Umbrella decorated with the constellations.

What it shows:

The aberration of starlight is the shift in the position of the image of a star due to the rotation of the Earth about the Sun, and is a consequence of the finite velocity of light. For a star directly overhead, a telescope will have to be angled by v/c to the vertical where v = velocity of the Earth in space, in order that the telescope be pointing at the star. The equipment necessary to show aberration is one umbrella.

How it works:

The best way to picture this effect is standing in the rain. If the rain falls vertically and you are not moving, you need to hold your umbrella directly above you to keep dry. But now start walking; the combination of your forward velocity v and the vertical velocity of the rain c, means that to keep dry your have to hold the umbrella ahead of you, at an angle of v/c. As the Earth moves through space at 31kms-1 describing a (roughly) circular orbit around the Sun, a star at the pole of the ecliptic will describe a small circle about the pole, and a star in the plane of the ecliptic will describe a straight line with an extension of ±20.49 arcsec. Between these two extremes, an ellipse is described.

figure 1. walking in a vertically falling rain shower - an analogy for the aberration of starlight.


To perform this demonstration, you simply need an umbrella (see Setting it Up) and an orbital path, such as a taped circle on the floor of the hall. If you can supply rain, so much the better.

Setting it up:

Although any umbrella will do, we have an Astrobrella® 1 , a large clear plastic umbrella decorated with the major constellations in fluorescent paint. This is used in conjunction with a 2m diameter circle taped to the floor.


This effect was discovered in 1725 by Astronomer Royal J. Bradley while attempting to measure stellar parallaxes. Rating**

1 Obtained somewhere in England for Harvard by Prof. Owen Gingerich