Download The Astronomical Unit

Objectives
How Big are things in Space?
measurement in space
• To identify the units we use to measure
different objects in space
• Define what an astronomical unit is
• Define what a light year is
• To describe parallax and identify how to
use parallax to measure distance in space
• To identify when to use the inverse square
law to determine the distance to a star
– Explain what the inverse square law says
The Astronomical Unit
• The distance between the earth
and sun is:
– 150,000,000 km or
– 93,000,000 miles
• To avoid using such large
numbers, we use the
Astronomical Unit, or AU.
• 1 AU = average distance
between the Earth and the Sun.
• 1 AU ~ 150 million km ~ 93
million miles.
• We can use AU to measure
things in our solar system.
Some approximate,
average orbital radii
sizes:
Mercury: 0.4 AU
Mars: 1.5 AU
Saturn: 10 AU
Pluto: 40 AU
Parallax
• Objects that are closer
to us will seem to
move faster and have
LARGER parallax
angles
Parallax
• Now that we have easy
units to use when we
measure objects in space,
now we need a way to
measure moving objects
in space
• We can use a little bit of
algebra and a concept
called parallax.
– We use telescopes to
chart a stars position in
relation to other stars in
the sky
– Six months later, we do
the same thing.
The star seems to
move because of
parallax.
Parallax Examples
– moon, planets, comets
• Objects that are
farther away from us
will seem to move
slowly or not at all
and have the smallest
parallax angles
– EVERYTHING ELSE
Parallax Animation
Parallax Animation
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Parallax Animation
Inverse Square Law
• Parallax works for objects
that are fairly close to us,
but not for stars that are
very far away from us.
Why?
– There is no detectable
change in the position of
the star over the 6 month
period
• We can use the Inverse
Square Law to help us.
The farther you are away from an object, the
dimmer it appears to be, because less light will
reach you
Inverse Square Law
Inverse Square Law Animation
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