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The Question

(Submitted October 29, 1996)

Hi,

  1. When a star (or light source) is moving away from you, does it emit blue or red light; and what about when it is moving towards you?
  2. With respect to the autumn and spring solstices: Do we get more "red" light in spring or autumn, more blue light .... etc?
We're just having a discussion at home and would like to clarify a point.

The Answer

  1. The fact that a star, or any light source, is moving toward or away from you does not affect what it emits - it affects what you (as the observer) perceive it emitting.

    The apparent shift of light toward the red when the emission source is moving away from us, or toward the blue when the emitter is moving toward us, is called the Doppler shift. Let us take a minute to try to explain what this is.

    Light consists of fluctuations, or waves, of the electromagnetic field. The wavelength (or distance from one wave crest to the next wave crest) of light is extremely small -- for visible light it ranges from four to seven ten millionths of a meter. The different wavelengths of light are what the human eye sees as different colors. The longest wavelengths appear in the red end of the spectrum and the shortest appear in the blue end. Now imagine a source of light at a constant distance from us, emitting waves of light at a constant wavelength. Obviously, the wavelength of the waves we receive will be the same constant wavelength at which they are emitted by the source. Suppose now that the source starts moving directly toward us. When the source emits the next wave crest, it will be nearer to us, so the distance we will see between the two wave crests arriving will appear to be smaller than when the star was stationary. This means that the wavelength of the waves we receive will be shorter (or shifted toward the blue end of the spectrum) than when the source was not moving. Similarly, if the source is moving away from us, the wavelength of the waves will appear slightly longer, or shifted toward the red end of the spectrum. The relationship between wavelength and speed is called the Doppler effect. We experience it every day -- like the engine sounds of a car approaching us having a higher pitch...and a lower pitch when the car moving away from us. Light and sound are both waves, so they both exhibit the Doppler effect.

    The Doppler effect is named after the Austrian scientist Christian Johann Doppler. He first predicted it must occur in a scientific paper he wrote in 1842.

  2. The summer solstice occurs about June 22 and the winter solstice around December 22. These 2 events are just special points in the orbit of the Earth around the Sun. On the summer solstice, the Sun shines most directly upon the northern hemisphere. On the winter solstice, the Sun's rays shine most directly on the southern hemisphere. This happens, of course, because the Earth is tilted 23.5 degrees on its rotational axis. So the plane in which the Earth revolves around the Sun is not coincident with the plane of the equator. The result is that the northern hemisphere is tilted toward the Sun in in June and away from the Sun in December.

    Given the orbit that the Earth takes around the Sun, we are moving slightly away from the Sun starting in January and we start moving slightly toward the Sun in July. However, given the tiny velocity with which these movements take place (relative to the speed of light), any Doppler shift this induces is imperceptible.

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