# astronomy
[toc]
## tools
### Angle Unit
1 degree = 1° = 1/360 of a circle
1 arcminute = 1' = 1/60 of a degree
1 arcsecond = 1" = 1/60 of an arcminute = 1/3600 of a degree

Angular Size ---------> Real Size
Small Angle Formula:
d=Dδ
angular size (δ)-> radian
Distance (D)
size (d)
### Distance Unit
1 AU = 149597870.691 km
1 ly ~ 63241 AU
1 pc ~ 3.26 ly //Parsec(pc)

1pc=1AU看起來1Arcsec大小的距離
D= 1/ θ
D= 1/ 1” = 1 pc
D= 1/ 2” = 0.5 pc

Redshift (z)
z ~ 4000000000 pc = 4000 Mpc

## inner Planets

### Mars:
* ~ 1/100 of earth air pressure
* mostly CO2
* no Ozone layer
* possible water in the past
* temporary surface water ?
* no current sign of life
* Water ice and CO2 ice

### Venus:
* Earth twin, almost the same size
* Atmosphere is very thick (~90 atm)
* Atmosphere is made of CO2 gas
* Surface is very hot ( ~400 C)
* No evidence of plate tectonics
* Rain of Surfanic Acid (H2S04)
* Runaway Green House effect?

### Mercury:
* Not much is known, because it is difficult to reach
* Very eccentric orbit (eccentricity~0.2)
* No satellite
* No thick atmosphere
* Similar to Earth Moon
* Dense (5.427g/cm^3: second highest in solar system)
* Relatively strong magnetic field
* Suspected large iron core
* Water Ice ?

## Outer Giant Planets
1. Jupiter 木星
1. Saturn 土星(環)
1. Uranus
1. Neptune

### Jupiter
satellites:
1. IO - Volcano
1. Europa - Ocean?
1. Ganymede -magnetic field
1. Callisto

* Atmosphere is mainly made of Hydrogen (H2)
* Liquid oceans may exist under Europa's icy surface.
* Galileo made the first observation of ammonia clouds in another planet'satmosphere. The atmosphere creates ammonia ice particles from material coming up from lower depths.
* Ganymede possesses its own, substantial magnetic field - the first satellite known to have one.
* Europa, Ganymede and Callisto may have a liquid-saltwater layer under the visible surface.

### Saturn
satellites:
1. Titan
1. Rhea
1. Iapetus
1. Dione
1. Mimas... 61<moons total

Some hightlights of what we know about Saturn
* Many many rings
* Many satellites (as of 2009: 61 satellites are known)
* Titan, the largest satellite, has thick atmosphere of nitrogen
* Surface of Titan is covered by many lakes of methane and ethane ?
* Titan may be able to harbour the life ?

#### Titan:
* The largest moon
* of Saturn.
* Only satellite in
* the solar system to
* have thick atmosphere.
* The second best
* cadidate to habour
* life in the solar system.

### Uranus
* ~ 20 AU away from the sun (P~ 84 yrs)
* ~14.5M星等, the lightest of giant planets (c.f. Neptune 17M星等)
* Cloud top is about 49K, the coldest giant p lanet, colder than Neptune
* Cloud pattern is featureless, though similar to Neptune otherwise
* Very high wind speed up to ~400 km/h
* "Tipped" spinning axis
* "Tipped" rings
* "Misaligned" magnetic field
* Atmosphere is mainly made of Hydrogen (H2)

### Neptune
* ~ 30 AU away from the sun (P~ 165 yrs)
* 17 M星等, slightly heavier than Uranus
* First planet discovered based on calculation
* Cloud pattern is relatively active: e.g. Great Dark Spot, wind speed up to 2000 km/h
* Can find nitrogen, methane in the atmosphere, similar to Uranus
* Some rings, 13 known moons
* "Misaligned" magnetic field
* Atmosphere is mainly maide of Hydrogen (H2)
* Neptune is slightly warmer than Uranus

## Comparison of Atmosphere

| celestial body | Atmosphere | bar |
| -------- | -------- | -------- |
| Mercury | (H2) | (~10^-14) |
|*Venus |CO2 |~100|
|*Earth|N2|~1|
|*Mars |CO2|~ 0.01-0.001|
|*Jupiter|H2 | >>>> |
|*Saturn|H2 |>>>>|
|*Uranus |H2 |>>>>|
|*Neptune |H2 |>>>>|
|Pluto|( N2 ) |(~10^-5)|
|Callisto |(CO2)|(~10^-12)|
|Europa|(O2)|(~10^-12)|
|Ganymede|(O2)|(~10^-12)|
|Io|(SO2)|(~10^-9)|
|*Titan|N2|~1 |
|Ceres|(H2O)| (<<<<<<) |