Download Determining Distances in Astronomy

Where is everything ?
Determining Distances in Astronomy
 Deriving distances to objects in space is fundamental to
determining the scale of the universe, from the solar system
to the observable edge of the cosmos.

Within the solar system, we can use radar to “bounce” signals (light or radio)
off of solid objects. Timing these signals gives precise information on the
distances to planets and asteroids. This process is analogous to locating
airplanes using radar; the same physical principles apply.

Distances to nearby celestial objects can be determined via the rules of basic
trigonometry. This method can be used for distances out to about 100 light
years.

Distances to planets and nearby stars are determined by measuring the
object’s parallax.

Parallax is defined as the apparent motion of a foreground object (planet or
star) against a background of more distant objects. Parallax occurs due to
the motion of the observer.

Parallax occurs for astronomical objects as a consequence of the Earth’s
motion around the Sun during a year. As the Earth swings around in its orbit,
objects appear to “swing” back and forth against the background of more
distant stars.

Many additional methods for estimating distances have been devised by
astronomers. Each method has a different range over which that method can
be applied. The entire sequence of methods is known as the distance
“ladder” or “pyramid”.

Once the distance to an object is accurately known, the true physical size of
an object can be determined by measuring its angular diameter.

The “small angle formula” (a trigonometry relation) relates distance,
angular size and true or physical size of any object. This formula is a
powerful, yet simple tool for answering the question: how large is that object ?
The small angle formula is expressed:
Physical diameter
=
(angular diameter) x 2 x (distance)
______________________________
360 degrees
When using this formula, the angular diameter must be expressed in degrees.
Distance is expressed in linear units, meters or kilometers, in most situations.