Unit 2: Scale of the Cosmos & Telescopes

venus transits

• venus stands between earth and sun

• rare: a pair 8 years apart every 121.5 and 110.5 yrs apart

• same situation reoccurs after 243 yrs

G = measuring the strength of gravity

G is small, gravity is weak

telescopes use…

lenses and mirrors to focus and collect light

rain/light collected is proportional to…

area of umbrella/mirror

refraction and reflection occur…

simultaneously

refraction

1. light bends when it passes from one material to another

2. light is slower in water/glass than in air

two types of telescopes

1. reflectors (mirrors)

2. refractors (lenses)

convex and concave lenses

magnification

1. determined by focal lengths of lenses

2. M = fobjective/feyepiece

does magnification = resolution?

no

chromatic abberation

1. different focus for different color light

2. image is distorted and has colored edges

3. effect is diminished for long focal lengths

4. 1760s cure (Dollond): achromatic lens, combination of two lenses

reflectors

1. no chromatic aberration

2. invented by newton, 1670

3. metal mirror, only reflects 66%

newtonian reflectors

1. long tubes

2. open at front

3. eyepiece on side

star points from mirror struts

struts for the 2º mirror don’t block light, but they do distort it

1610

keplerian and galilean telescopes invented

17th century

• aerial telescopes

  • long focal length to reduce lens errors

1672

newton invents reflector

1750

dollond patents the achromatic lens

late 1700s

herschel builds speculum reflectors

1858

foucault invents silverized mirrors

galilean telescopes

1. coverging and diverging lens

2. upright virtual image

keplerian telescopes

1. 2 converging lenses

2. inverting

3. real image

cavendish

measures gravitational attraction between 2 lead balls

measuring astronmical unit (sun-earth distance)

1. project known distance on earth onto sun

2. deduce venus-earth distance

3. parallax

how to measure an astronomical unit

observe an event in the solar system from two different location to triangulate the distance

maselyne weighs the earth by…

force ratio + mountain mass