Observing the sky Flashcards
Celestial equator ( CE )
- Extending Earth’s equator onto the celestial sphere
- Stars move parallel to it
North and south celestial poles
(NCP and SCP )
- Extend Earth’s rotational axis, we call the points at which it intersects the celestial sphere
- Celestial objects rotate around the celestial poles
Celestial sphere (CS)
Celestial objects as distributed on the inside of a huge celestial sphere (CS) that has its centre concentric with Earth’s centre
Explain north pole observer
- Standing at the north pole, the NCP is directly overhead and the CE on the horizon
- The stars will move in circles centred on your zenith, and will never set
Explain equitorial observer
Standing on Earth’s equator, the CE will pass overhead through your zenith, and NCP and SCP will lie on the horizon. Stars will rise in the east, move straight up, then straight down to set in the west
Circumpolar zone
Stars within 34° of the NCP will never set
***not in this zone but Stars within 34° of the SCP will never rise
Meridian
- Great arc running from the NCP to the SCP and passing through the zenith
- A star’s angle relative to the local horizon is highest when it crosses the meridian
Zodiac
The 18°-wide belt centred on the ecliptic within which the planets, Moon, and the Sun are found
Explain astronomical parallax
- A ristarchus of Samos (310 - 230 BCE) suggested the Earth was moving around the Sun.
- His idea was rejected due to the expectation of being able to observe the shifting of nearby stars relative to the
background stars as the Earth moves in its orbit. - This does happen! The effect is known as astronomical parallax. However, the effect is so small, mankind had to wait for thousands of years until large telescopes could observe the effect.
Retrograde motion
Is the apparent westward at the correct speed and for the correct interval of time, thus replicating ret westward movement of a planet caused by Earth model moving between it and the Sun
Epicycle
- Each planets moves on a small orbit called an epicycle
- The epicycle moves around Earth on a circular path called a deferent
- By choosing the correct combinations of speeds and sizes (radii) of epicycles, Ptolemy’s model could replicate the retrograde motion of planets
Ptolemy
- Produced a geometric representation of the solar system that predicted the positions of the planets for any desired date and time
- Retrograde motion
- all the planets (and the Sun) move around Earth on circular orbit
Copernicus
- Heliocentric, model of the solar system
- Using this concept, he was able to work out the correct general picture of the solar system. He placed the planets, starting nearest the Sun, in the correct order: Mercury, Venus, Earth, Mars, Jupiter, and Saturn.
- Further, he deduced that the nearer a planet is to the Sun, the greater its orbital speed
- He was able to explain the complex retrograde motions of the planets without epicycles and to work out a roughly correct scale for the solar system
