Constellations and Earth's orbit.

[Daecon]

Why don't the constellations change every six months?

 

Surely at one part of the year the stars you would see overhead are in the exact opposite direction to the starts you would see 6 months difference, because those same stars would be in daytime and the night sky is facing the opposite way.

[swansont]

Why don't the constellations change every six months?

 

Surely at one part of the year the stars you would see overhead are in the exact opposite direction to the starts you would see 6 months difference' date=' because those same stars would be in daytime and the night sky is facing the opposite way.[/quote']

 

The constellations don't change every six months because they change more often than that. The rotation is continual, and the arbitrary divisions mean that they change monthly.

[Sisyphus]

Why don't the constellations change every six months?

 

Surely at one part of the year the stars you would see overhead are in the exact opposite direction to the starts you would see 6 months difference' date=' because those same stars would be in daytime and the night sky is facing the opposite way.[/quote']

 

Well... they do......

 

Of course, it's not like the stars that were right overhead are directly beneath you, unless you live on the equator. They "rotate" about the Earth's axis, so, for example, Polaris is always in the same place, and every other star looks like it's rotating around it. If you keep track of them, it takes about 23 hours and 56 minutes for one full rotation. That is, 24 hours for the Earth's rotation about its axis minus about 4 minutes for the Earth's revolution around the Sun. It therefore takes a full year for the same star to be back at the same place at the same time of night.

[Daecon]

So Polaris is at an exact right-angle to the line of Earth's orbit around the sun?

              [b].[/b] (Polaris)
              i
              i 
              i
(Earth)[b]o[/b]       [b]O[/b]       [b]o[/b](Earth)
            (Sun)    

What about when you do observe stars directly overhead at midnight on the Equator? There's no room for rotation there...

[swansont]

So Polaris is at an exact right-angle to the line of Earth's orbit around the sun?

              [b].[/b] (Polaris)
              i
              i 
              i
(Earth)[b]o[/b]       [b]O[/b]       [b]o[/b](Earth)
            (Sun)    

What about when you do observe stars directly overhead at midnight on the Equator? There's no room for rotation there...

 

No, Polaris is just very close to being on the rotational axis, which is tilted at ~23.5 degrees to the orbit. Due to precession, this angle changes over time.

 

An observer on the equator sees rotation for overhead constellations. Polaris is the only star that appears ~stationary, all others appear to rotate.