eli5: If the sun revolves around the galaxy, why do we still see the same constellation that was discovered by the Romans (probably 1000s of years ago). surely they should have been scattered by now due to revolution of the sun combined with the revolution of the earth around with sun


Thnx to all, for the answer. I had a good time discussing and clearing my doubt.

In: 179

Distances in the universe are incredibly huge, so that even the great speed at wich stars move around the galaxy is extremely slow in comparision.

One “galactic year” is 230 million earth years. The last time the sun was in this position was before the first dinosaurs.

Those 5000 years humans have mapped stars are just the blink of an eye on that timescale. Stars changed positions, but not noticable for us.

All the stars that we can see in the sky are relatively close by, within about a hundred light years (the galaxy is a hundred _thousand_ light years across). This local group of stars travels around the center of the galaxy at about the same speed, so the speed of the Sun relative to the nearby stars is low, compared with the distance to these stars. Also distances in space are very, very large. Unimaginably large. You literally have no idea how big interstellar space is.

that’s a bit like asking “why hasn’t the season changed in the last ten minutes even though the earth has been orbiting the sun for that whole time?”

the sheer scale of the galaxy is incomprehensibly large. 5000 years is nothing on that scale, the stars have all barely had a chance to move

Each rotation around the sun has a small effect on stats, it is used to measure distance, it is called [https://en.wikipedia.org/wiki/Stellar_parallax](https://en.wikipedia.org/wiki/Stellar_parallax)

The effect is tiny because earth orbit around the sun is small compared to the distance to the start. The closes star Alpha Centauri is 138,000 times the orbital diameter it has a parallax of 750mas. 1 mas =1 Milliarcsecond = 1/3600000=0.00000027 degrees so 750 mas =0.0002 degrees. You need to take images and compare it to stars faster away to see it.

The sun orbits the galactic core but so do the other star. We are around 26,000 lightyears from the galactic center and an orbit take 230 million years. So we have moved less then1/125000 of a revolution in 200 years. One revolution ago was before dinosaurs emerged on earth, earth have obit less than 11 times since it was founded.

Orbital time depend on the distance so object close to the sun will orbit at quite a similar speed. All start you can see with you naked eye are with 4,000 light years.

If you look at the 300 brightest start the median is 185 light year with the distribution below

* <10 light years = 2 stars (Sirius and Rigel)
* 10-100 light years = 79 stars
* 101-1000 light years = 200 stars
* 1000 light years = 19 stars

The result is a consolation change but quite slowly. It is called https://en.wikipedia.org/wiki/Proper_motion and is measured. most start motion is less the 10 mas per year so it is not a lot.

Barnard’s Star moves faster at 10,000 mas per year it is 6 light years from us but it is to dim to see with your naked eye.

61 Cygni A is the faster visible at 5281 mas per year but in 2000 it is still just 3 degrees. It is not a bright start but visible.

The brightest star Sirius moves at 461mas/year is 0.25 degrees in 2000 years

So constellation does change but it is quite slow.

The signs of the zodiac and their applicable dates have changed since they were set around 1000 years ago:


|Star sign|Was From|Was Days|Now From|Now Days|
|Aries|Mar 21|30|Apr 19|25|
|Taurus|Apr 20|31|May 14|37|
|Gemini|May 21|31|Jun 20|31|
|Cancer|Jun 21|32|Jul 21|20|
|Leo|Jul 23|31|Aug 10|37|
|Virgo|Aug 23|31|Sep 16|45|
|Libra|Sep 23|30|Oct 31|23|
|Scorpio|Oct 23|30|Nov 23|7|
|Ophiuchus|||Nov 30|18|
|Sagittarius|Nov 23|29|Dec 18|32|
|Capricorn|Dec 22|29|Jan 19|28|
|Aquarius|Jan 20|30|Feb 16|24|
|Pisces|Feb 19|30|Mar 12|38|

“Zodiac” is both an astrological **and** astronomical term. You probably know that the solar system is disc-shaped; if you extend that disc until it meets the next galaxy, then that galaxy is on the zodiac. *Which* galaxy is the one you would see during the day if the Sun didn’t exist, or the one which is visible in the southern night sky, but six months early (or late).