ASTR1002 Mystery Object Tute

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ASTR1002 Mystery Object Tute
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Patterns in the Night Sky

• Imagine, once again, that you live in Mog, where
  the clouds recently cleared (for the first time
  in history) giving you a view of the sky for the
  first time.
• You now know that Mog is a sphere, spinning on
  its axis and orbiting its Sun.
• You know that stars appear to circle the
  celestial pole roughly once a day (actually
  travelling 361 degrees around in a 24 hour
  period).
• But now, youve seen something rather strange in
  the twilight sky. Every two weeks, during the
  last month or so, youve been out just after
  sunset, looking towards where the Sun has just
  set.
• You live on the equator.

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Six weeks ago
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Four weeks ago
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Two weeks ago
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Tonight
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Explain what you see!

• Your job, working in groups, is to explain what
  you see.
• You should thing about the pictures and what they
  mean for a bit. Once youve got everything you
  can out of them, you should turn to the next page
  for a few more clues.

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Clues

• The people of Mog are remarkably unobservant.
  None have ever noticed these particular
  constellations before. However, you did meet a
  fisherman who reported seeing a cross of bright
  stars six months ago. However, he saw the cross
  at dawn, just above the Eastern horizon. There
  was no fifth bright star within the cross.
• All these stars look just like dots to the naked
  eye - there are no obvious colour differences
  between them. Some might argue that the middle
  star in the cross doesnt twinkle quite as much
  as the others, but thats pretty marginal.

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Telescope Observations

• Mid-way through the tute, I release the following
  data. It turns out that another group of
  scientists have recently been observing this part
  of the sky with their new invention, a collection
  of lenses they call a far seer, or
• Tele - Scopos. They looked at this cross
  constellation six, four and two weeks ago.
• Most of the stars in this constellation look much
  the same when viewed through the telescope
  (they still look like dots of light, only
  brighter). The middle star of the cross, however,
  the one that moves with respect to the others,
  looks quite different.

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Shapes of the Mystery Object.

• Here is what it looked like six, four and two
  weeks ago.

Four weeks ago
Six weeks ago
Two weeks ago
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Enough Clues

• That should be quite enough clues to allow you to
  work out what this thing is. In fact, you dont
  really need the telescope data - theres enough
  information in your naked eye views of the sky.
• One last clue - there is an object that behaves
  just like this mystery object in our own solar
  system.
• The next slide gives the answers - but dont read
  them until youve tried good and hard to figure
  this out for yourself (the process of figuring
  this out is far more important and examinable
  than the answer).

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The Answer

• The mystery object is a planet, closer in to the
  Sun than Mog (like Venus in our solar system).

As you are looking towards where the sun has just
set, you must be looking this way.
As it is just after sunset, you must be here.
Six weeks ago, the mystery object was here
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Four weeks ago

• Two weeks later, both the mystery object and Mog
  have moved around the Sun. The mystery object,
  being closer in, moves faster.

It is now slightly higher in the sky (relative to
the background stars) but closer to Mog (and
hence bigger) and seen edge-on to the terminator
(so it appears half full)
Four weeks ago, the mystery object was here
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More recently

• It is now closer to us (and hence larger), but is
  moving towards the line between mog and the Sun,
  making it appear lower on the horizon.

Its dark side is now facing us, so it appears
mostly dark, with only a crescent of light.