A magnificent rendering of the Mediterranean winter sky crowns the concourse of Grand Central Terminal in Manhattan. It's a 1940s reworking of the original one designed by Paul César Helleu for the terminal's 1913 opening. Helleu consulted a well-known astronomer, but a month after the station opened, a starwise commuter claimed that the sky was backwards.

Scrap the skylights – paint a skyscape
The original plan called for skylights over the concourse, but that was scrapped. Yet in keeping with the grandeur of the rest of the building, the ceiling needed to be decorated. Helleu was asked to design a skyscape. A ceiling full of stars wouldn't have impressed, but the mythological figures of the constellations could make a splendid design. Helleu turned to Columbia University astronomer Dr Harold Jacoby.

Helleu's ceiling deteriorated badly over the decades and was replaced with one of almost the same design by experienced mural painter Charles Gulbrandson.

The view from Greece
Although we can't be teleported to the Mediterranean, the planetarium feature of the website Heavens-above.com can show us the stars. The Grand Central mural represents the autumn/winter Mediterranean sky, and Greece seemed an appropriate place to see the ancient constellations. So I've chosen a sky map from Athens on December 21, 1900 at 10.00 p.m.

We're looking to the south towards the ecliptic, the apparent path of the Sun through the background of stars during a year. It's shown by the curved purple line on the star map. (The other purple line seems to be an artifact of the planetarium program.) The zodiac constellations are the ones through which the Sun seems to move during the year. The celestial equator, the projection into the sky of Earth's equator, isn't shown on this map.

A star map is somewhat different to a road map. You're looking up at the sky, so the map needs to represent half a sphere. The stars directly overhead are in the center of the map, and the 360 degrees of the horizon form the edge of the map. Compared to a road map, east and west are reversed. This is because you orient the map towards the part of the sky you're looking at. If you're facing south, east is to your left and west to your right.

Our star map shows six zodiac constellations. Reading from left to right, Cancer (the crab) is low in the east. Next is Aries (the ram), then high in the sky are Gemini (the twins) and Taurus (the bull), followed by Pisces (the fishes), and Aquarius (the water carrier) would be low in the west.

There are three other constellations of interest on this map. Orion (the hunter) is below the ecliptic, but is on the celestial equator. Having crossed the ecliptic between Aquarius and Pisces, the celestial equator goes through the right-hand star of Orion's belt. Pegasus is above the ecliptic near Aquarius and Pisces. The small constellation Triangulum is just above Aries.

The mural sky
What does the Mediterranean sky look like in Grand Central Terminal? To be blunt, as the commuter said, it's backwards. If you look up at the ceiling, as you would the sky the zodiac sequence is reversed. It starts with Aquarius in the east and ends up with Cancer in the west.

With reference to the mural's reversed zodiac, Orion and Pegasus are in about the right places. Triangulum is shown as two triangular constellations, and there is a small nebulous figure between Aries and Triangulum, which was not on the star map. Polish astronomer Johannes Hevelius (1611-1687) introduced the second triangle, calling it Triangulum Minor. It appeared on some star maps, but is no longer used. The small blob is another obsolete constellation. It was called Musca (the fly), and has since been absorbed into Aries. The existing constellation Musca is in the southern hemisphere.

The figures representing the constellations in the mural are based on the drawings in the star atlas Uranometria by Johann Bayer (1572-1625). However the figures are – except for Orion – facing in the opposite direction to the way in which Bayer drew them. It's more like the configuration Hevelius used. Hevelius followed the convention for celestial globes, with everything reversed right to left.

Celestial globes and the celestial sphere
Long ago people thought that the stars were located on a celestial sphere surrounding the Earth. A popular way of representing the sky was on a globe, but of course the outside of a globe wasn't the Earthly view. It showed “God's view” from outside the sphere. This is why the convention for such globes is the reverse what we would see in the sky.

Hevelius notwithstanding, the reversal is obviously not useful for sky observers. As in Bayer's atlas, other major star atlases represented the constellations as they would look to us.

Backwards? How did that happen?
The only constellation which isn't turned the opposite way to Bayer's drawing is Orion. Even that isn't as puzzling as the fact that Bayer drew Orion the wrong way around in the first place. The stars are in the right places, but Orion is holding his club in his left hand, looking away from Taurus, and menacing Gemini. John Flamsteed and Johann Bode, amongst others, showed the club in his right hand, his shield between himself and Taurus.

We don't know why Bayer's Orion is reversed, or why the ceiling mural is reversed, except for Orion. Explanations abound, none entirely convincing, but in the final part of this story, we'll look at some of them.