Motions of the Planets (“Wanderers”) Planets move on celestial sphere - change RA, Dec each night - five are visible to naked eye Mercury, Venus, Mars, Jupiter, Saturn www.universetoday.com Days of the week: - named after 7 “Power” (moving) objects in the sky (Sun, Moon, 5 planets) English Spanish French Roman/Latin Origin Germanic/ Norse Origin Sunday Sunday Sunday Sunday Domingo Dimanche Sun Sun Monday Monday Monday Monday Lunes Lundi Luna (Moon) Moon Tuesday Tuesday Tuesday Tuesday Martes Mardi Mars Tyr Wednesday Wednesday Wednesday Wednesday Miercoles Mercredi Mercury Wodan Thursday Thursday Thursday Thursday Jueves Jeudi Jupiter (Jove) Thor Friday Friday Friday Friday Viernes Vendredi Venus Frigg Saturday Saturday Saturday Saturday Sabado Samedi Saturn Saturn Motion of the planets: - move near ecliptic (through zodiac) - most of the time, move eastward → “Prograde Motion” - occasionally: change direction and move westward → “Retrograde Motion” http://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.html Models of the Universe: Aristotle: philosophy of “Idealism” (∼ 2000 years ago) - circles and spheres are ideal shapes → structure of universe based on circles and spheres Geocentric Cosmology - Earth is at center, stationary - Sun, Moon, 5 known planets → revolve around Earth on concentric circles - Earth at center of rotating Celestial Sphere
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Motions of the Planets Days of the week: (“ Wanderers ...sites.pc.gsu.edu/wlahaise/files/2015/10/Lec-History4-oc4k3m.pdf · - circles and spheres are ideal shapes ... - planets
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Motions of the Planets (“Wanderers”)
Planets move on celestial sphere - change RA, Dec each night - five are visible to naked eye
Mercury, Venus, Mars, Jupiter, Saturn
www.universetoday.com
Days of the week:
- named after 7 “Power” (moving) objects in the sky (Sun, Moon, 5 planets)
Observations cannotcannotcannotcannot be explained by Geocentric Cosmology!
Isaac Newton (1642 - 1727) - described what causes motion
→ FORCE - a “force” is a push or a pull
Newton’s Three Laws of Motion:Newton’s Three Laws of Motion:Newton’s Three Laws of Motion:Newton’s Three Laws of Motion:
First Law (Law of Inertia)First Law (Law of Inertia)First Law (Law of Inertia)First Law (Law of Inertia)
if no net force, an object: - will be at rest - or moves at a constant speed, in straight line, forever Second LawSecond LawSecond LawSecond Law (F = ma)F = ma)F = ma)F = ma)
If a net force acts on an object: - it changes its motion (accelerates)
- speeds up, slows down, or turns - depends on mass of object - more mass, harder to accelerate
Third Law (“Third Law (“Third Law (“Third Law (“actionactionactionaction----reaction”reaction”reaction”reaction”)))) If two objects apply forces on each other:
- the forces are exactly the same strength, but opposite in direction
Planets move on elliptical orbits - change direction, speed
� due to force on planets! Newton: Universal Law of Gravity
FG = G m1 m2
r2
G - gravitational constant = 6.67 x 10-11 m3/kg s2
- proportional to masses (m1, m2) - inversely proportional to square of distance (r)
Mass versus Weight Mass - amount of matter Weight - force of gravity on matter If you weighed 150 lbs on Earth: Mass Weight Sun 68 kg 4000 lbs Jupiter 68 kg 400 lbs Mars 68 kg 60 lbs Moon 68 kg 25 lbs Asteroid 68 kg 1/2 oz.