Earth’s Moon

Origin

Step 1: A Mars-sized object hit Earth about 4.5 billion years ago. It hit so hard that debris was shot out into space around Earth.

Step 2: The debris settled into orbit and its gravity started to pull it together to form the moon.

Step 3: Meteoroidskept hitting the surface of the new moon, melting the surface to make a ‘magmaocean.’ Light stuff floated to the top, while heavier stuff sunk into the core.

Step 4: The outer crust cooled and solidified. Meteoroids continued to hit, forming impact craters. Some hit so hard that magma erupted from beneath the crust to fill the craters. This solidified lava is what makes the blackmaria(or ‘seas’) on the Moon’s surface.

Step 5: For the past 3 billion years, the moon has been almost completely solid. Only small meteoroids still hit, forming tiny craters.

Characteristics

Lower gravity – because it is smaller & less dense, you would weigh 1/6th of your Earth-weight

No magnetic field – because it has no spinning liquid core, compasses won’t work

No atmosphere – no weather or wind, so footprints remain for centuries

Moon’s Motion

The moon revolves around Earth in an elliptical orbit that is tilted about 5° from Earth’s orbit and that has a period of 27 1/3 days.

Due to Earth’s rotation, from here on Earth the moon appears to rise in the east & set in the west, but 50 minutes later each day

Rotation = Revolution, so the same side of the moon is always facing Earth

Moon’s Phases

Half of the moon is always receiving light from the sun at any time-except during a lunar eclipse.

Phases- the varying amounts of the lighted moon seen from Earth

oThe phases of the moon are cyclic.

oThe period from one fullmoon to the next is 29 ½ days.

oDue to moon’s revolution around Earth

New Moon: When the moon is between us & the sun, we only see the ‘dark’ side of the moon (it’s all dark – we usually don’t see anything!)

“Waxing” Moons: When the part of the
moon we see lit by the sun is increasing
(include waxing crescent, 1st quarter & waxing
gibbous)

Full Moon: When the Earth is between the
moon & the sun, we see the full ‘lit’ side
of the moon (we see a full circle – half the surface of the moon)

“Waning” Moons: When the part of the moon we see lit by the sun is decreasing (includes waning gibbous, 3rd quarter & waning crescent)

It takes the moon 27 days to make a trip around the Earth. Then why is it more than 29 days between full moons?

It takes 27 days for the moon to orbit the Earth with respect to the stars. However, the Earth orbits around the sun as the moon orbits around the Earth, so the moon must travel through an extra distance to make one orbit with respect to the Sun. Thus it takes 29.5 days for the moon to reach the same position with respect to the Sun.

Eclipses

Eclipse- when a celestial object partly or completely comes into the shadow of another celestial object

Lunar eclipse- when the moon revolves into the shadow of the Earth at the FULL moon phase.

oPartial eclipse-part of the moon is covered

oTotal eclipse- all of the moon is covered

oAverage of 2 eclipses per year, 100min long

oSeen by anyone on the night side of Earth

  • When the Earth’sshadow falls on the moon so we can’t see the moon
  • Solar eclipse- when the moon blocks out the sun during the NEW moon phase
  • Average of 1 every 200years or so for a specific location, lasting 7.5 minutes
  • Seen only by a small area on the day side
  • Umbra: central area of fulldarkness
  • Penumbra: area around the umbra of partialdarkness
  • when the Moon’s shadow falls on the Earth and we don’t see the sun