FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Thanksgiving and the Great Lights

Thanksgiving is sadly becoming just another one of those excuses to get some time off, like Memorial Day. And like Christmas it suffers from so much busy work, from family to food to football, that the entire day can pass us by and we can completely forget to celebrate it.

But now the family is gone, the extra food is in the Tupperware, and the football is… well, that will never go away I guess. So let's have our own mini-Thanksgiving! And we'll concentrate on the cosmos - specifically the Sun and Moon - to see what there might be to be thankful for.

There of course is a poetic way to look at this dynamic duo. The greater light to rule the day and the lesser light to rule the night have been revered - sometimes as gods - for millennia. They are amazingly reliable timekeepers and season markers. And there is something about the sunrise and sunset, or the crescent Moon sinking in the west, or one rising full in the east, that has this way of drowning some of us in thanksgiving and praise.

But allow me to put on a pair of cold, clinical scientific spectacles and see if there might be even more in those celestial spheres to be thankful for.

When we look deeper into the Sun and Moon we see that they are the exact same size in the sky, a phenomenon that occurs on no other planet in the solar system. This allows for those mind-altering solar eclipses, but is a wonderful thing in its own right.

It also means they are both at just the right distance to be life-sustaining orbs and not the killers they could be. What?
We don't want the Sun to be bigger or smaller in the skies above. "Bigger" could mean several things; that it is really a larger ball of gas or it is the same size as our Sun, just closer. Both would spell doom.

A naturally bigger star would burn much faster, spew out more of the lethal wavelengths of light like ultraviolet and x-rays, and die much sooner. Nothing but bad news in that.

And if our Sun appeared bigger because it was closer - closer even just by just millions of miles - we'd be headed for a heat wave that would shut this planet down. (see Venus)

Contrariwise, we don't want our Sun smaller in the sky, as in farther away. We'd freeze. (see Mars) And a legitimately smaller star, one with a smaller radius, would not give us enough energy to live. We'd have to snuggle in closer to that fire to stay warm. But in doing that the small star would, because of the laws of physics, slow down our own rotation to a near stop. And daytimes that last dozens and dozens of hours followed by equally long nighttimes are not conducive to life.

The Sun is not too big, not too small, not too close, not too far - and I am thankful for that.

And we do not want the Moon any bigger or smaller. It was much closer and bigger in the sky when it was first formed more than 4 billion years ago. Back then our planet spun so fast that our days were about 5 hours long! Winds were breakneck. And the Moon, after its miraculous formation, must have been a terrible and awesome sight in the sky, very close and thus many times bigger.

But through the eons the Moon has been slowly moving away from us, diminishing in apparent size. During this time it has tugged on us with its own just-right gravity because of its just-right mass to slow us down to a just-right 24-hour spin.

It will continue to get farther away and smaller in the sky, and will also continue to slow us down. But longer days and nights mean more extreme temperature changes, from way above 100 degrees to well below freezing. We are here at exactly the right time.

The Moon is not too big, not too small, not too close, not too far - and I am thankful for that.

There is a lot more about those two heavenly bodies that would give us a greater appreciation of them both. Suffice it to say now that they are wonderful daily reminders of how, in spite of our attempts to ruin the place, we have truly been blessed with a wonderful home.

Mr. Blue Sky, Please Tell Us Why...

Most astronomers - but not all - do their observing at night. It is then that the skies are see-through black and the objects that make up our starry heavens are at their best.

But once the sun comes out, observing time for those astronomers is over. It is then of course that our skies metamorphose from transparent and black into translucent and blue.

So why is the sky blue?

This question is a classic, but few people really know the answer. It's almost as if it were enough just to ask, but not to find out. Let's answer it now.

One reason for the blueness of the daytime skies that I have heard many times is that the sky is reflecting the oceans. Good guess, but if we took that to its logical end we'd have to ask why the sky isn't also brown and green and white, other colors which dominate the Earth's surface.

I'll bet we can find another, more logical answer. Let's try. The sky is blue during the day, so the sun must play some role. Here are another couple helpful clues: There is no blue sky on the Moon, ever. And have you noticed that when the Shuttle astronauts are working on things up there, they are being bathed in sunlight, but the "sky" all around them is pitch black? So the sun must have an accomplice; it cannot be working alone.

Immediate hypothesis? The sun and an atmosphere are needed to make a blue sky. The two work together somehow to bathe us in a sea of blue.

Consider this also. The Sun, as you know, bleeds out all the colors of the rainbow, from deep blue all the way over to the red end of the visible spectrum. Might there be something about the atmosphere that monkeys with just the blue parts of the spectrum sent down from the Sun? If you think so, you would be right.

The photons of light that make up the visible spectrum differ by wavelength. The blue end has the shorter wavelengths, the red end the longer.

The molecules of oxygen and nitrogen that make up our atmosphere are just the right size - that is, incredibly small - to interact with the short-wavelength blue guys. Those photons are absorbed by the air molecules and re-emitted in random directions all over the sky. This happens so much, they are scattered in so many directions, that everywhere we look we are getting hit by those redirected blue photons.

All of this works to give our inquisitive minds the impression that the blue photons are coming from everywhere, and ta-da! the sky is blue.

The other parts of the spectrum come on through with relative ease, but even some of these are challenged if there is a lot of atmosphere to go through. This of course is what happens at sunset.

As the sun sets, the light pouring from it has to travel through a lot more sky - and a lot more schmutz - to get to us. Much of the spectrum gets pealed away from the light stream leaving only the heartiest and longest of the wavelengths, those on the red end, to make it through, and giving us those spectacular sunsets.

You may have noticed that when the sky is smoky from a fire that the sun appears red. Same reason as above: the longer-wavelength red photons get though, the rest are scattered.

And now you know what happens to the stars when the sun is out. This whole effect -the scattering of the blue wavelengths - essentially fills the skies with that familiar blue glow, and the relatively dim stars beyond, still traveling over us, get blotted out. If you could suddenly take away our atmosphere during the day, the skies would be transparent and black again, except for the vicinity of the Great Star, our own Sun.

But then it would be kind of tough to breathe, our blood would begin to boil, and the thrill of seeing the stars during the day would soon be replaced by excruciating pain, followed by death less than a minute later.

I'll settle for studying the stars at night, and keeping the pretty blue skies for the daytime.
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me