Transcript The Expanding Universe - Australian National University

ASTR1002
Mystery Object Tute
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, you’ve seen something rather strange in the
twilight sky. Every two weeks, during the last month or so,
you’ve been out just after sunset, looking towards where
the Sun has just set.
You live on the equator.
Six weeks ago
Four weeks ago
Two weeks ago
Tonight
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 you’ve got everything you can out of them,
you should turn to the next page for a few more clues.
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 doesn’t
twinkle quite as much as the others, but that’s pretty
marginal.
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.
Shapes of the Mystery Object.
Here is what it looked like six, four and two weeks ago.
Six weeks ago
Four weeks ago
Two weeks ago
Enough Clues
That should be quite enough clues to allow you to work out
what this thing is. In fact, you don’t really need the
telescope data - there’s 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 don’t read them until
you’ve 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).
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.
Six weeks ago, the
mystery object was here
As it is just after
sunset, you must
be here.
Mog
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
Mog
hence bigger) and seen edgeon to the terminator (so it
appears half full)
Four weeks ago, the
mystery object was here
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.
Mog