Download Parallax

Parallax
L/O to explore
the phenomenon
of parallax
Determining the distance to a star is
difficult because we cannot actually
travel to the star and measure the
distance directly. Instead, astronomers
must be very clever and measure the
distance indirectly. One of the ways
they do this is by the method of
Parallax.
Parallax measurements take advantage of the
fact that, as the Earth orbits around the
Sun, relatively near-by stars appear to move
with respect to the fixed, very distant stars
Notice in the cartoon
that the closer water
seems to move a lot
quicker than the
background water
although they are
moving at the same
speed
This is the same thing that happens when you look at a
close object with first one eye and then the other.
For example, hold your thumb at the tip of your nose. Look
at your thumb with first your right eye and then your left.
Your thumb appears to move because your eyes are not at
exactly the same place, so each eye views the thumb from
a different angle. Now hold your thumb at arm's length
and repeat the experiment. Your thumb will still appear to
shift, but will not appear to move as much as it did when it
was closer. The same thing happens to stars. The closer
stars appear to shift more than the farther stars. The
"fixed" background stars are not really fixed; they are
just so far away that we cannot distinguish their apparent
shift.
Calculating the angle of parallax
b
L
sin p
L
b
Now attempt the work sheet questions
p
The Scale of Things
Parallax angles are usually measured in
fractions of a second of arc
a second of an arc is
1
of a degree.
3600
Astronomers use a unit based on this: the
parsec.
An object whose parallax angle is 1
second of arc it is at a distance of 1
parsec
Because a smaller angle means it is a
bigger distance an object whose
parallax angle is 2 seconds of arc is at a
distance of 0.5 parsec.
A parsec has a distance of 3.1 x 1013km