Eye Astronomy #12: The Dark and the Bright
by Dale E. Lehman
At HCAS, we have shirts with our organization’s name and logo. On the back they say, “If you can read this, it’s not dark enough.”
To see much of anything at night, it must be dark. The sun must be down. (Duh.) Rather far down, in fact. Few astronomical objects are visible during morning or evening twilight.
Did you know there are three types of twilight? Yep, there are!
· Civil twilight: The sun is between zero and six degrees below the horizon. It’s bright enough for most outdoor activities without artificial light.
· Nautical twilight: The sun is between six and twelve degrees below the horizon. Sailors can still see the horizon and use it for navigation.
· Astronomical twilight: The sun is between twelve and eighteen degrees below the horizon. It’s dark enough for some astronomical observations, though fainter objects aren’t visible.
Night arrives once the sun is more than eighteen degrees below the horizon. The earth turns fifteen degrees every hour, so full darkness lasts from an hour and twelve minutes after sunset until an hour and twelve minutes before sunset. That’s when the stars really shine.
Sort of. Two hundred years ago, night was dark. Today, not so much. Light pollution brightens the sky and hides a lot of interesting stuff, like the Milky Way. Most living creatures never experience full darkness anymore, which spawns environmental and human health effects. But for the moment, let’s think about bright things in a dark sky.
The stars are not, in the main, all that bright. Oh sure, step outside on a clear, dark night, and a handful do seem very bright. The brightest stars are collectively called first magnitude stars. There are only 22 of them, including both northern and southern hemisphere stars.
The magnitude scale was invented by the Greek astronomer Hipparchos around 150 B.C. He divided stars into groups by brightness, calling the brightest first magnitude, the next brightest second magnitude, and so on down to sixth magnitude, which covered the faintest stars visible. The system wasn’t precise. After all, he was only working with his eyes and brain.
In 1856, English astronomer Norman Pogson brought precision to Hipparchos’ system. In Pogson’s refinement, a difference of five magnitudes corresponds to a difference of 100 times in brightness. That choice wasn’t arbitrary. Hipparchos’ first magnitude stars were about 100 times brighter than his sixth magnitude stars. Unfortunately for the non-mathematically inclined, Pogson’s scale is logarithmic. A single magnitude difference is a difference of about 2.51 in brightness. The brightness difference between any two stars is 2.51 raised to the power of the difference in their magnitudes. Yeah, you’re gonna do that every time, aren’t you?
The magnitude scale is open-ended in both directions. Hipparchos had six magnitudes, but with telescopes, we can see fainter. A lot fainter. Space-based scopes can detect objects of magnitude 31! Plus, some objects are brighter than first magnitude, such as the bright northern star Vega. Vega proved very nearly magnitude zero. Astronomers needed a zero point anyway, so they defined Vega’s brightness as zero. It thus is the reference for measuring the brightness of everything else in the sky.
This is a good point to pause and look up at the night sky. Right now, Vega shines overhead, along with another star that rivals it:
Look east-southeast of Vega (right and a bit down in the image). There’s the bright orange star Arcturus. Modern measurements put Arcturus ever so slightly brighter than Vega, at magnitude -0.04. That’s right, it has a negative magnitude. Anything brighter than Vega will! And that includes some very bright things, indeed:
· Sirius, the brightest star in Earth’s sky. Magnitude -1.46, 3.8 times brighter than Vega. Only four stars are brighter than Vega: Sirius, Canopus, Alpha Centauri, and Arcturus.
· Saturn, sometimes. Depending on distance, it varies from -0.55 to 1.17.
· Mars, sometimes. Depending on distance, it varies from -2.94 to 1.86.
· Jupiter, always. Depending on distance, it varies from -2.94 to -1.66 and is the second brightest point-like object in the sky.
· Venus, always. Depending on distance and phase, it varies from a blazing -4.92 to -2.98, the brightest point-like object in the sky.
· The moon. When full, it’s -12.7. When a sliver, it can be as dim as -2.5. But think about it. The full moon is 119,085 times as bright as Vega! It casts obvious shadows on the ground and overpowers nearly every faint object in the sky.
· The sun. The sun is dangerously bright. Looking directly at it can cause serious eye damage. And viewing it with a telescope? Without a proper filter, you may as well shove a burning brand into your eye. I’m not joking. True story: an amateur astronomer’s telescope set his deck on fire when a gust of wind pulled the shroud partially off and dragged the instrument around to the sun. The sun’s magnitude is a whopping -26.74, 48.7 billion times as bright as Vega, 408,714 times as bright as the full moon. That’s a lot of light! Sunlight scattered by the atmosphere washes out everything except the moon and Venus. (Some people can locate Venus in broad daylight by knowing exactly where to look and blocking sun with a building.)
Sunlight isn’t the only light the atmosphere scatters. Any light entering it from above or below is also scattered: moonlight, starlight, planetary light, artificial light. All of it. The sky itself has a magnitude.
Naturally, the night sky’s magnitude is between 6.5 and 7.4. Hipparchos couldn’t see anything dimmer than his sixth magnitude stars because the sky hid anything dimmer. But he was lucky. Under suburban light pollution today, the sky might be as bright as magnitude four. Urban light pollution? Magnitude three, if you’re lucky. City nights can be as much as 23 times brighter than natural nights.
That’s why you can’t see the Milky way or anywhere near as many stars as Hipparchos could. Moreover, our bodies aren’t adapted for a darkness so bright. Nor is the rest of the biosphere. Something to think about, no?
