Science in the Sky

Recently Coastal Carolina University, my employer, released a bunch of wallpaper designs to promote the University.  Admittedly there’s some pretty cool designs, especially those that feature Chauncey the Chanticleer.  There’s one particular wallpaper that really caught my attention

Of course it caught my attention.  It’s astronomy related.   Besides, my favorite color is purple.

Overlooking the fact that there’s no Chauncey constellation, there are a number of notable errors with this wallpaper that drive me crazy.  Let’s start with the stars.  If you look at a star through a telescope then it’s true that they usually have spikes radiating away from the central star.  Take a look at this image of the Pleiades (also known as the Seven Sisters).

Notice how the stars have four spikes emanating from the stars.  Also, the spikes are more pronounced for the brightest stars.  The origins of these spikes actually has to do with the type of telescope used to view the stars.

A reflecting telescope works by bouncing light off a larger primary mirror at the back of the telescope up to a smaller secondary mirror toward the front of the telescope.  The secondary mirror is held in place by what’s called a spider. A typical spider has four vanes.  Shown to the right is a typical reflecting telescope.  As you look down the barrel you can see the primary mirror at the bottom, while the spider is the crosshair-looking device toward the opening.  As the starlight passes through the telescope it will diffract around those vanes.  In other words, the light is deflected from its ordinary path.  To make a long story short, this change in path is made evident by the four diffraction spikes we see in pictures of stars, like what’s shown above for the Pleiades.  The issue is more pronounced for brighter stars because more starlight is diffracted.

This brings me to my first point of contention with the wallpaper.  If you look at the stars, many of them have diffraction spikes.  However, diffraction spikes are an artifact of looking at stars through a telescope, specifically reflecting telescopes.  The wallpaper image covers a very wide field of view, and it includes a foreground palmetto tree and grass, which altogether implies that this view does not represent what someone would see looking through a telescope.  So why do some of the stars have diffraction spikes?

Problem two with the Chauncey Constellation wallpaper are the “shooting stars” seen on the left side.  I purposely called them shooting stars because in the wallpaper it’s clear from the diffraction spikes that these are meant to be stars.  First off, shooting stars have nothing to do with stars.  They’re actually small pieces of debris that is flying around in our Solar System.  Second, when a shooting star passes through our atmosphere—which is what astronomers call a meteor—it’s path is not curved like that shown in the wallpaper.  It’s mostly straight with maybe a slight arc.  Check out the August 16, 2010 APOD shown below, which shows a meteor shower over Quebec.

Notice how the incoming meteor trails are straight, not curved.  So what’s the deal with the highly curved path of the “shooting stars” on the wallpaper?

Now for my biggest beef with the Chauncey Constellation wallpaper.  Take a close look at the Moon.  Inside the crescent moon you can see stars!!!  This would imply that the Moon physically changed shape during the course of its lunar phase cycle.  Sometimes its full.  At other times it looks like a boomerang, or a semi-circle, and at yet other times a bloated semi-circle.  In reality the lunar phases are due to the Moon’s position relative to the Earth and the Sun.  At any moment half of the lunar surface is lit up by sunlight.  Since the Moon orbits the Earth, how much of this lit half is visible to Earth observers changes.  A crescent phase, like the one depicted in the wallpaper, occurs when the Moon has moved in its orbit just a bit past the point where it’s aligned with the Sun’s direction.  The image to the left shows a photograph of a real crescent Moon.  Notice that the dark side of the Moon is actually visible.  This is due to light that has reflected off the Earth, traveled to the Moon, and returned back to the Earth for us to see—an effect known as Earthshine.  More importantly it’s not transparent.  In my Astro 101 class we spend a considerable amount of time discussing the lunar phases.  It’s a difficult topic but I wouldn’t expect any of the students to make such a grievous error as shown in the wallpaper.

So I have to ask the question, where was I when this wallpaper was made?  Why wasn’t I consulted?  I know the purpose of this is particular wallpaper is to be fanciful but geez, let’s get some things right.  Especially the part about the Moon.

By the way, even with it’s flaws, I’m still using this as my wallpaper on two computers and my iPad.