Comments on: Darkness at the edge of town

Question: Darkness at the edge of town

nightengine — Fri, 26 Aug 2005 11:53:49 -0800

How would I determine sunlight levels for the planets in the outer solar system?

I've always been fascinated by the exploration of the outer solar system, from Pioneer to Cassini. As an amateur photographer, I've always been particularly interested in the available light levels used to image the planets and their satellites. I realize I can determine light levels with a little physics, but I'm more interested in a source that describes those levels in terms I can relate to. Instead of describing sunlight at Pluto as being 1000 times dimmer than on Earth, the ideal reference would compare it to a moonless night, a dark room with a candle, etc. My Google-fu has failed me, so I've not been able to find such a practical source.

By: norm

norm — Fri, 26 Aug 2005 12:32:10 -0800

This site discusses where the four spacecraft that are escaping the solar system are located, and the apparent brightness of the sun from their positions.

The brightness of the sun at Pioneer 10, for example, is -17.0.

Brightness is expressed in a logarithmic scale where each notch is 2.5 times the next one. -17 is therefore about 250 times (as I calculate it, bear in mind that I'm definitively NOT a math guy) brighter than a full moon on earth, which is -11 (on average).

For further reference, the sun on a clear day on earth is -26.

By: smackfu

smackfu — Fri, 26 Aug 2005 12:32:56 -0800

At the bottom of this page is a useful chart giving numerical luminance values for conditions like "full moon" or "starlight".

By: DevilsAdvocate

DevilsAdvocate — Fri, 26 Aug 2005 13:01:39 -0800

Even if you don't want an answer in physics terms, the easiest way to get to where you want is via physics.

For levels of light on earth, I'll go with the various levels as described on this page.

The illuminance is inversely proportional to the square of the distance from the light source. (For the sake of argument, I'm going to ignore how much light is absorbed by the atmosphere of the various planets, but obviusly this would make a difference if you were, say, well into the deep atmosphere of one of the gas giants--it may well be completely dark there.)

Mars is 1.52AU from the sun, so the illuminance is 43% of what you get on earth. Sounds like quite a reduction, but maybe it wouldn't be all that noticeable subjectively--notice there's a tenfold difference between "indirect sunlight" and "overcast day" on the chart.

By way of anecdotal evidence to support that, I once observed a partial (but fairly near full, something like 80% of totality) solar eclipse on a cloudless day. I noticed that I didn't feel the need to put my sunglasses on, as I normally would on a clear day--so it was noticeable to some extent--yet even during the eclipse the day was still what I would call "bright."

Jupiter: 5.20 AU from the Sun. Illuminance is 3.6% of what it is on earth. Direct sunlight on Jupiter is still a bit brighter than an overcast day on earth. Indirect sunlight on Jupiter (say, in the very high atmosphere so you get scattering of light, but not much absorption) would be about equivalent to indoor light.

Saturn: 9.54 AU from the sun. Illuminance is 1.1% of what it is on earth. Direct sunlight on Saturn is about the same as an overcast day on earth.

Uranus: 19.2 AU from the sun. Illuminance is 0.27% of earth's. Direct sunlight on Uranus would be like office lighting.

Neptune: 30.1 AU from the sun. Illuminance is 0.11% of earth's. Direct sunlight on Neptune would be like a "very dark day" on earth.

Pluto: Pluto has a more eccentric orbit than most; at times it is closer to the sun than Neptune. At its farthest point from the sun it is 49.3 AU from the sun. There, the illuminance is 0.04% of what it is on earth. Direct sunlight on Pluto is still a bit brighter than twilight on earth.

On preview, from norm's link: the furthest of those four vessels, Voyager 1, is 96.55 AU. Illuminance is 0.01% of earth's; direct sunlight there is like twilight on earth.

By: teece

teece — Fri, 26 Aug 2005 13:02:47 -0800

If you are only interested in relative measures, you can easily figure this out yourself.

The intensity of the light from the sun goes down as the square of the distance from the sun. S = P /( 4*pi*r²).

Let P = the power output of the sun. It doesn't matter what the number is. S is the intensity: power per unit area. Look up the value for r of the planets you are interested in (r = average radial distance from the sun). Plug the numbers in Excel or a calculator, and use earth as a baseline to make the measurement meaningful.

By: teece

teece — Fri, 26 Aug 2005 13:03:27 -0800

Oops, DevilsAdvocate beat me to it.

By: Wolfdog

Wolfdog — Fri, 26 Aug 2005 13:43:30 -0800

DA gives one of the best answers I've ever seen on the green.

By: nthdegx

nthdegx — Fri, 26 Aug 2005 14:32:02 -0800

First of all bear in mind that human appreciation of light levels is massively sensitive at low levels and relatively insensitive at high levels. For instance, the illuminance of the Earth's surface in broad daylight is 10,000 lux, but in direct sunlight is 100,000 lux. I'll use the latter figure as a basis of comparision by which to work out the illuminances at the surfaces of other planets. I'm not an astrophysicist. I'm not taking into account differences caused by planets' atmospheres or any other phenomenon, so I'll assume direct sunlight is a fair assumption for the normal state of other planets. I am sure this will not be the case in many instances.

I'm simply using the principle that illuminance obeys the inverse square law: an object twice as far away as another from a light source will be four times as dim. Looking up the distances of the planets from the sun and applying the inverse square law, I arrive at the following illuminances in lux:

Mercury 650,000
Venus 190,000
Earth 100,000
Mars 45,000
Jupiter 3,500
Saturn 1,100
Uranus 270
Neptune 110
Pluto 60

Bear in mind we're not thinking about looking at light sources, but at a surface illuminated by them.

Mercury: 6 times as bright as direct sunlight on Earth. Sounds very very bright, doesn't it? But given that the difference between broad daylight and direct sunlight is a factor of ten, such a difference at this level of brightness isn't going to be off the charts. I know of no Earthly light source capable of brightness in this order, however.

Venus: Twice as bright as direct sunlight on Earth: see above.

Earth: You're familiar.

Mars: Still a bright bright Summer's day, but whispy cloud is taking the edge off. An overcast day will give you 10,000 lux (so will a doctor's examination lamp at close range) on Earth, and you're still at 4 times that.

Jupiter: You're into artificial lighting levels, now; but still very bright. The problem now is that artificial light levels create a very different impression and possibly a sense that things are brighter than in fact they are. Imagine a small white room with sunshine streaming through the window, giving the room a bright lively appearance.

Saturn: Imagine your local supermarket. Look down at the floor beneath your feet. Saturn is this bright, but without the warmer glow of artificial lighting.

Uranus: You've bought your lunch from the supermarket and go back to work in your office. Yoy again decide to look at the floor. This is how bright Uranus is.

Neptune: A windowless stair well (again, don't be tempted to think about looking into the bright fluorescent light sources... they'll dazzle)... look at the walls and the floor. Neptune is something like this.

Pluto: You're still well within the limits of being able to see comfortably. Imagine a boiler room with a couple of broken light fittings, or a dimly but comfortably lit bar you frequent.

All much much brighter than moonlight, assuming direct sunlight applies. Take all of this with a pinch of salt.

By: Zed_Lopez

Zed_Lopez — Fri, 26 Aug 2005 23:43:57 -0800

Hunh. I was totally sure Pluto had to be darker than moonlight, but the more I check on it, the righter nthdegx looks. Thanks, nthdegx.