18 May 2008
'Tis the time of the year that our poor public school children are subjected to those dreaded California State Tests. To help us all feel their pain, if only for a few minutes, I'd like to subject you to some questions similar to those that are plaguing our children during these weeks. They are all astronomy related, so regular readers here should find all this easy like Sunday morning. Got your Number 2 pencils sharpened and ready?
Question 1: Which sequence correctly lists the relative sizes from smallest to largest? (A) solar system, universe, Milky Way Galaxy, (B) solar system, Milky Way Galaxy, universe, (C) Milky Way Galaxy, solar system, universe, (D) Milky Way Galaxy, universe, solar system.
Most if us know the solar system is the smallest of the triad, which, in good test taking practice, eliminates (C) and (D) and narrows it to (A) or (B). But many people are not too sure of what's next in size. Both are huge, but one is huger - more than 275,000 times huger.
The Milky Way, our home galaxy, is generally believed to be about 100,000 light years across. The visible universe is more than 13 billion light years in all directions! And that is just the visible universe; the actual size may be much greater. Correct answer: A resounding (B).
Question 2: Stars begin their life cycle in (A) a black hole, (B) a nova, (C) a nebula, (D) star eggs.
Star eggs? What the...? No, not star eggs. A black hole is how some stars end their lives, not begin it. A nova is how some other stars sputter out towards the end, and although we have rarely talked here before about novae, there is no need to know about one to eliminate it as an answer. Remember the Orion Nebula? It is there that countless baby stars are presently being born. Nebulae, you may recall, are the celestial clouds that can condense down to form new stars. Correct answer: (C)
Question 3: The seasons of Spring, Summer, Winter, and Fall are a direct result of which phenomenon? (A) Earth's proximity to the sun, (B) shifting ocean currents, (C) the 23.5° tilt of the Earth, (D) global warming.
Global warming? Uh...no. But you would not believe how many students, even graduates from university, answer with some variation of (A). In their defense, it does seem almost self-evident: Close to the sun implies hotter, thus summer, farther means colder, thus winter. But then how do people in the southern hemisphere celebrate just the opposite seasons as we? Why, when it is summer here, is it winter Down Under?
If you have been even the casual reader here over the last decade you know that one point that has been pressed home is that our seasons are due to our perfect tilt. Our tilt with respect to the sun allows us to get more solar exposure during part of the year, and less six months later. Tilted towards = summer. Tilted away = winter. Spring and Autumn are the in-between points. And you may also remember that our Earth tilted any more or any less results in global misery. That magical 23.5 is our very special tilt angle. Correct answer (C).
Question 4: The highest tides are due to (A) sun and moon working together, (B) sun and moon working against each other, (C) the moon only! (D) global warming.
Well, despite the fact that global warming is now The Hot Topic (pun intended), it is not the catch-all answer to every earthly phenomenon. Many of us know the moon plays a role in the tides. But did you know the sun plays a role as well? Its gravity also tries to yank the water off this planet, but with a little less strength than that gift of a nearby satellite we call the moon.
And when they act together, as when they are on the same side of the Earth (or opposite), the tides are extra high. We call that spring tide. They work against each other when at right angles to each other, like the 12 and 3 positions on a clock with us at center. That resulting not-so-high tide is called neap tide. Correct answer: (A).
How did you do? If you are a regular here, or just love the subject, you probably aced it. If not, fear not! There is always time to get yourself educated in the best of scientific disciplines on or off this planet, to wit, astronomy.
Until next time, clear skies!
04 May 2008
Last time here we took a deeper look into the great Lion of the Sky - Leo. We saw there a new spot on the celestial cat, a spot more familiar to us as Saturn.
Saturn happens to be parked in Leo this year, although we might more accurately call it a rolling stop. The giant ringed planet is traveling very slowly through Leo taking its sweet time to get to Virgo. But because it is so far away - and the laws of planetary motion tell us that the farther away you are from your star the longer it takes to get around it - Saturn will be in Leo until August 2009. Saturn is no Mercury.
But it would be a good thing to see Saturn this year, not next. The next time we come around to its side of the neighborhood, about a year from now, it will not be the Saturn we all know and love. The rings will almost seem to have disappeared.
You see, Saturn has a tilt like we do. By next year the planet will be at that point in its orbit where its position from our point of view will give its rings almost no tilt at all. And the rings are paper thin (actually only tens of meters thick). Bad news for ring junkies. Imagine someone down the street tilting a piece of paper so it is edge-on with your line of sight. Voila! The paper seems to vanish.
And Saturn without its rings is like a lion without its mane. It's not much more than a giant, featureless ball. That ranks low on the Exciting Things To See In The Sky scale.
But it will still have that one bizarre quality that planets possess. It will not twinkle. Yet the stars around it, like bright Regulus right next to it, will. Why?
It has to do with distance, and that annoying outer layer of our planet called the atmosphere.
Regulus is "ginormous," as my young son might say. It is a star over 4 times bigger than our sun. Imagine a sun the size of that sucker in our skies. But it is 78 light years - over 458 trillion miles - away.
Now in your mind’s eye, take Regulus from where our sun is and move it farther and farther and farther away. It shrinks and dims, and shrinks some more. By the time Regulus gets to its actual position in the galaxy, its size in the sky goes almost to a true point. And that, my friends, is the key.
The miniscule shaft of life that hits your eye from that distant star is unimaginably small and subject to even the slightest changes. What changes? Changes it encounters as it hits our atmosphere. It is then that the poor shaft hits our wall of air, air filled with countless pockets of different temperatures.
These different temperatures cause the tiny shaft to change direction every so slightly, but enough so that by the time it hits you the shifting shaft of light makes the star appear to jump around or twinkle.
But Saturn and the rest of the planets don't twinkle. Why? Now you have the tools to figure that out.
The planets, although very much smaller than stars in size, are not nearly as far away. They do not reduce in apparent size down to points of light. They still have a visible disk.
And yes, the atmosphere messes up their light paths, as well. But instead of one little shaft of light slapping you in the eye, here we have light from all parts of the planet's disk firing away at you - from its top, bottom, middle, and sides. They all get bent out of shape as they race through our atmosphere, but all the planet's flood of light more than compensates for any wayward photons that might get bent out of the way of your eye. Overall effect: No twinkling.
Is there anything you would be interested in reading about here? Any burning questions? Let me know.
Until next time, clear skies!