Friday, April 13, 2012
The Wondrous Night Sky
When my daughter recites twinkle, twinkle, little star, I am tempted to answer her question regarding what they are: "Giant collections of particles moving in all directions colliding into one another at high speeds forming larger particles in fusion reactions that give off heat and light..." And then I remember that she turns three this summer.
I got to thinking about stars when a couple of callers rang me up the other day on my call-in show. OK, I don't have a call-in show. It was just a couple of friends calling me at home with questions about stars. They were either genuinely curious about these glowing masses or were poking fun at my passion for all things science. I am going to assume it was genuine curiosity, and answer their questions below.
Why can't we see stars during the daytime?
The cheeky answer to this is that we can. We see one star in particular, the Sun. So close is it to Earth as compared to all other stars (283,000 times closer than the next closest star, Proxima Centauri) that its apparent luminosity, aka brightness, is so much greater than any other. For the same reason that sun rays through a window make the image on an indoor screen hard to make out, when the Sun rises, and peers through our atmosphere, it outshines all other stars, making them next to impossible to see with the naked eye. The other stars do not go anywhere by day, they just get overpowered by the Sun, much like when a quiet teacher speaks in a loud classroom.
A tag-along question to this one might be, "Why is the sky blue during the day?" Many people believe this is due to the "blue" water that covers the Earth, but the causality in this situation is actually the reverse. Water itself is colourless, and appears blue because it reflects the sky's colour. The blue colour we observe in the sky is the result of the particular make-up of our atmosphere.
The Sun emits white light, that is, light consisting of all colours in the visible spectrum. The molecules in our atmosphere allow all of these colours to pass with the exception of blue. Blue reflects upwards, and is projected to form the blue backdrop we are all familiar with.
If the atmosphere of the Earth were to disappear, there would be a black sky by night, but also by day. Well, that, and we'd all be dead.
To what extent does the visual landscape of stars change over the course of the day/year?
First, it is worth considering just how many stars we can actually make out with the naked eye on a given night. In the absence of light pollution and any clouds, it is possible to see about one thousand distinct stars. These stars are some of the nearest to us among the billions in our galaxy. We can see just our neighbourhood of the galaxy - a small chunk of the outermost spiral of the Milky Way. Still, one can't complain. The view of one thousand distinct stars at once is magical. To a city dweller, who is accustomed to a night sky with ten or twenty scattered stars, the sight of one thousand stars at once is truly breathtaking - it is surely one of the highlights of a wilderness camping trip far from the city.
But, does this starry picture with its familiar constellations change? It really depends on where on Earth you are. We must keep it mind that at any given time, the Earth blocks half of the universe from our sights.
The universe is very dynamic. Galaxies are separating from one another, while, simultaneously, stars orbit about the center of their respective galaxy, while, simultaneously, planets orbit around their governing star, while, simultaneously, each planet spins about some axis. A lot is going on at once.
The fact that galaxies separate from one another does not affect those stars that we see with our eyes, as they all reside in our galaxy. That our solar system and billions of others revolve around our galaxy's center also does not affect our night sky view to a noticeable extent. It takes more than 225 million years (Earth years) for our solar system to make one full revolution around the Milky Way core. Put otherwise, our solar system sweeps through less than two degrees of its 360 degree orbit in one million years. From day to day or year to year, the change in the relative angles between our solar system and other stars in our galaxy is extremely minute.
Due to this, we can actually consider the stars to be static with respect to our solar system when it comes to viewing them. Then, the night sky picture becomes exclusively dependent on the position and orientation of the Earth within its solar system, and where on Earth you so happen to be standing.
The Earth makes one revolution around the Sun in one year, but what distance does it actually cover? The Earth's orbit is nearly circular (very low eccentricity) with a radius of 150 million kilometers. As such, the maximum discrepancy in distance from one location on the orbit to another is the diameter of this near circle, or 300,000,000 km. This may sound like a large distance, but it is just one 240,000th of that separating the Earth and Proxima Centauri. Therefore, the Earth's orbital position around the Sun does not affect the angles between the Earth and other stars; it merely determines what direction a given spot on Earth is facing when it happens to be night time there.
Imagine you live on the axis of rotation of the Earth - let us choose the Geographic North Pole. The night sky that you see is unchanging. All that changes throughout the year is the length of time that the night sky is visible. During the winter, the Sun is never visible, and the other stars are always visible. Throughout the summer, the reverse is true.
Now, pretend that you live somewhere on the equator, like Uganda. Every day throughout the year, independent of the season, the sun dominates the sky for precisely the same amount of time. However, the direction that your view of the sky points at night changes with the season. The night sky you see tonight will be entirely different from the one that you saw one half year ago. If a variety of constellations is what you like, then the equator is the place to be. There, the night sky is a constantly evolving landscape. Even over the course of a single night, some stars pop into view as others disappear over the horizon.
Most of us live somewhere in between these two extremes, and thus both the degree to which the night sky changes and the ‘length’ of the day are variable. In the northern hemisphere, many northern stars are night sky fixtures throughout the year (take the Big Dipper constellation, for example), although their positions in the sky do alter to a certain extent. The particular direction that the Earth faces at night is dependent on the season, and the extent of the change in direction is greater at lower latitudes.
New York and Rome are two cities at approximately the same latitude (about 41 degrees). Their longitude, however, is different by about 90 degrees, and so, while the night sky appears the same from either city, it is experienced about six hours apart.
In summary, the night sky varies more the closer you get to the equator, and is dramatically different at high southern latitudes as compared to high northern ones.
This appears to have been a long answer to a simple question, but such is often the case when it comes to astronomy. I hope it satisfies my friends' respective curiosities and those of my readers. Please don't hesitate to phone in to my call-in show.