Activity: Tracking the Planets

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Activity Tracking the Planets
The Celestial Sphere
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Summary

• In this Activity, we will investigate
• (a) planetary motion along the ecliptic,
• (b) phases of the innermost planets, and
• (c) retrograde motion of the outer planets.

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Tracking the planets

• The apparent motions of the planets (or
  wanderers) across our nighttime sky does not
  coincide with the regular rotation of the stars
  around the celestial poles.

Instead their motions fall in a narrow band
around the ecliptic, which, as we saw earlier,
is the Suns track across the sky.
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As we have seen, the plane of the ecliptic is an
imaginary planar surface in space containing
the Earths orbit and the Sun
Earth
Sun
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• The other planets orbits are in or close to the
  ecliptic too, which is why they seem to follow
  the Suns path from East to West across the sky.

planetary orbits
(This is not to scale! For example, Plutos
average distancefrom the Sun is actually 100
times that of Mercury.)
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Distances in the Solar System are very large!
To compare the average distances between the Sun
and each of the planets, its convenient to do
it in terms ofthe average Earth - Sun separation.
Remember the Astronomical Unit? The AU
(astronomical unit) average distance between
Sun and Earth 1.496 x 1011 m
1 AU
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• In order of distance from the Sun, the planets
  are (not to scale)

Mercury, 0.39 AU from the Sunon average
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Venus, 0.72 AU from the Sunon average
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Earth, 1.00 AU from the Sunon average (by
definition!)
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Mars, 1.52 AU from the Sunon average
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Jupiter, 5.20 AU from the Sunon average
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Saturn, 9.54 AU from the Sunon average
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Uranus, 19.2 AU from the Sunon average
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Neptune, 30.0 AU from the Sunon average
Pluto its companion Charon, 39.5 AU from the
Sunon average
Pluto is usually the furthest planet from the
Sun, but itseccentric orbit brings it closer
than Neptune on occasion - for example, between
Jan 21, 1979 and Mar 14, 1999.
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Phases of the innermost planets

• The innermost planets, Mercury and Venus, never
  stray very far from the Sun from our vantage
  point on Earth.

The Sun illuminates one side of each planet
dependingon where Mercury and Venus are in
relation to the Earthand the Sun, they exhibit
phases just like the phases of the Moon.
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For example, here is Venus viewed side-on from
the Sun, photographed by the Hubble Space
Telescope in ultraviolet light
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• To see how this comes about, follow this link
  (/essmovs/h13.htm) to a simulation which
  demonstrates the phases of Venus.

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Retrograde Motion

• Mars, Jupiter, Saturn ( Uranus, Neptune and
  Pluto) wander far from the Sun, showing no phases
  but retrograde motion instead.

For example, if we keep track of the position of
Mars inthe sky at the same time each night, over
a period of many months, it will appear to move
along the ecliptic, then, at some stage, it will
appear to loop the loop
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Retrograde motion caused great difficulties in
the pastto natural philosophers who tried to
model the SolarSystem as being centred on Earth.
However retrograde motion is easily explained in
the heliocentric model, where the planets travel
in elliptical ( nearly circular) orbits around
the Sun with each planet travelling more slowly
as we move out from the Sun.
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• Then retrograde motion is analogous to the effect
  ofpassing another car while travelling on the
  inside lane of a freeway - the other car appears
  to be going backwards.

To see how retrograding comes about, follow this
link (/essmovs/h14.htm) to an animation
illustrating the retrograde motion of Mars.
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Orbital Rotational Periods

• Just as the Earth rotates around a rotational
  axis ...

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• , so do the other planets. In most cases, this
  rotation produces day and night on these planets
  too, but as we will see the length of the day -
  the rotational period - can be quite different on
  other planets to that on Earth

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• In order of distance from the Sun, the planets
  are (not to scale)

On Mercury, the length of the sidereal dayis
59 Earth days.
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On Venus, the length of the sidereal dayis 243
Earth days.
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On Earth, the length of the sidereal dayis
(almost) 1 Earth day.

In the last Activity we saw that a sidereal day
is about 4 minutes shorter than a mean solar day
on Earth.
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On Mars, the length of the sidereal dayis 1.03
Earth days .
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On Jupiter, the length of the sidereal dayis
0.41 Earth days.
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On Saturn, the length of the sidereal dayis
0.43 Earth days.
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On Uranus, the length of the sidereal dayis
0.72 Earth days.
Note the angle of the rotation axis of Uranus-
Uranus rotates on its side, which gives it very
unusual days nights!
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On Neptune, the length of the sidereal dayis
0.67 Earth days.
On Pluto its companion Charon, the length of
the sidereal dayis 6.4 Earth days.
(Pluto rotates almost on its side too.)
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• As you can see, there is no particular pattern in
  the length of days on planets in our solar system.

However the lengths of planetary sidereal years
- their orbital periods - do show a general
trend, and so do the speeds with which they orbit
the Sun
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If we express each planets orbital period as
multiples of Earth years ...

• Planet (Sidereal) Year
• Mercury 0.241
• Venus 0.615
• Earth 1.00
• Mars 1.88
• Jupiter 11.9
• Saturn 29.5
• Uranus 84.0
• Neptune 165
• Pluto 249

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and also compare their average orbital speeds
...

• Planet Orbital Speed (km/s)
• Mercury 47.9
• Venus 35.03
• Earth 29.79
• Mars 24.13
• Jupiter 13.06
• Saturn 9.64
• Uranus 6.81
• Neptune 5.43
• Pluto 4.73

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• we can see that the length of planetary years
  increases and the orbital speed decreases as one
  moves out from the neighbourhood of the Sun.

Well investigate this trend in the next Activity.
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Image Credits

• NASA Mercury
• http//pds.jpl.nasa.gov/planets/welcome/thumb/merg
  lobe.gif
• NASA Venus
• http//pds.jpl.nasa.gov/planets/welcome/thumb/veng
  lobe.gif
• NASA Earth http//pds.jpl.nasa.gov/planets/welco
  me/earth.htm
• NASA Mars
• http//pds.jpl.nasa.gov/planets/welcome/thumb/marg
  lobe.gif
• NASA Jupiter
• http//pds.jpl.nasa.gov/planets/welcome/thumb/jupg
  lobe.gif
• NASA Saturn
• http//pds.jpl.nasa.gov/planets/welcome/thumb/2moo
  ns.gif

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• NASA Uranus
• http//pds.jpl.nasa.gov/planets/welcome/thumb/urag
  lobe.gif
• NASA Neptune http//pds.jpl.nasa.gov/planets/welc
  ome/thumb/nepglobe.gif
• NASA Pluto Charon http//pds.jpl.nasa.gov/plane
  ts/welcome/thumb/plutoch.gif
• NASA Ultraviolet image of Venus' clouds as seen
  by HST's Wide-Field/Planetary Camera 2. (NASA
  Photo Numbers STScI-PRC95-16, 95-HC-114)
• http//nssdc.gsfc.nasa.gov/image/planetary/venus/h
  st_venus95.jpg

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