24 Jun 2007
You are sitting in your backyard after sunset watching the evening sky begin its fade to black. Even before the sun fully sets you notice a small bright light gradually appearing in the west. Of course that's Venus. As the sky darkens a gleaming spot in the southwestern skies seems to brighten, as well. That would be Jupiter. Then slowly over the next half hour or so, stars seem to just show up from out of nowhere.
If you are away from the city lights and the Moon is not out at the moment, the sky will seem to fill with stars over the next hours.
If you didn't before, you now get an idea of how varied the brightness of stars can be: from celestial bodies so bright you can see them just as the sun sets, to stars so dim you can only see them after the sky goes pitch and your eyes have adapted for the darkness.
Of course the brighter stars have always played a major role in the construction of constellations, and in the mythologies and oral traditions of many people groups. But it wasn't until just a couple thousand years ago that some people tried to take a more scientific look at all stars, bright to dim.
Over two millennia ago, Hipparchus, one of the great astronomers of the ancient days, categorized the stars into a brightness scale. The brightest stars in the sky were classified magnitude 1. The dimmest, nearly imperceptible stars were magnitude 6. All the rest of the stars fell somewhere in between.
We still use basically the same scale today, but not without some ado.
For example, what is disturbing for modern minds is that the brightest stars up there have the lowest magnitudes. So a star of magnitude 5 is considerably dimmer than a magnitude 2. That's not a big problem, but being counterintuitive it can be a little annoying.
More exasperating, though, is what happened as our knowledge of the skies grew.
What about objects in the sky that were brighter than the brightest stars - like Venus? What magnitude should they be assigned? Well, inconveniently astronomers agreed to take those brighter objects down to zero and then lower, into the negative numbers! So, for example, Venus, the brightest object in the sky besides the sun and moon, has a magnitude of -4.4!
But why stop there? The Moon, when full, has a magnitude of less than -12. And the brightest star in the sky, the sun, is magnitude -27! Yikes!
When telescopes were invented and could open their bigger and more sensitive eyes to the heavens, they could pick up light from objects our tiny eyes could never see. The scope was like a battering ram that broke open the door on the other side of Hipparchus' scale. With their help we can now see objects with magnitudes near 30! To give you an idea of how dim that is, the Hubble Space Telescope had to keep its shutter open for more than a day just to pick up enough light from those distant objects to get an image!
This whole crazy magnitude scale, like that embarrassing uncle at family gatherings, is here to stay, but once you get used to it, it isn't that bad.
Sadly, today, because of light pollution we are losing the higher magnitude stars to the bright lights of malls and casinos. For those living near cities today, it is a good night if one can see down to magnitude 3 stars. To the disappointment of many, the starry hosts of yesteryear are becoming the starry handful for today's generation.
Until next time, clear skies - and dark nights!
10 Jun 2007
The end of the year for teachers and students doesn't really happen in December; it occurs in the merry month of June. In keeping with the scholastic spirit of the season, let's take a short final exam on some of what we've learned here in this column for the last 10 months. Ready?
1. The sky is blue because: a) it reflects the oceans, b) molecules in the atmosphere bounce the blue photons from the sun all over the sky, c) the sky has water in it.
Well, the sky does have water in it, but you can't see it unless it bunches up together into those puffy things we call clouds. And no, the ocean is not reflected in the sky. But the tiny gas molecules in the air do cause the bluer photons from the sun to bounce about in erratic paths giving us the impression the whole place is lit up in blues. Answer: b
2. The summer Milky Way is bigger and brighter than the winter Milky Way because: a) in the summer we face the star-studded center of the galaxy, b) the sun is out longer making the night sky brighter, c) the summer planets just make it appear that way.
There are no "summer planets," so that one is out straightaway. And the sun itself plays no role in a nighttime sky. But our night sky during summer does face toward the billions of stars in the galactic center. During the winter we face the dreary and not-as-well-populated outskirts of the galaxy. Answer: a
3. Which of the following wavelengths of light do we use in astronomy? a) Just the ones we can see, b) Just the sun's wavelengths, c) Gamma rays, d) All wavelengths we can get our curious little hands on.
In the old days, up until the last century, all we could use were the very limited "visible" wavelengths of light, the colors of the rainbow. Now we have telescopes that can tune into just about any wavelength; from long radio waves, through the infrared, ultraviolet, and x-rays, over to the ultra-small and ultra-dangerous gamma rays. And with that, in the last century we have "seen" more of the universe - visible and invisible - than any generation before us in the history of humankind. Answer: d
4. How many stars are responsible for illuminating the Great Nebula in Orion? a) Just one, b) Billions and billions, c) What's this Great Nebula thing?
Seeing images of Orion's great cloud on sites such as Astronomy Picture of the Day, there just has to be an army of stars lighting the whole place up. Alas, no. It is just one hyperactive whiney star in the center that is responsible for providing the energy necessary to ionize the entire region. Answer: a
5. Venus is always seen in the sky: a) on the opposite side as the sun, b) near the sun, c) nobody really know where she will pop up next, that crazy vixen!
Thanks to Newtonian mechanics we know exactly where Venus will be tomorrow - and 100 years from today. And because she is on an inside lane around our star, she will always be in the same vicinity as that great ball of gas, and never on the opposite side of the sky. Answer: b
6. Easter and Passover set their dates by: a) the phases of the moon, b) the phases of the sun, c) the spin of a dreidel.
Well, the sun has no phases. And of course there are no dreidels involved in choosing the dates for high holidays. Both Easter and Passover are dependent on the phases of the moon, specifically the first Full Moon after spring equinox. Answer: a
How did you do? If you are a regular reader here, I'll bet you did well. You can always go back and read the full explanations in our archives of The Skies Above at firstlightastro.com/skiesabove.
Until next time: a) clear skies, b) go learn more about the universe, c) tell others about the wonders of our sweet Home, d) all the above. Answer: d