My mind is wandering...

[Draconiator]

I'm too lazy to log in, heh.

Anyway...I was wondering how bright the Sun would appear on each planet in the solar system, considering the rocky ones like Pluto, Mercury, ect. have an atmosphere it can shine through. For example, I have HEARD that a day on Saturn would be as bright as a night on Earth during a full moon.

But what about Neptune? Uranus? Etc?

[Bob Hegwood]

Now that *is* an interesting question...

I was wondering the same thing during the Venus tour because the daylight
side of Venus is really difficult to look at because it is so bright. I realize that
we're looking at a radar-imaged surface, but in reality wouldn't Venus' clouds
block a large part of the daylight since they're so thick? Had the same kinds
of questions for Mars and some of the Jovian satellites? Wouldn't they be so far
away that they would get very *little* light from the Sun? Please understand
that I don't know beans about the physics or the actual dynamics of sunlight,
but it does seem that the daylight would have to be diminished by a lot on the
outer planets if for no other reason than distance. Yes?

Thanks, Bob

[Evil Dr Ganymede]

For starters, illumination drops with the inverse square of the distance. So a planet twice as far from the sun as the Earth receives 1/4 of the energy from the illumination. You can use Celestia to see how the apparent magnitude of the sun drops with distance.

Mercury is at 0.4 AU from Sol, so it gets 6.25 times the illumination. Sol's magnitude is about -29.3 here.

The top of Venus' atmosphere receives roughly twice as much illumination as the Earth does, being 0.7 times the distance from the sun (Sol's magnitude is -27.44 here) . But the thick atmosphere absorbs some of the light, so by the time it reaches the surface, it's probably not unlike an overcast day on the Earth.

At Earth, Sol's magnitude is -26.71, according to Celestia.

Mars is at 1.6 AU, so it gets about 0.39 times the illumination as Earth. Still not too dark - Sol's mag is about -25.65 here.

Jupiter is at 5.2 AU, and receives 0.037 times the illumination as Earth. So the sun is considerably dimmer there. Sol is at apparent magnitude -23 there, according to Celestia.

Saturn is at about 9.5 AU, so it receives about 0.011 the illumination as Earth. The apparent magnitude is about -22, so that's still vastly brighter than the full moon is from Earth (-12)

Uranus is at about 20 AU, so illumination is 0.0025 that of the Earth. Sol's magnitude is still about -20 though - much brigher than the Moon.

Neptune and Pluto are about 30 AU, so illumination is 0.0011 that of the Earth. Apparent mag of Sol is about -19 there - still brighter than the Moon from Earth.

Even out at Sedna (at its current location), Sol is still magnitude -17. Only when Sedna is furthest from the sun, at about 1000 AU, does the Sun's magnitude drop to about -11.7 - comparable to a full moon on Earth.

[tony873004]

Sunlight at Saturn would be much brighter than a full moon. I've heard that a full moon is about 1/400,000 as bright as the Sun. Here's the brightnesses of the planets:

Mars ~ 1.5 au: 1/(1.5*1.5) = 0.44 * the brightness of Earth
Jupiter ~ 5 au: 1/(5*5) = 0.04
Saturn ~10 au = 0.01
Uranus ~ 20 au = 0.0025
Neptune ~ 30au = 0.0011
Pluto ~ 40au = .000625
Sedna at its closest ~ 90au = 0.000123
Sedna at its farthest ~ 850au 0.00000138
Brightness of a full moon = 0.0000025

The sunlight at Sedna when Sedna is closest is much stronger than a full moon. The sunlight at Sedna when Sedna is farthest is about half as strong as a full moon, or about as bright as a gibbous moon.

Here's an experiment. Get an SLR camera with a built in light meter. Place a white piece of paper on the ground when the Sun is high in the sky. Set your camera's shutter speed to its fastet position. Aim the camera at the white piece of paper (fill the frame) and adjust the apature until the light meter shows a proper exposure. Make a table using the above values. Multiply each planet's number with your shutter speed. In the evening, go to a room in your house that has a light on a dimmer switch. Put the same piece of paper on the floor. With the apature at the same setting, adjust the shutter speed so it matches the computed value for a planet from your table. Aim the camera at the paper and adjust the dimmer switch until the light meter reads a proper exposure. That's about how bright it is on that planet.

You probably can't turn the dimmer switch high enough to simulate Mars or Jupiter. Pluto and Sedna at its closest will have pleanty of light to read by.

Instead of a dimmer switch, you could wait until around sunset, and do the second part of the experiment outside. As light fades, look at the light meter to see when Earth's twilight is the same brightness of the other planets.

[Bob Hegwood]

Thanks for the explanations gentlemen...

If I understand correctly then, a person standing on the surface of Europa
would actually have enough light to view his surroundings with an un-aided
eyeball. Is this correct? I understand that the light would be considerably dimmer,
but I would not have thought that there would be enough light to view the
landscape.
Interesting stuff. Of course, on Europa the ice might enhance the view a bit
but this is still more than I would have thought possible at the distances we're
contemplating. Cool...

Thanks again, Bob

[tony873004]

You'd have no problem on Europa viewing your surrounings with the naked eye. Europa receives 4% of the sunlight that Earth receives, but 4% of the Sun is still a very bright light. On Earth, you pupil turns into a small dot on a bright sunny day. On Europa your pupil would open wide and compensate for almost all the difference. I'm guessing it would be about as bright as as Earth moments before sunset. I've noticed from playing golf that after the sun sets we can still play for about 1/2 hour, until Sedna-like lighting force us to stop.

[julesstoop]

Good comparison would be the almost totally eclipsed sun in my town Leiden in '99. About 97% of the sunlight was blocked, but most people told me afterwards (I was at totality in France, clouded grrr...) that you couldn't really notice if you glimpsed through the window from the inside of non-artificially lit home: the contrast was the same and the eyes accomodate for the diference in overall lighting. When staying inside though, with blinds open, it was markedly more difficult to read a book without the aid of artificial lighting than on a normal bright day. Evidentely there are thresholds to what our visual system can manage.

[tony873004]

Your eclipse example is a good one. I was in Arizona in 1994? for an annular eclipse. We arrived at a campground near the centerline, and I saw a bunch of people with cameras and zoom lenses. But when I asked where were their solar filters they gave me a puzzled look. These people were not here for the eclipse. This was one of the biggest birding places in America (Cave Creek Canyon). My brother and I were the only people watching the eclipse. At mid eclipse with about 96% of the sun covered, it still didn't look much different that a normal sunny day. None of the birders looked like they noticed at all. With only 4% of the sun remaining, that's probably about what Europa would experience.

[Starman]

Hmm, I have a question then guys, how in the heck can we even see Uranus or Neptune? Both seem to emit a glow of their own, if the light does not in fact come from Sol.

If that's true, then the two twin giants are a heck of a lot more interesting then I previously thought.

--Starman

[selden]

Starman,

As described above, we can easily see objects that are illuminated by very dim light. Planets in the solar system are seen in the wavelengths of visible light only because of reflected sunlight.

In a dark sky, the human eye can see objects that are as dim as about a magnitude of +6. Saturn has a maximum brightness of about -0.3, which is easily visible. (Remember brighter objects have smaller astronomical magnitudes.) Uranus has a maximum brightness of about +5.6, so it's very hard to see. Neptune is +7.7, so it can't be seen without a telescope.

Does this clarify anything?

[Matt McIrvin]

Human vision (and hearing) are extremely nonlinear; you can think of them as more or less logarithmic. When light is bright, large differences in light intensity look relatively small; when light is very dim, we can easily detect fine differences in intensity. Cutting the intensity of full sunlight in half barely creates a noticeable difference; cut it down by a factor of ten and it still looks pretty bright. Sunlight is many times brighter than ordinary room lighting, as you can easily see by turning the lights on at noon. Yet we see all right with our indoor lights.

Our eyes also have a separate system of detectors just for faint light-- the rods, which can't see in color, which is why we don't see colors well when it's dark. The rods are extremely sensitive; it takes, if I recall correctly, just a few dozen photons to create a visual sensation (the rod itself can respond to a single photon hit, but retinal circuitry knocks the sensitivity down somewhat as a noise-reduction mechanism).

[piellepi]

Hi!
Here is my little contribution to this discussion...
Without too much mathematics (I'm back to the first posts, about the values of sun's illumination on planets surfaces) you must consider that the sun we see every day is by far the brightest light source we can observe.
But be careful! Obviously we cannot observe it directly because of its monster brightness!
You know that in order to see black spots on the sun's surface we must use very dark filters or we get blinded in a moment!
So I don't wonder that even standing upon Pluto's surface the sun is still bright!
I would like to go to Pluto just to witness the sun's brightness!
In the meanwhile I use Celestia ... and dream!
Good bye!
Pierluigi