In addition to the recent missions to Mars, several others are scheduled to explore and study the red planet in the coming years, and missions to Jupiter's moon, Europa, and to Pluto are on the docket for later in this decade. We're a long way from sending someone to Mars, and we may never have human exploration of our most distant planets, but assuming that we'll eventually visit some of the more exotic addresses in our solar system, how bright would the Sun appear in their skies?

We've all been warned about the permanent damage that can be done to our eyes if we look directly at the Sun. However, there are certain times when we can look at the Sun and not be so concerned about having to use a cane and guide dog for the rest of our lives. At sunup or sunset, when the Sun is low in the sky, and its brilliance dimmed by the thicker atmosphere, the reddened disk of the Sun can be looked upon without fear of scalding your peepers. Hundreds of years ago, the Chinese were known to have counted sunspots when the Sun hugged the horizon. Mid level clouds or smoke from distant fires will also reduce the glare of the solar disk and permit viewing of the Sun. Another time when you can look directly at the Sun is during a total solar eclipse. However, for those of you who witnessed the partial solar eclipse on Christmas Day, you probably realized that even the partially eclipsed Sun was still to strong to view without the aid of special eclipse glasses or a welders shield. Unless old Santa put one of these in your stocking, you likely had to revert to using a piece of cardboard and a pinhole to see the Sun's disk change. For all practical purposes, from Earth, the Sun is just too bright to gaze upon safely.

On cloud-shrouded Venus, presuming an astronaut could withstand the 850 degree temperatures and crushing atmospheric pressures on its surface, the Sun could be probably stared at without much risk of damaging your eyeballs since the dense Venusian atmosphere would cause it to appear as a blurry blob of light. However, except under special circumstances, the Sun's too dazzling to look at directly from any place in the inner solar system (inside the orbit of Jupiter), at least with human eyes. Is this also true for the outer planets in the solar system?

Jupiter's moon, Europa, is 5 times as far from the Sun as is the Earth, but even at 485 million miles, the Sun is far too brilliant to observe safely. The same is true for Saturn's moon, Titan, which is nearly 10 times the distance from the Sun as is Earth. Furthermore, with hardly any atmosphere to buffer the Sun, the sunlight on almost any moon in our solar system would be blinding. Titan is an oddity in that it has an atmospheric (nitrogen and chlorine), and it's more dense than ours. What about the brightness of the Sun from Pluto?

It has been thought by many people even perhaps a few astronomers that from Pluto, the Sun would seem no brighter than does Venus from our skies. If we use as the distance from the Sun to Pluto a value of 3.5 billion miles and apply the inverse square law, we can estimate how much light actually strikes the surface of little Pluto and its puny satellite, Charon. Think of the inverse square law this way; if an object, such as the Sun, were to somehow suddenly move twice as far away as it is now, it would appear four times dimmer. Conversely, if it were to move twice as close as it is now, or about 47 million miles away, it would be four times brighter. Since Europa is 5 times more distant from the Sun than are we, an astronaut on its surface would see a Sun that is only 1/25 as bright - no wonder Europa is cloaked in ice. For someone on Titan, the Sun would appear only about 1/100 as bright as it does from Earth, and for someone on Pluto, the Sun would only be about 1/1400 as bright as the Sun when viewed from Earth. It would seem that an astronaut would need a good pair of binoculars just to see old Sol.

But even something 1/1400 as bright as the Sun is still pretty darn bright. Celestial objects are assigned apparent magnitudes that give us a relative sense of their brightness. The more negative the number, the brighter the object. An object with a magnitude of -1 is 2 1/2 times as bright as an object with a magnitude of -2, and about 6.25 times brighter than a object that has a magnitude of -3. A magnitude difference of 15 amounts to a difference in brightness of 1 million. Using this scale, from Pluto the Sun would have a magnitude of approximately -20. From Earth, the Sun has a magnitude of -26.7, whereas, the brightest star we can see in the Northern Hemisphere, Sirius, has a magnitude of -1.4 and Venus can attain a magnitude of -4. Thus, the Sun from Pluto would be more than a million times brighter than Venus looks like from Earth and 1,000 times brighter than our full moon!

One reason our full moon seems so bright is that when we're seeing it, the Sun is hidden from view. We're only seeing reflected sunlight, which is considerably dimmer than the real thing. The bright full moon is well-resolved by the retina, but even if the Sun had the same magnitude as the moon, it's an unresolved source, so, the energy delivered to each rod or cone in the eye's retina would be far greater than for the moon - like looking into a laser beam.

Of course, from Pluto, the Sun won't look as big as it does here. The disk of the Sun and moon cover about 1/2 degree of the sky - 360 Suns placed end to end would be needed to stretch all the way across the sky from one horizon to the other. On Pluto, the Sun would only appear as a point of light, like any other star, except much, much brighter. Despite the fact that its light comes from a mere point in the sky, from Pluto the Sun would be bright enough to cast deep shadows on its surface. Because the source of light is so far away, the shadows would be particularly noticeable, more so than those here on Earth.

So even on Pluto, the Sun is probably too dazzling to look at safely. From the Sun, Pluto is still only about 1/6500 the distance to Proxima Centauri (4.2 light years away), the nearest star other than the Sun. In order to view the Sun safely, we wouldn't have to go all the way to Proxima Centauri, but we would have to travel to the far reaches of our solar system, perhaps 5 times the distance that Pluto is from the Sun.

For more about this see Secrets of the Night Sky by Bob Berman (HarperPerennial Publishers) and the Astronomy Cafe by Sten Odenwald (W.H. Freeman Publishers).