Imagine that you are suspended at a point in space, well protected from cosmic radiation, camera at hand.
93,000,000 miles behind you, the furnace that is our Solar System's Sun burns. And burns. Its surface temperature is about 10,000 degrees F (5500 degrees C). At its core, 27,000,000 degrees F (15,000,000 degrees C). Note: the three prevalent temperature scales used commonly in science ~
- Farenheit (defined by two fixed points - the temperature at which water freezes into ice is defined as 32 dF (degrees Farenheit), and the boiling point of water is defined as 212 dF, with a 180-degree separation, as measured at sea level and at normal atmospheric pressure.
- Celsius (once called Centigrade - commonly defined by two fixed points, with 0 dC (degrees Celcius) being the freezing point of water and 100 dC being the boiling point of water. The 100-degree separation, being more easily, infinitely divisible by 10 than is the Farenheit 180 degree separation, makes the Celsius scale an measure of temperature in the Metric System with its standard set of interrelated base units and a standard set of prefixes in powers of ten.
- The Metric System is the official system of measurement in almost every country in the world. The United States, Libya and Myanmar remain the only natons which cling to antiquated systems of measurement such as the Imperial system.
- Kelvin (defined as an absolute scale using as its null point being Absolute Zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. One dK (degree Kelvin, or simply one Kelvin) is defined as the fraction 1/273.16 of the temperature of the triple point of water. (The Triple point of a substance is the temperature and pressure at which the three phases (solid, liquid, gas) coexist in thermodynamic equilibrium. Note how closely water's Triple Point approximates its freezing point. The Kelvin is the primary unit of measurement of temperature in the physical sciences.
Confused? Here's a comparison chart which should help your understanding of how the three scales relate to each other ~
That still plenty of light for humans, with our Earth-evolved eyesight, to sense, and to make sense of the objects we see.
Speaking of objects, recall that you are suspended in space, the sun at your back. Now imagine that before you, perhaps a million miles away, the Earth rotates about its axis (the velocity of rotation producing our familiar periods of day and night), and revolves in its orbit around the sun (one revolution defining our year). Since you "stand" directly between Sun and Earth, the entire Earth surface facing toward you is illuminated for your viewing pleasure.
But wait. What's this? (See Image below, click to enlarge) It looks a little like the 'Star Wars' Death Star. In fact, it is our very only Luna, Earth's only moon. And the size proportion between Moon and Earth is actually quite close to how both bodies compare in size to each other in real life. No! No?
Earth's diameter is about 7,917 miles. The moon's diameter is about 2,159 miles. Do the math ~ in diameter (not circumference, not volume, not mass) the moon has about 1/4 the diameter of the earth.
So why does the moon look so small in the night sky? Easy ~ we don't see the Moon parked right next to the Earth. The photo only appears that way because the camera was positioned on a satellite or space probe located much farther away from Earth than the Moon is in the image. When you use a telephoto lens from such a distance, two objects which in reality are separated by a considerable distance (like the average of 238,900 miles separating Earth from our Moon's orbit (illustrated below), may appear to be much closer to each other.
It is an illusion, a visual effect called foreshortening. If you've ever photographed two objects (one large, one small) whose distance apart is a fraction of the distance between them and you, the image produced will depend upon the lens used ~ wide angle, normal, or telephoto. With a telephoto lens the two objects may appear to be much closer to each other in size. You've just witnessed foreshortening (see image below).
Here's an example from Space ~ in the image, two of Saturn's moons, Epimetheus (left) and Janus (right), appear close because of foreshortening. In reality, Janus is about 40,000 km farther from the observer than is Epimetheus.
All the above is intended to provide the gentle reader with a different perspective from that presented in last Saturday's post. That hypnotic image might leave one feeling humbled, insignificant, a mite in the starry immensity of cosmic space ~ a place where Earth's nearest neighboring star is our own Sun, 93 million miles away. After that, it is a wee bit of a hike to the sun's nearest neighboring star, Alpha Centauri ~ actually a triple-star system, the stars bound together by gravity ~ 4.37 light years distant.
Here is what the Alpha Centauri system looks like ~ and what our Sun looks like from AC ~
But I'm leaping ahead of myself. Let's take a step back, and consider our Earth and our Solar System ~
- the Inner Solar System containing the Sun, the Interplanetary Medium (composed of the vacuum of space plus dust, plasma and cosmic rays), the four terrestrial inner planets (having dense, rocky composition but only one moon among them) and the Asteroid Belt.
- the Outer Solar System containing the four giant outer planets (having a higher proportion of volatiles like water, ammonia and methane, and many more moons), and icy minor planets called Centaurs.
- the Trans-Neptunian Region containing the Kuiper Belt (a great ring of icy debris, many dwarf planets like Pluto, some with multiple satellites and most having orbits which take them outside the plane of the ecliptic).
- the Farthest Regions containing the Heliosphere (a Stellar Wind Bubble dominated by the Sun's gravity and stellar wind, acting in opposition to the the wind of the Interstellar Medium. The outer boundary of the Heliosphere, the point at which the stellar wind terminates and is the beginning of interstellar space, is called the Heliopause. Farther out are various detached objects and the Oort Cloud, a spherical cloud of up to a trillion icy objects and breeding ground for trans-solar-system comets. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System.
So what does all this tell us about the size of the entire Solar System? For starters, rather than measuring in miles or kilometers, which introduce many, many zeros into any distance figure, let's adopt a convenient encompassing unit of measure, the AU or Astronomical Unit. One AU is simply the average distance from the Sun to the Earth (93,000,000 miles), a concept we can readily identify. (See below)
- Working outward, the distance from the Sun to the Asteroid Belt, the outer boundary of the Inner Solar System, is about 3.2 AU.
- The distance from the Sun to Neptune, the outer boundary of the Outer Solar System, is about 30.1 AU.
- The distance from the Sun to the Kuiper Belt extends from 30 to 50 AU.
- The distance from the Sun to the heliopause, as reported by the Voyager 1 and Voyager 2 spacecrafts, may range from 84 to 94 AU.
- Finally, the distance from the Sun to the Oort Cloud ranges from 50,000 AU (cloud's inner limit) to 100,000 AU (cloud's outer limit).
Here are several illustrations of scale. The first simply compares the relative sizes of the sun and its planets.
The second portrays the sun and planets in order of orbit (this is what many of us were taught was the entire Solar System), with size proportional but not distance.
And now we come full circle (as it were), to an image portraying the true entire solar system from Sun to Oort Cloud, and our nearest stellar neighbor Alpha Centauri included (with its distance massively foreshortened. (remember foreshortening?)
Recall when I wrote that Alpha Centauri lies 4.37 light years away from our Sun? Now we've transitioned from miles to AU to the light year (which is the distance which a photon of light traveling at 186,000 miles per second covers in one Earth year. Doing the math, there are roughly
- 93,000,000 (93 million) miles in an AU,
- and about 64,156 AU in a light year.
- How many miles in a light year? Oh, 6,000,000,000,000 or 6 trillion miles, give or take.
I know, the numbers become dizzying at times, don't they? And just think, the incomprehensible distances we've described so far are only a drop in the ocean compared to the distances which measure the following:
1. our Milky Way Galaxy
2. the Milky Way's local galactic group (a neighborhood made up of up to 54 entire galaxies)
4. and where our Milky way galaxy fits into the local galactic group)
5. the Virgo Supercluster , a mass concentration of at least 100 local galaxies and clusters,
- Finally, we would be remiss in not including the classic 1980's National Geographic universe map,, with each small-scale image projected into its position in the next-large scale image, from our Solar System up through the entire Universe -
My mind is spinning. Or rotating. Or revolving. Or orbiting. Or ... never mind.
So. We started our journey with a simple, unassuming image of the Earth and Moon, and expanded our view ever-outward until we were seeing all that there is possible to see .... so far. Imagery technology and advances in our understanding of the structure and physics of the Universe are constantly improving. Who knows, someday soon you may be able to point your handheld camera set to super-zoom and get a clear shot of the license plate on hat unfamiliar maglev speeder that has been parked illegally, blocking the driveway to your summer home on Saturn's moon Titan. Or simply check around for any nude sunbathers of the super-sized, submarine, tentacled, bioluminescent or hyper-intelligent variety.