Seeing the sky

One or two generations ago, you could find people with the ability to name stars in the sky, tell time with them, find directions, and even gauge their latitude; but in this era few people can point to and name a single star. In my work with students, I’ve come to believe that the ability to ‘see’ something often begins with the knowledge that a person can see that thing. 

Here is one example:  a couple of students from the Harvard Graduate School of Design submitted an art proposal to the Radcliffe Institute for Advanced Study as part of a competition for an outdoor sculpture.   The proposal was to display stars that were no longer visible in Cambridge due to light pollution.  The judges were happy with the proposal and invited me to view their work as I teach star identification in my class.   When I saw their write-up, however, I was startled. The stars that they named as missing-in-action were the very same stars I teach my students to identify from the roof of the Science Center on clear nights.   Had the GSD students even attempted to find the stars they said were lost due to light pollution?  Doubtful, but maybe it was simple case of a self-fulfilling prophecy – the stars cannot be seen because they didn’t think that they could be seen. 

This above tale gives credence to the notion that much of the world in front of our eyes goes unnoticed.   If we cannot ferret out truths that are in front of our eyes, how can we verify more distant and abstract propositions?  Can the knowledge that we can see something help us to see it in ways that were previously hidden?   How do we see the sky?  How could we see the sky if we knew how?  Here are some different, literal, perspectives.

Some people think of the sky as a dome.   Perhaps this is due to how we view it – swiveling the head left, right, back, and forth – this rotation allows us to take in the stars from the horizon to the zenith and across all the compass points.  A part of the brain responsible for processing the head direction, the anterior thalamic nucleus, has connections to other parts that allow us to develop a sense of spatial orientation. The head motion maybe becomes ingrained? 

In Ancient Egypt, the sky, the night sky in particular, is associated with the got Nut – a woman.  She is depicted in an arched position, with her arms and legs forming the pillars of heaven, and touching the four cardinal points of east-west-north-south.   Her arching body is the vessel of the heavens.  In many cultures, the sky is seen as something that’s held up by pillars, as if it were a tangible thing that has weight.   People have difficulties with the concept of space in general, seeing it as something that must resemble the earth in some form. 

Figure 1 - The Egyptian god Nut

I asked the students in my Freshman Seminar how they viewed the sky, expecting that they would say it was a dome, like I did.   Much to my surprise, the said that it was rectangular, shaped like a shoebox in their minds eye.   At first it was one student who offered up this thought, but then I went around the room and all ten students agreed that they visualize the sky as shaped as an upside-down shoebox.  The long axis of the shoebox runs north-south.

Although I was first surprised by this, but I then remembered that in Kiribati, the sky is described as an A-frame roof of a hut, with the top beam running north-south.

The sky-as-dome visualization is central to modern celestial navigation, where the positions of stars are labeled with two coordinates: altitude and azimuth.  The horizon is treated as a plane – locally, which makes sense.  Altitude is the angle from the horizon up to the star on the ‘dome’.  Azimuth is the angle along the horizon, starting with north as zero, and going clockwise to 360^o.    To a good approximation, this ‘works’ in the sense that stars are so far away, the change in their position as the earth orbits the sun is quite small and can only be discerned with precise telescopes.    

Figure 2 Altitude-azimuth system.

Even though I know in my rational half that space is truly three-dimensional and the earth is a sphere, I mostly visualize the sky as a dome and the earth as flat, not perceiving any curvature. I had, however, three “ah-ha” moments when my visualization changed.  This is not to say that I learned something new, I saw things in a different way.

The first ah-ha moment happened when I was out in the mountains of the coastal range in northern California.  It was summertime, and the Milky Way was visible, as a diagonal slash in the sky.   I tilted my head sideways and saw the disk of our galaxy more or less level with my eyes.   Although this is not precise, there is a relation between the brightness of stars and their distance.   There is a well-known relation between the brightness of stars and distance: brighter stars are closer on average, and stars farther away are dimmer.   The ‘milk’ in the Milky Way comes from a huge density of distant stars that blurs into a continuum.   Although I knew all this, I couldn’t ‘see’ this.  However, on this particular night, I tried to shift my perspective and my view turned three-dimensional  - the sky was no longer a dome, but instead was a vast panorama – bright stars just in our neighborhood, but their numbers growing higher and higher and dimmer off into the distance.  The most distant stars finally merged into the continuum of the white cloud that is the Milky Way.  The sky had ceased to be a dome and had not only an extra dimension added to it, but lost the dome-like shape I always envisioned.  In addition, I no longer perceived the earth as a plane, but truly felt that I was on a sphere peering out into the far distance.   With some effort, I can regain this perspective if I work at it.  

In the 1920’s there was a debate between the astronomers Harlow Shapley and Heber Curtis about the nature of the universe.   Shapley believed that the Milky Way was the entirety of the universe, whereas Curtis believed that the Milky Way was one of many galaxies – kinds of spiral nebulae that had been seen through telescopes, like the Andromeda galaxy.  In some ways, the perception that we were inside one of those spiral nebulas helped Heber’s perception. The implication of Heber’s hypothesis is that the spiral nebulae would not be in our local neighborhood, but a vast distance away.  This prediction was confirmed by astronomer Edwin Hubble who used a kind of variable star called a Cepheid, as a distance marker. 

Figure 3 Milky Way

Figure 4 Andromeda galaxy

My second change of perspective was when I could visualize the ecliptic in the sky.  The ecliptic is a plane where the sun, moon, and planets move.   They’re restricted to this plane due to physics:  the solar system evolved out of a flat disk of dust that coalesced into the planets.  The planetary orbits are restricted the same region the flat disk once occupied. Now, the word ‘planet’ comes from Ancient Greek for a ‘wandering star’.  

The motion of the sun, moon, and planets is quite complicated to see from the earth’s surface and may very well be why it took so long for the emergence of a sun-centered solar system.  The earth orbits the sun, with its rotational axis tilted with respect to the plane of its orbit. Already, this is tough to visualize.  Seen from a fixed earth, celestial objects appear to rotate in paths through the sky at a speed of 15^o per hour.  This is due to earth’s rotation.   In addition to this, the relative position of the sun with respect to the stars shifts by about 1^o per day, with the stars moving east-to-west.  This second effect is due to the earth’s orbit. It takes quite some time to be able to actually perceive this motion.   When we add in the motions of the planets, things really become complicated.  Like the earth, the planets also orbit the sun, in a plane. When seen against a fixed background of stars, the sun, moon, and planets all appear in the same plane.  The stars along that plane are the Zodiac.   Visualizing the plane of the ecliptic can be difficult and was one of the motivations for a device called an armillary sphere: a model used to teach students the motion of the planets, and shown below.

Figure 5 Armillary sphere

In the armillary sphere, the paths of the planets are indicated as a ring that’s tilted with respect to the celestial equator.   The ecliptic is marked by the constellations of the zodiac – e.g. Pisces, Aries, etc.    For the model of an earth-centered universe, the planets are all restricted to this ring.  

For me, for the longest time, the planets just seemed to be randomly placed in the sky, with no order. I knew what the solar system was, how it worked, but I couldn’t see it in the sky. Slowly, however, I began to learn the stars, and constellations, I started to become more adept at catching the planets and recognizing the ecliptic.  This was more of a rational process and not an immediate vision of the ecliptic in the sky.   Then one night, I looking at the sky and there was an alignment of the Sun, Moon, Venus, Mars, Jupiter, and Saturn, all strewn across the sky.  Knowing that this “marked” the ecliptic, I could ‘see’ the plane of the ecliptic in the sky for the first time, and could understand the motions in an intuitive sense.  Like my view of the Milky Way in three dimensions, the solar system and its motions became apparent in a way that I hadn’t seen before.  

The third change in sky perception came to me on a recent trip to Chile.   Again, I knew things in principle, but seeing/perceiving them is another matter. I know that the stars I see to the south in the Northern Hemisphere would look upside down when viewed to the north from the Southern Hemisphere.   It’s quite one thing to know about this perspective, but quite another to actually experience it.  At 51^o south latitude in Patagonia, the familiar stars: the Pleiades, Aldebaran (Taurus), Rigel, Betelgeuse (Orion), Sirius (Canis Major), and Procyon (Canis Minor) were flipped upside down and left to right.   It took me awhile to get adjusted to the view, and I had to step rationally through it to be able to again ‘see’ the sky, but eventually I did.   What this particular exercise brought home to me was the perceptions that Polynesian navigators may have had with long-distance voyaging. The information gleaned by these navigators from the stars were less step-by-step practices but a more intuitive vision of the sky.

Figure 6  Orion as seen from the Northern Hemisphere, looking south

Figure 7 Orion seen from the Southern Hemisphere, looking north