3. Order of Magnitude
1 Gm = 1 gigameter = 10^9 meters = 1 000 000 000 m
1 nm = 1 nanometer = 10^(-9) meter = 0.000000001
4. Order of Magnitude
nucleu
s
1000 kg 1 Kg
ObservableSystematom
Interstellarof an
Nucleusuniverse
Solar distance
Humans
Bacteria
Atom
Galaxy
Sun
Earth
5. Order of Magnitude
Filling of space by the matter is essentially
incomplete, at both tiny (atom) and big (cosmos)
scales
(essentially void around us)
6. Speed of light
Speed of light in vacuum is a phyical constant :
c = 3.10^8 m/s = 300 000 000 m/s
The light is composed of photons
and is propagated in straight line
A.Einstein 1879 -
1955
Speed of light (speed of propagation)
depends only on the medium
speed(vacuum) = speed(air) = c = 3.10^8
m/s
speed(water) < c
Refraction index n : quantity without unit
7. Speed of light
Galilée, Romer, Cassini, Newton,
Michelson..
using astronomy
The meter is the distance travelled by the
light in the void during 1/299 792 458
second
m
v = c (m/s) s
8. Speed of light
d = 1m
d = 150 000 000 Km
s km
s m
km/
s
m/s
Even if the light of speed is great, the gigantic distances at the
cosmos scale cause delays (we don’t see things in ‘real time’)
9. Light-year
A light-year is the distance travelled by the
light in vacuum during one year
1 ly
m
roughly
s
m/s
1 year converted in seconds
The light-year (ly) is not a new unit, it’s used only because
it’s handy with the gigantic distances we have in the
cosmos
10. Light-year
How far are we
from this
Extinction of the dinosaurs planet?
: 65 millions years ago
I see dinosaurs
The aliens are at least 65
000 000 light-years away
from us
Watch far = Watch in the past
