Transcript Midterm 1: - Physics @ IUPUI

The Moon part deux
•
Goal: To understand the interactions
between the earth and the moon.
• Objectives:
1) To understand Eclipses – solar and lunar
2) To learn about what causes theTides
What path does the moon make in
the sky?
• A) It makes a circle along the ecliptic
plane.
• B) It makes a circle along the celestial
equator.
• C) It does not make a circle.
• D) None of these.
The Moon’s orbit
• The moon’s orbit is offset from the ecliptic by 5
degrees.
• So, most of the time when it is in front of or
behind the earth, it does not fall directly in line
with the sun (for example in the new moon it
would pass directly above or below the sun).
• There are 2 places in the orbit called the “nodes”
where the plane of the moon’s orbit intersect
with the ecliptic plane.
• When these intersect at either the new moon or
full moon….
Lunar Eclipse
• At the full moon, if it lines up onto the
node, will be eclipsed.
• This happens twice per year (with less
than half of the earth being able to witness
it – which means you can witness about 1
lunar eclipse every 1-2 years on average).
• There are 3 types of lunar eclipses.
• This why the moon turns red during a total lunar eclipse.
• Next lunar eclipses viewable from N. America will be 6 AM, October
8th, 2014 (yes that is a week from tomorrow!), April 4, 2015, and Sept
28, 2015
Solar Eclipse
• Similarly, the moon can block the sun when it lines up
correctly in the new moon.
• However, since the moon is much smaller than the earth,
the shadow is much smaller, and only a small part of the
earth is cast in the moon’s shadow.
• While there are still 2 eclipses per year, the main effect
is limited to smaller areas, so to view them from a
specific city is far more rare.
• Next solar eclipse viewable from the lower 48 U.S. states
will be August 21, 2017 (might want to head to Kentucky
that day)
• There are also 3 types of solar eclipses.
• Partial eclipse:
The moon only covers up part of the sun.
• Annular eclipse:
Since the moon’s orbit is ELLIPTICAL, in some
portions of the orbit it will appear smaller.
If the angular size of the moon is less than the sun,
it will create a dark circle on the sun, but NOT
entirely cover up the sun.
Annular Eclipse vs Total Eclipse
Tides:
• What are tides and how are they formed?
Tidal forces
• Tidal forces are the result of a net difference in
the gravitational force on 2 sides of an object.
• So, the part of the earth facing the moon feels a
greater tug than the part away from it.
• This creates a net force for the parts of the earth
near to and far away (near to towards the moon,
and far away is away from the moon).
• At the parts of the earth in the middle, there is no
net force.
• So, material (water mostly) goes towards the two
points of greatest force. Thus the high tide!
Basic tidal facts
• For most places on the earth, there are 2 high
tides and 2 low tides a day.
• Since the earth rotates the position of the tides is
about 10 degrees in front of the moon, and on
the opposite side of the earth from that.
• Due to local coastlines, the sizes of the tides
vary, but average to be about 1-2 m in height.
• The moon also affects land! The land tide is
about 6 inches, but is so gradual we don’t feel it.
• Finally, since the moon orbits the earth, the time
of the tides changes by about 50 minutes each
day – the same as the rise/set times of the
moon.
Tidally locked!
• The moon is tidally locked. That means
that the position of the “high tide” for the
ground on the moon is locked roughly into
place.
http://csep10.phys.utk.edu/astr161/l
ect/time/tides.html
Tide from
the sun is
10% that
of the
moon, but
enough to
affect the
Magnitude
of the
tides.
Yearly variations:
• The Moon’s orbit is ELLIPTICAL!
• So, at some points it is closer (closer = stronger
tides), and some further away.
• The phase it is closest and furthest depends on
time of year.
• So, at some times of the year you will have
stronger high tides, and some weaker.
• Strongest tides occur when moon is closest to
the earth at either the full moon or new moon.
But wait, there’s MORE!
• Since the bulge is pulled forward (due to
tidal friction), that bulge will tug on the
moon, and the moon on the bulge.
• This will result in a force which slows down
the earth’s rotation ( our days increase by
2 milliseconds per day per century).
• Since energy must be conserved, the
moon gains orbital energy, and gets 4 cm
further from the earth each year!
How do we know?
• Sorry hoax enthusiasts, but we know this
by shining laser light onto mirrors we
placed on the moon (i.e. we had to be
there to do this, sorry). By finding the time
it takes to come back, they can measure
the distance to the moon accurate to a few
millimeters.
Bizarre fact of day
• Billions of years ago, the earth’s rotational
period would have been 18-20 hours, and
the moon would have been 10X closer
than it is now.
Conclusion
• There are 3 types of lunar and solar eclipses.
• There are 2 of each eclipse each year, but much
less that we here in Indianapolis get to see.
• Tides happen twice a day due to changes in
gravity on opposite sides of the earth.
• The strength of high and low tides change based
on the elongation of the moon and the moon’s
distance to the earth.
• These tides change the moon’s orbit, and the
earth’s day.