Terminal Badness

Here's where the Bad Astronomy comes in. The sign shown here is a note to commuters on the original painting. The sign says "Grand Central's famous zodiac ceiling depicts a Mediterranean winter sky with 2500 stars. Said to be backwards, it's actually as seen from a point of view from outside our solar system. You can watch the stars come out as we clean the ceiling in sections with a simple solution of soap and water." (emphasis is mine)

This is so bizarre it's hard to know where to start.

The sign is quite wrong for two main reasons: stars are at all different distances from the Earth, and they have different intrinsic brightnesses. If stars were embedded in a giant transparent sphere a few billion kilometers in diameter, then you certainly could travel outside it, turn around, and see the constellations backwards. But they aren't; what looks like a flat painting of stars on a canvas is actually a three-dimensional tapestry of stars at varying distances, the least of which is still tremendously far away. If you go just outside the solar system, the stars will look largely the same as they do here on Earth. Stars are very far away, much farther away than any object in the solar system. You would have to travel light years before seeing any change in star positions, and the solar system is tiny indeed on that scale. You could travel all the way across the solar system to chilly Pluto, and the stars will look exactly the same to the naked eye (although the Sun will look only like a very bright star-- very bright, about 100 times brighter than the full Moon as seen from the Earth).

Look at the constellation of Orion, shown on the left as seen from the Earth. There are eight or so bright stars that make up the constellation, all of which are at different distances from us. You cannot simply travel a thousand light years to the "opposite side" of Orion, look back, and see it backwards (as shown on the right)! The stars are different brightnesses intrinsically. In other words, Rigel, the bright blue star at the bottom right of Orion (his "left foot", as he is usually depicted facing us), is one of the most luminous stars visible to the naked eye. On the other hand (haha), the star at the bottom left of Orion (his right foot), called Saiph, is only a fairly luminous star. Although much less luminous than Rigel, Saiph is one-fifth as far, so it looks about as bright as Rigel. Say you were to travel to the "other side" of Orion, going as far behind Rigel as Rigel is from the Earth. Rigel will look the same brightness as it does from here, but Saiph will be about 1/100^th as bright! It would not be visible to the naked eye. All of the stars in Orion would look different at that distance, since they are all different brightnesses.

Another, more subtle, problem is that as you move around, the stars will appear to change position. Just as nearby trees appear to move past you faster than farther away trees when you are in a car, nearby stars would appear to move more quickly as you magically fly around to the "back side" of Orion. This would change the relative positions of the stars, which in turn changes the shape of the constellation.

So you cannot go "outside the solar system" and look back, seeing the stars backwards. I suppose the people who wrote that sign (or perhaps the original artist who painted the ceiling) thought that stars are painted on a crystalline sphere a few billion kilometers in diameter and centered on the Sun. In a few years, perhaps, the Voyager probes will collide with it, smashing the sphere into millions of falling stars.

To be fair, it has been pointed out to me that old maps (say, before 1500 AD or so) showed the sky backwards, to represent "God's view". So it's possible that the ceiling was painted to represent that. However, the sign posted does not mention it, and merely states as fact that the sky looks backwards outside the solar system. I may be willing to give the artist some license, but not the person that wrote the sign!

I wonder... how many thousands of people see that sign every day during their commute? Nodding their heads, they think to themselves, "Oh, I get it!", but actually they are only more victims of Bad Astronomy.

I found a wealth of info on the web about constellations. Chris Dolan maintains an excellent site with many links to other pages.

A great page about astronomy with lots of constellation drawings (it's where I got the (original) Orion image above) can be found at The College of Saint Benedict and Saint John's University Thanks to Tom Kirkman for his permission to use the drawing of Orion, and for his comment about the backwards sky on old maps.

My special thanks to Bad Reader Thomas Fenton for suggesting this page and for sending the great picture of the sign!