1. THE MEASUREMENT OF DISTANCE

2. TRIANGULATION
Method of determining distance based on the principles of geometry and trigonometry
Visualize an imaginary triangle by sighting a distant object from two separate positions
The simplest possible triangle is a right angle, in which one of the angles is exactly 90o
Baseline-the distance between two positions (A & B) that makes up the base of the imaginary triangle
Knowing the value of one side (AB) and two angles is all that are necessary to ascertain the object’s distance

3. BASIC GEOMETRY IS USED This is a scaled estimate
Nothing more complex than basic geometry is needed to infer the distance, the size, and even the shape of an object too far away or too inaccessible for direct measurement

4. MEASURING GREATER DISTANCES
This figure shows a triangle having a fixed baseline between two observation positions at points A and B
The triangle becomes longer and narrower as the object’s distance from A increases
Narrow triangles cause problems because the angles at points A and B are hard to measure accurately
The measurements can be made easier by "fattening" the triangle—in other words, by lengthening the baseline

5. PARALLAX
(a) This imaginary triangle extends from Earth to a nearby object in space (such as a planet)
The group of stars at the top represents a background field of very distant stars
(b) Hypothetical photographs of the same star field showing the nearby object's apparent displacement, or shift, relative to the distant, undisplaced stars

6. The observer at point A sees the planet at point A'
The observer at B sees the planet at point B'
If each observer takes a photograph of the appropriate region of the sky, the planet will appear at slightly different places in the two images
The planet's photographic image is slightly shifted relative to the field of distant background stars
The background stars themselves appear undisplaced because of their much greater distance from the observer
Parallax is measured as an angle on the celestial sphere-the third, small angle shown

7. PARALLAX
The apparent displacement of a close object relative to the background as the observer changes locations
The closer an object is to the observer, the larger the parallax
Small parallax-large distance
Large parallax-small distance
Knowing the amount of parallax (as an angle) and the length of the baseline, we can easily derive the distance through triangulation

8. LIGHT-YEARS
Unit introduced by astronomers to help them describe immense distance
Distance traveled by light in a year
300,000 kilometers per second X 86,400 seconds/day X 365 days
=10 trillion kilometers (about 6 trillion miles)
Earth’s diameter = 13,000 km =1/20 of a light-second
Light travels 186,000 miles in one second
Light from the sun takes about 8 minutes to get to the Earth (about 93 million miles away)
In one year, light travels about six trillion miles
The nearest known star, other than the Sun, is 4.22 light years away

9. Astronomical Unit (AU)
The mean distance between the Earth and the Sun
Used to indicate distances within the solar system
One AU is approximately 150 million km or 93 million miles