In the mid-nineteenth century Christian Doppler discovered what proved to be one of the most important physical phenomenon's known to present day astronomy, the Doppler effect. "A change in wavelength proportional to any line-of-sight velocity between observer and the source." In simplified terms, the color of many celestial objects directly approaching or directly receding earth will shift towards red or blue depending on their direction and speed. This change is commonly called redshifting or blueshifting and allows astronomers to calculate the speed and direction of the objects in space.
To better understand the Doppler effect it becomes necessary to consider a general definition of what light is and how astronomers determine colors. Light visible to the naked eye or not, is a from of electromagnetic radiation that carries energy outward in all directions form a given point. A star for example, is radiating energy (electromagnetic radiation) outward from it's mass in the form of fluctuating electric and magnetic fields we call waves. To calculate the speed of these waves we depict a continual S-formation maintaining a maximum parallel top and bottom in the formation. We then measure the amount of time it takes for each wave to re-establish its maximum height or depth, which is labeled a wave cycle. Radio, infrared, ultraviolet, X-rays, gamma rays and visible light waves all have a different wave cycle allowing astronomers to label their properties. In short, all colors, visible or not, have a unique wave pattern that can be viewed by a process called spectroscopy. Using this process, we evaluate all colors by their signature wave patterns and even decide what elements the source objects are composed of.
To get a practical idea of how these wave cycles work here on earth, imagine that you are in a very small boat in the ocean anchored several miles from shore. The wind is blowing somewhat hard and the brisk crests of waves are continually hitting the bow of your boat about every two seconds. You become tired of this annoyance and decide to return to land but must steer in the direction the waves are coming from to return. As you build up speed into the waves you notice that the wave crests are now hitting your bow twice as fast. (About 1per second) This proves to be annoying so for an experiment you choose to direct your boat away from the direction of the waves. Now you notice that the bumping wave crests have decreased dramatically and only hit your bow approximately every four seconds. The problem is that you are now heading in the opposite direction from shore. Finally you conclude that you will just have to put up with the increased frequency of wave crest bumps (wave cycles) in order to reach your destination. Just as the water crest cycle increased as you traveled in their direction, light wave crest cycles increases when you or the source begins moving towards each other.
Astronomers and scientists have cataloged millions of wavelength cycle patterns and placed them in huge databases for reference. The term "frequency" is used to measure the number of times a complete wave cycle intersects a certain point per second. The result of this computation is normally labeled "Hertz", named after a German scientist Heinrich Hertz who experimented with properties of radio waves. In the example above, that anchored boat was receiving a complete wavelength cycle (frequency) every two seconds. That equates to a frequency of .5 Hz. Per second. (Hz. Is short for Hertz). As the boat powered in the direction of the wave crests it was receiving wavelength cycles of 1 per second or 1 Hz. per sec.
Another example of increased or decreased wave cycles here on earth is the sound of a horn of a fast moving train. If you are standing near a railroad track the sound of a train's horn changes dramatically as it passes by. As the train blows it's horn, the wave lengths frequency is constant but the train is physically rushing them towards you resulting in an increase of complete cycles hitting you per second. The result is a continual increase of frequency while it is approaching and a continual decrease as it is receding. In space however, wavelength crests are not that sensitive to change.
Unlike sound and water waves, light radiation does not require a material medium to travel. Electromagnetic radiation waves travel through the vacuum of space at a specific speed. The speed of light travels at the approximate rate of 300,000 km. per second no matter where you are in space. In the length of time it takes you to blink your eye, light could travel about one third of the way around the world. Since the speed of light is constant in space, it provides a very convenient way of tracking the speed and direction of a source object provided that is moving towards or away from us. For example, a certain source object in space has a known spectral wave signature and is receding the observer. It would have a slightly longer wavelength because it is physically moving away from the observer and appear redshifted. That is to say that the color of the source object would be more of a reddish appearance than one would expect. The exact opposite is also true for objects in space that are traveling in the direction of the observer. It would be blueshifted and appear to have more of a bluish cast than normal. It should be noted here that the observer of a Doppler shift couldn't tell whether it is the source or his observation point are moving. Because of this fact, several know calculations must be performed before determining the nature of the movement. Since the earth orbits the sun, it is in effect traveling towards and away from a source object at different times of the year causing a variation in color.
One of the most noticeable examples of the Doppler effect can be seen in a binary star. A binary star really consists of two stars, one orbiting the other. As the orbiting star travels in its path it becomes redshifted and blueshifted depending on it's direction in relation to earth. The actual change in color alerts astronomers to the direction and speed of both stars. As a side note astronomers can also determine the mass or size of the star by its orbital period. Since the redfhifting and blueshifting will be a repetitive occurrence it is easy to determine the time length of orbit.
Astronomers use the Doppler effect to discover speeds and directions of many celestial objects just by looking at their colors. Even distant galaxies and the expansion of the universe itself are continually monitored for change and conformation. A planet could be tugging a distant star closer or further from earth and it would be impossible to discover if it were not for its change in color.
There is one catch to this would be perfect measurement, the angle. Evidence of redshifting and blueshifting will not be present if we are perpendicular to the plane of orbit. Reconsider the example of the small boat and the ocean waves. The waves hit the bow faster or slower depending on the direction and speed traveled by the boat. But what if the boat was traveling parallel to the direction of the waves? Almost no effect would be felt in either parallel direction the boat traveled as it moved. It is for this reason that the Doppler shift cannot be measured in objects that are at 90-degree angles from each other.
Although it is currently impossible to go fast enough here on earth to notice a Doppler shift, it is interesting to note that the Arizona Highway Patrol seems to be somewhat advanced in their thinking. Imagine that you have a futuristic car that can travel faster than the speed of light. Therefore both you and the observer are experiencing redshifting and blueshifting. It would not matter whether you are accelerating away from, or the patrol officer was accelerating on your position as he attempts to stop you. On the top of his car are two emergency warning lights that are conveniently colored red and blue.