1. Doppler Shift:
* Concept: Similar to the changing pitch of a siren as it passes by, light from objects moving towards us is "compressed" (blueshifted), and light from objects moving away from us is "stretched" (redshifted).
* Observables:
* Spectral lines: Astronomers analyze the light from distant stars and galaxies. The spectral lines (unique patterns in the light) will be shifted towards the blue end of the spectrum for objects moving towards us and towards the red end for objects moving away.
* Radial velocity: The amount of shift in the spectral lines directly corresponds to the speed of the object's motion towards or away from Earth. This is how we measure the radial velocity of stars and galaxies.
2. Proper Motion:
* Concept: This refers to the actual movement of a celestial object across the sky as seen from Earth. It's like watching a star slowly crawl across a celestial canvas over long periods.
* Observables:
* Star positions: Astronomers carefully chart the positions of stars over many years. Changes in these positions reveal the object's proper motion.
* Parallax: The apparent shift in a star's position against the background of distant stars as Earth orbits the Sun provides information about both the star's distance and its proper motion.
Key Point: These two effects are complementary. Doppler shift reveals the speed of an object's motion towards or away from us, while proper motion reveals the object's movement across the sky. Together, they provide a complete picture of a celestial object's motion through space as observed from Earth.
