Phases of the Moon
• Half of the moon is always illuminated
• Half of the moon is always illuminated
• We see all, none, or some of that illumination
The same side of the moon always faces the Earth.
The same side of the moon always faces the Earth.
The same side of the moon always faces the Earth.
Time for one
rotation
(axis)
=
Time for one
orbit
(Earth)
• Half of the moon is always illuminated
• We see all, none, or some of that illumination
• Half of the moon is always illuminated
- But it’s not always the same half!
• We see all, none, or some of that illumination
A few notes for the take home...
2
4π
________
3
2
a
p =
G(MEarth)
p2
C
___
=
MEarth
p: period (time for Moon to go around Earth)
a: Moon-Earth distance
Take home #1:
If M becomes 0.5M,
what is the new value of p?
A few notes for the in-lab...
A few notes for the in-lab...
A few notes for the in-lab...
A few notes for the in-lab...
A few notes for the in-lab...
A few notes for the in-lab...
A few notes for the in-lab...
2
p
=
3
a
p: period in Earth years
for one orbit around the Sun
a: distance in AU
from the planet to the Sun
2
p
2
4π
________
3
a
=
G(M1+M2)
G = gravitational constant = 6.67 x 10-11 m3/kg/s2
MSun = 1.99 x 1030 kg
MPlanet << MSun
2
4π
________
3
2
a
p =
G(MSun)
seconds  years
meters  AU
2
4π
________ = 1
G(MSun)
GM1M2
_______
F=
d2
F = force that the Moon feels as it orbits
the Earth
M1= Earth’s mass
M2 = Moon’s mass
d = distance between Earth and Moon
If d stays the same, what happens to F
if Earth’s mass decreases?
2
p
2
4π
________
3
a
=
G(M1+M2)
G = gravitational constant = 1.475 x 10-34 AU3/kg/day2
MEarth = 5.97 x 1024 kg
MMoon = 7.36 x 1022 kg
p = period in days of the Moon’s orbit around Earth
a = distance in AU between Moon and Earth
1 AU
_____________
a = 400,000km x
= .002667 AU
150,000,000km