophotographs that get published and what you actually see. Lots of scope for artistic expression - even if it's just the false colour that's used to accentuate the features of astronomical objects.

1oldman2
Nice picture but it is not 'as seen'. The clouds would have moved much more than Mercury during the time for the multiple exposures.
I am still having to come to terms with the difference between the astrophotographs that get published and what you actually see. Lots of scope for artistic expression - even if it's just the false colour that's used to accentuate the features of astronomical objects.
Very good point, thanks.

Let's be honest...Why were planets visible to the ancient people and not to us humans any longer? Is it because the planets distanced? How can some planets distance themselves while others don't? It's said that light pollution has affected the visibility of planets in the sky, whereas there still reports of people seeing at least one planet in the sky. So how come ancient astrology was on right before the telescopes?