Download day05parallax

Day 5 notes
Parallax
measurements
Small angular
measurements
Units of large
distances
We have been describing the position of stars,
the sun, and the orientation of the Earth
using the celestial sphere and coordinates.
• These coordinates are described in terms of
angles (declination and right ascension).
• To fully understand the position of the sun
and stars, we need to also know the
DISTANCE to these objects.
• This is similar to saying that Chicago is
northeast of here; we need to also say how
far it is, to know its location.
Question ?
• How do we measure the distance to an
object if we cannot reach it?
• For example, we look across a river and
see an object. We can walk around on
our side of the river, and make
measurements of directions.
The method is called Triangulation, since there is a triangle
involved in obtaining the distance. Notice that angles are
also needed to estimate the size of the sides of the triangle.
Parallax is an angle that
describes the change in
your “line of sight”
when you look at
a distant object
from TWO points of view,
or from TWO positions.
Parallax Geometry can be illustrated by using two pencils.
Look at some distant object with one eye, then the other.
More on parallax
• Motion parallax is seen
when you travel. (link)
• The measurement of
parallax can be used to
estimate the distance to
the Moon if observations
are made from different
parts of the Earth.
Parallax can also be used to estimate the distance to
relatively near-by stars by observing at opposite points
in the Earth’s orbit (of course, you have to wait 6 months
between measurements). We will mention this again
when we study stars in Ch. 9.
Notice that the parallax angle is larger for stars that are closer.
This simulation shows what the sky might look like
if the orbit of the Earth were huge, say 16 light-years.
The stars close to us would appear to wobble back
and forth during the year. The actual motion is much
smaller and requires careful measurement with
telescopes or similar instruments.
Proper motion of the stars
• Over very long periods of time, even the
constellations will change shape, since the
stars have REAL motion, called “proper
motion”, but this is difficult to see in one lifetime
because there is little relative movement.
• Here is a drawing of what the Big Dipper would
look like if you went back 100,000 years from
the present.
Measuring Distances with Geometry
and angular diameter
Very small angles are used in Astronomy
Small angles are defined
using the number 60
A full circle has 360 degrees.
One degree has 60 arc minutes.
One arc minute has 60 arc seconds.
So, one arc minute is 1/60 of a degree.
One arc second is 1/60 of an arc minute
and it is also 1/3600 of a degree.
An example would be 5o 23’ 45”
Units of Astronomical Distance
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Astronomical Unit – AU
1 AU = 1.5 x 108 km = 9.3 x 107 miles
This is the distance from Earth to Sun.
Light Year is abbreviated ly
1 ly = 9.46 x 1012 km = 63,000 AU
The nearest star is 4.2 ly away (over
265,000 AU).
Units of Astronomical Distance
• Parsec – defined as the distance at
which 1 AU perpendicular to the
observer’s line of sight make an angle
of 1 arcsec
• 1 pc = 3.09 x 1013 km = 3.26 ly
• The nearest star is 1.3 pc away (over
265,000 AU).
Astronomical Distances are huge compared
to the size of the solar system
• Make a model using a golf ball to represent
the Sun.
• The Earth is a grain of sand 1 m away.
• One meter represents one A.U.
• The nearest star is over 265,000 AU away
so this would be 265,000 meters or 265
kilometers in our model.
• This is about the distance from Macomb to a
big city like St. Louis or Chicago.
The first exam is on Thursday, Feb. 4 (this week!)
We will have about 30 minutes of class before the exam.
Then you will take the exam (which uses a Scantron).
The exam is multiple choice and true/false questions.
Coverage is Chapters 1 and 2 in your textbook.
To review, look at the chapter summaries, day notes,
and a study guide that I posted over the weekend.
2009 was a special year for
Astronomy because we celebrated
the 400th anniversary of the first use of the
telescope to look at the sky (by Galileo).
We will see some short movies that
were made to celebrate that year.
Eyes on the Skies is an mp4-coded
HD1080p movie in several parts.
It is also available on DVD.