1. PTYS/ASTR 206 – The Golden Age of Planetary Exploration Shane Byrne – shane@lpl.arizona.edu Solar System Scales

2. PYTS/ASTR 206 – Solar System Scales 2 l Measuring length, mass and time l Angular sizes and real sizes l Distances between solar system objects l Timescales of the solar system l Scientific notation In this lecture…

3. PYTS/ASTR 206 – Solar System Scales 3 l Units are vital! n E.g. How far away is it? wTwo hundred – a meaningless answer! wTwo hundred meters – this means something n How old is it? w46 – means nothing wSeconds or years?? l In science we use the metric system (SI units) n Length is in meters (1m = 3’ 3.4”) n Time is in seconds n Mass is in kilograms (1Kg = 2.2 pounds on Earth) n MKS units wWe’ll use these throughout the course n CGS units wMetric system using centimeters, grams, seconds wWe won’t use this. Length, Time and Mass

4. PYTS/ASTR 206 – Solar System Scales 4 l 1s n The time needed for a cesium-133 atom to perform 9,192,631,770 complete oscillations. l 1m n Originally 1/10,000,000 of the distance between the equator the pole n 1983: The distance traveled by light in a vacuum in 1/299,792,458 sec l 1 Kg (1000g) n Originally defined as the mass of 1 liter (0.1m x 0.1m x 0.1m) of pure water n Now defined by a standard platinum-iridium block in Paris. l Temperature – in Kelvin n Starts at absolute zero (-273.15 Celsius), directly measures thermal energy n Interval is the same as Celsius, 1/273.16 of the triple point of water Length, Time and Mass l You can make any other units by combining these basic ones… n Speed = distance/time or m/s n Acceleration = Change-in-speed / time or m/s 2 n Weight (or force) = mass * acceleration or Kg * m / s 2

5. PYTS/ASTR 206 – Solar System Scales 5 freemars.org

6. PYTS/ASTR 206 – Solar System Scales 6

7. 7 l Small things can appear large n Angular size depends on physical size and distance Lunar Transit - Stereo mission Real Lunar Size

8. PYTS/ASTR 206 – Solar System Scales 8 l Small angle formula n angular_size = size / distance n Moon = 1737km / 384,440 km w0.0045 radians n Sun = 695950 km / 149,000,000 km w0.0047 radians

9. PYTS/ASTR 206 – Solar System Scales 9 l Measured in astronomical units for convenience n 1 AU is the Earth-Sun distance l That’s pretty hard to measure n Relative distances are easy n E.g. Mars is 1.5 AU from the Sun. Distances E M SUN l First measured by Richer & Domenico 1672 using parallax of Mars and size of the Earth l Later methods used transits of Venus Value of 149,597,870,700 m

10. PYTS/ASTR 206 – Solar System Scales 10 l Solar system structure n Inner rocky planets wMercury0.39 AU wVenus0.72 AU wEarth1.00 AU wMars1.52 AU n Asteroid belt wHundreds of members wSeveral groups n Gas Giant planets wJupiter 5.2 AU wSaturn9.6AU n Ice giant planets wUranus19.2 AU wNeptune30.1 AU n Kuiper Belt (> 30 AU) wContains Pluto wSeveral groups n Oort cloud (50,000 AU) wLong period comet reservoir w1/5 of the distance to the nearest star.

11. PYTS/ASTR 206 – Solar System Scales 11 l Numbers to keep in mind… n The universe is probably about 10-15 billion years old n The solar system is ~4.5 billion years old – middle aged n The Sun rotates every 30 days n Most planets rotate in a day or less wMercury and Venus are special cases n Terrestrial planets orbit the sun in 1/4 – 2 years n Gas giants orbit the sun in 10-170 yrs n Most moons orbit their host planets in a few days to 1 month Solar system timescales

12. PYTS/ASTR 206 – Solar System Scales 12 l Full numbers in planetary science are awkward… n The sun is 150,000,000,000 meters away n The wavelength of visible light is 0.0000005 meters n The solar system is 4,600,000,000 years old l Much more convenient to say n The sun is 1.5 x 10 11 m away n The wavelength of visible light is 5 x 10 -7 m n The solar system is 4.6 x 10 9 years old l How about a useful example? n There are about 31 million seconds in one year (1 yr=3.1 x10 7 s) n We can replace 4.6 x 10 9 years with 4.6 x 10 9 x 3.1 x10 7 s n The solar system is 14.26 x 10 16 s old. (or 1.426 x 10 17 s) Scientific Notation More replacements 10 -3 m = mm 10 9 yr = Gyr etc..

13. PYTS/ASTR 206 – Solar System Scales 13 Not all the answers are known…. There are plenty of exciting mysteries out there

14. PYTS/ASTR 206 – Solar System Scales 14 l Measuring length, mass and time l Angular sizes and real sizes l Distances between solar system objects l Timescales of the solar system l Scientific notation In this lecture… Next: Exploring the solar system from the Earth l Reading l Chapter 1 to revise this lecture l Chapter 3 for next Tuesday