Planets

1. Move with respect to background -> Word planet comes from greek for “wanderer”
2. planets don’t twinkle

Aristotle (circa 350 BC) -> geocentric model of universe

The Moon

Visible phase determained by:

1. Current phase

2. Position of the sun in the sky (time)
3. Position of the moon in the sky

At sunset: full is rising, first quarter is high in the sky, new is setting (third quarter sets at noon)

Moon is offset from Earth Sun by .5*

Orbital plane of moon around earth is different than that of the earth around the sun

Only solar eclipse when phase is new

penumbra = outershadow of moon (only part of the solar eclipse)

umbral shadow = inner shadow of moon (full eclipse) about 300km on earth at a time and never longer than 7 minutes

annular eclipse (when moon doesn’t completely cover the solar disk), umbral shadow is not complete

Lunar eclipse is longer (earth is bigger)

1. Takes hours
2. Penubral shadow has almost no effect
3. At totality, it looks red due to refraction through earth’s atmosphere

Light

1. Light is a form of electromagnatism
2. Light has both Wave and Particle features (wave particle duality)
3. Photon
   1. A Partical that carries the electromagnatic radiation
   2. Wave length: linear distance between two crests of the wave of a photon (lambda)
   3. Frequencey: Number of crests that pass over a unit of time (nu)
   4. A long wave length will have few crests over a unit of time
   5. Wave length and frequency are related by the speed of light: 3×10^5km/s or 3×10^10cm/s
   6. c = lambda * nu (where c is the speed of light)
      1. if lambda goes up then nu goes down (and vise versa)
   7. Visible light has very short wavelengths (on order of angstroms, 4000-7000)
   8. Hertz = one unit per second
   9. The energy of a photon = h*nu (where h is a constant)
   10. Spectrum
       1. Gamma (0 -> .1 A) Harmful, found in nuclear explosions, Very high frequency thus energy
       2. X-rays (.1 -> 100A) Harmful, but not in very small doses
       3. Ultraviolate (100 -> 4000A) Harmful with continued exposure
       4. Visible (4000-> 7000A) Small range from blue (4000) to red (7000)
       5. Infrared (7000A -> 1mm) Skin feels as heat
       6. Microwave (1mm-> 10cm) Like a microwave oven (which uses alot of microwave, like a bright light)
       7. Radio (10cm -> infinate) Need long wave length to get through water
   11. S
4. Spectroscopy -> breaking up light into its components (like a prism)
   1. Using this we can plot the intensity of light as a function of wavelength
5. Light can tell us about:
   1. Tempurature
   2. Movement
   3. Conditions
6. Continuos spectrum (all wavelengths in a range)
   1. Stars often aproximate this
   2. Black body object (absorbs all light and reflect none, however does emit its own)
   3. Stars are blackbody because they emit light, but don’t reflect light (the moon does the exact opposite)
   4. The  characteristics of a blackbody is that its tempurature is observable from its spectrum
      1. Plots (wv vs brightness) aproximates poisson distribution
      2. As the object cools, its “peak” moves from ultraviolet to infrared
   5. Higher tempurature emitis more radiation at all wave lengths
   6. Hotter may emit more blue than red (making it look cool) but it still emits more red than the cooler (red) one
   7. Wien’s law (lambda max * T = 2.9×10^7AK)
   8. Humans are 310k so lambda max = 100,000A, making us blackbody in the infrared spectrum (which is why the military likes infrared stuff)
      1. We’re visible in the visible spectrum because we reflect light
   9. Emited by solid, liquid or pressurized gas (star)
7. Absorption line specturm
   1. If we put a cool cloud of gas between a star and ourselves we’ll notice certain wavelengths are missing
   2. Most stars have an atmosphere around them which causes them to apear as an absorption line spectrum
8. Emission line spectrum
   1. Put the same gas agains the background of the universe we’ll see only certain lines
   2. These lines are the same lines which were absorbed when we looked at the sun through said gas
   3. These are due to electrons absorbing photons an jumping up into higher orbitals (thus ions absorb and emit differently than once with full valiance shells)
9. 90% of the universe is made of H, so we talk about that alot