On Neptune, day & night are the same: dark & cold

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

04 December 2011

Wilson Wahome thinks that “because the sun is not obstructed in outer space, there is always light and never darkness. Is that so?” For some one who has lived on Earth all his life, that would be an expected assumption. After all, when you look up in the sky during a clear cloudless day, it all appears very bright (actually, blue). Now I have explained in a past article that the blue sky is simply the colour of the air in the atmosphere, thus we are left to wonder what we would see if we went into outer space.

It common knowledge that the farther away a source of light is the dimmer it appears. But the relationship between brightness and distance is not direct: the intensity decreases in proportion to the square of the distance.

For example, the intensity of solar radiation on Earth (150 million kilometres from the sun) is about 1,000 watts per square metre. On Mercury (60 million km from the sun), the intensity is about 6,250W per square metre.

We get that answer by squaring the ratio of the distances and multiplying the result by the intensity on earth. That is: 150 divided by 60 is 2.5; the square of 2.5 is 6.25; the product of 1,000 and 6.25 is 6,250.

The same calculation applies when we go planets that are farther away. The farthest is Neptune and it is located about 4.5 billion km from the sun. This is 30 times the distance to the Earth. Therefore, the intensity of sunlight on Neptune is about one nine hundredth (one divided by 30x30) of that on earth. That is, only one watt per square metre – very dim and very cold as well.

There is another factor that we must consider. This is that the farther away an object is, the smaller it appears to be. The sun is about 1.4 million km in diameter and the moon only 3,400km, yet the two seem as if they are the same size when viewed from Earth. The reason is that the sun is much farther away (150 million km) than the moon (380,000km).

An interesting question then arises: at what distance does an object begin to appear like a dimensionless point? The answer is when the distance from the observer is about 3,400 times its diameter. Thus if you viewed the sun from about 4.8 billion km away (i.e. 1.4 million times 3,400), it would appear like an ordinary star – a bright point in the sky.

For this reason, I expect that the sky in Neptune (4.5 billion km from the sun) during the day looks like that on Earth at night. That is, very dark with many stars dotted all over. From that distance, the sun is indistinguishable from the other stars!

Thus the answer to Wahome’s questions is no; out space is not bright. It is mostly dark and cold. In fact, it is estimated that if you were dropped at a random place in the universe (and don’t ask, “Dropped by whom?”!) you would not see any stars. They would all be too far away to be visible.