Exam 2 answers -- Fall 2011


Multiple Choice

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Short answer

27.  Explain why it is possible for you to view an artificial
satellite while just standing outside looking up at the sky even
though the satellite might be quite small and hundreds of miles away.
A sketch will work a lot better than lots of words.

The basic idea is that in the early evening or pre-dawn morning 
you are standing in the dark at the surface of the Earth while
space above you is in sunlight.  The brightly lit satellite
stands out against the dark sky.

See, for example:

http://www.astro.virginia.edu/class/skrutskie/images/art_vis.gif




28.  Why are the worlds largest telescopes of the “reflecting” rather
than “refracting” type?


Refracting telescopes use transparent lenses which much be internally perfect
and can only be supported by the edges - and thus subject to "sagging" if
they get too large.  Mirrors only require a polished front surface and can
be supported from behind, enabling much larger reflecting telescopes.


29.  If you are experiencing a high tide at your location on Earth
should you expect to see the Moon high in the sky?  Why or why not?

Two misconception seemed to trip a lot of people - namely that high
tide is associated with New and/or Full Moon.  Any phase of the Moon
produces high tides.  New and Full Moon do produce slightly higher
high tides than normal.

The key to the question is to understand that the Moon raises high
tides BOTH on the side of the earth facing the Moon and on the
side AWAY from the Moon. 

See this figure.

http://www.astro.virginia.edu/class/skrutskie/images/tides_forces2.jpg

The dashed line represents the stretched oceans of the Earth.  The
Moon (at any phase) stay largely in place while the Earth turns
carrying a person through the two tidal bulges each day.
One bulge is on the side of the Earth facing the Moon (so you expect
to see the Moon there), but the other tidal bulge is opposite
the Moon and you will NOT see the Moon at that high tide.

30.  Why do astronauts appear weightless while in orbit around the Earth?

Astronauts, their spaceship, and everything around them are simultaneously
falling around the Earth in orbit.   Everything falls at the same rate
so it seems like gravity has been turned off, when in fact gravity is
pulling on everything just about as strongly as it ever did.

31.  Define in a sentence or less the following terms:

    Parallax: The apparent shift in the position of a nearby object
         relative to the background scenery as the position of
         the observer moves.

    Redshift:  A Doppler shift toward longer wavelengths produced when
      a source of light is moving away from the observer.


    Geostationary satellite:  A satellite at an altitude such that 
       it orbits the Earth once each day - hovering above a point on
       the Equator.

    Weight: The force of gravity on an object.

    Eccentricity: The amount an elliptical orbit is "elongated".