Constellation Changes

Science fiction spaceships zoom to the farthest stars. If we could travel across light-years, the stars would shift position around us. A reader asks how that would alter the constellations?

Bill Dockery in Tennessee asks, “How far would you have to travel through space before you noticed a change in the shape of the constellations?”

If we could race across light-years, close stars would appear to shift across our field of vision more quickly than distant stars. You’ve seen this happen in a car — nearby trees whip past, while a distant building shifts slowly against the background.

The brightest star in Earth’s sky, Sirius, is only nine light-years away. Its location would shift relatively quickly with respect to more distant stars. Most of the stars in the Big Dipper lie about 80 or 100 light-years away. The Dipper pattern would become unrecognizable some 25 light-years from Earth. The stars of the constellation Cassiopeia lie between 100 and 600 light-years distant. That constellation would distort before 100 light-years.

Now, Bill, consider the nearest star besides our sun, Alpha Centauri, only four light-years away. Only an expert stargazer would notice differences in the constellations seen from there — with one exception.

In Earth’s skies, Cassiopeia is a distinctive five-star zigzag. In Alpha Centauri’s skies, Cassiopeia has six stars with the sixth being our sun. Great question, Bill.

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