How can the color index give the star's temperature? What will ...

[gbarreiro]

How can the color index give the star's temperature?
 
  What will be an ideal response?

Question 2

How did Galileo's observations of Venus disprove the Ptolemaic model?
 
  What will be an ideal response?

[cegalasso]

Answer to Question 1

Color index requires observing the star's brightnesses (magnitudes) through blue and visual filters. By taking the difference between these two magnitudes, the color index is calculated. The color index depends directly on the temperature of the star; the brighter it is in the blue image, the hotter it is.

Answer to Question 2

Ptolemy said that the epicycles for both Venus and Mercury were centered on the Earth-Sun line, so they must always lie between us and the Sun, so would always appear as crescents in the telescope. Galileo found Venus went through the entire phase cycle, proving that she revolved completely around the Sun, as predicted by Copernicus in his heliocentric model.

[gbarreiro]

Thanks for the timely response, appreciate it

[LVPMS]

Gracias!