Download What Causes Tides?

Lec 11: 3 October 2011
Chapter 7: Overview of the Solar System
LAST – Orbits & Spacecraft; Intro to Tides
TODAY – Overview of the Solar System
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What Causes Tides?
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Census of the Solar System
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How do we Measure Planet Properties?
NEXT – Formation of Solar System
(Chapters 7 & 8 combined)
Lab This Week: Outside! Dress Warmly, Bring
Flashlight, Star Chart, & Sky Ruler
Close-Toed Shoes!!!
What Causes Tides?
•  DIFFERENCE in gravity between front side and
back side (front side of Earth is closer to Moon)
•  Sun’s gravity is MUCH stronger than Moon’s,
but difference between front and back is 4 times
larger for the Moon than for the Sun, because
Moon is closer
•  Water flows toward Moon (and Sun)
–  “tidal bulge” rotates with Earth
•  Both Sun and Moon cause tides on Earth
•  When solar and lunar tides line up -> Spring Tides
•  When solar and lunar tides 90o apart -> Neap Tides
What Causes Tides?
•  DIFFERENCE in gravity between front side and
back side (front side of Earth is closer to Moon)
•  Sun’s gravity is MUCH stronger than Moon’s,
but difference between front and back is 4 times
larger for the Moon than for the Sun, because
Moon is closer
•  Water flows toward Moon (and Sun)
–  “tidal bulge” rotates with Earth
•  Both Sun and Moon cause tides on Earth
•  When solar and lunar tides line up -> Spring Tides
•  When solar and lunar tides 90o apart -> Neap Tides
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Other Effects of Tides
•  Tides on Moon due to Earth stronger than Earth tides
due to Moon (but no water tides on moon)
•  Tides have synchronized Moon’s rotational and orbital
periods, so it always keeps its tidal bulge pointing
toward Earth (“same face”)
•  Tides are slowing Earth’s rotation
•  Tides are increasing the distance to the Moon •  If Moon was big enough, eventually Earth’s rotational
period would equal Moon’s orbital/rotational period!
•  Other Effects
–  will cause Triton to spiral into Neptune
–  produce rings of outer planets
–  heat interiors of satellites
Review: Orbits of the Planets
•  all planets orbit Sun
in the same direction
and in almost the
same plane •  ecliptic plane =
equatorial plane of Sun
•  most planets have
nearly circular orbits
•  mostly rotate in same
direction; axis mostly
perpendicular to plane
•  note Pluto’s weird orbit
Census of the Solar System
1.  The Sun (a typical star)
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gravitational center of the solar system
contains most of the mass
everything else orbits the Sun
principal source of light and heat
we see everything else by reflected sunlight
hotter closer to Sun
2.  8 Planets
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Mercury, Venus, Earth, Mars
[ “Terrestrial”]
Jupiter, Saturn, Uranus, Neptune [ “Jovian” ]
Two Types of Planets
•  Terrestrial (inner) planets have high average density
–  smaller than Earth
–  made of rocky and metallic materials
–  we see solid surface (except Venus, but it has one)
correct relative size; NOT correct relative distance!
8 Planets (continued)...
•  massive enough to pull themselves into spheres (gravity)
•  generate heat internally
•  undergo “geological activity”
•  solid, liquid, and gaseous parts
•  massive enough to hold on to atmosphere
•  do not emit visual light (do emit radio and infrared)
•  Jovian (outer) planets have low average density
–  much larger than Earth
–  composed primarily of hydrogen and helium (but
they have all the heavy elements, too)
–  we see only the cloud tops
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The Terrestrial Planets
The Jovian Planets
3.  > 160 Natural Satellites (“Moons”)
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orbit the planets; 7 “planet sized” moons
largest are spherical and have atmospheres
some might even harbor life!
smallest are irregularly shaped
all are rocky and/or icy (not gaseous or liquid)
most orbit in planet’s equatorial plane in same
direction as planet rotates; keep same face to planet
4.  Debris
A.  Asteroids (hundreds of thousands)
•  large, rocky debris (6 are over 3000 km diameter)
•  most in “belt” between Mars and Jupiter
•  some can/will collide with Earth
•  source of many meteorites
•  mostly “primitive” material; some “primordial”
•  but some are mostly nickel/iron
C.  “Trans-Neptunian” Objects; Dwarf Planets
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B.  Comets
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several hundred “short-period” comets
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  orbit in ecliptic plane
  sublimate near the Sun; leave trail of debris
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Pluto now recognized to be prototype Several other objects like Pluto recently discovered
Largest are spherical and as massive as the major
satellites of the planets (but smaller then Mercury)
Orbits not strictly confined to ecliptic
Would be considered comets if icy and passing
through inner solar system
Largest asteroids (Ceres) also spherical, but rocky
billions orbit in outer reaches of solar system
  might never approach Sun; unpredictable when they do
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icy, primitive material left over from solar system
formation (or even interstellar space)
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D.  Meteoroids
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miscellaneous junk orbiting around Sun
some can orbit planets too (very minor satellites)
mostly tiny (dust particles)
“meteors” when they pass through our atmosphere
“meteorites” if part of them survive trip through our
atmosphere and collision with Earth’s surface
Note: asteroids and comets hit the Earth, too
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