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Name: Casey
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A

When in deep space, beyond the point at which the sun's light is significant, are objects easily visible? For instance, how visible would a passing object be if the only light on it are the stars? Would this be similar to being on earth at night, during a new moon?

Dear Casey,

Without any sunlight, an object will be virtually black and invisible. It is not likely that you would see it alt all.


David Levy

One needs to generalize the "visible" to include wavelengths (or equivalently, frequency) of electromagnetic radiation outside the very narrow range (about 400 to 700 nanometers) that the human eye can see. Every object gives off radiation whose frequency distribution depends upon its temperature -- so called "black body radiation". For "hot" objects such as stars, this is approximately what produces their "brightness". An object that is too cool to radiate energy due to its temperature, can reflect radiation that falls upon it. The obvious examples being the moon and planets that reflect sunlight. The amount of radiation emitted will depend upon what is between the source and the observer. So an intervening object can cause the amount of energy that can be detected to vary. An eclipse of the moon is a classic example of the Earth blocking radiant energy from the Sun from reflecting off the Moon. There are other objects that may be faint, or even invisible, between 400 and 700 nanometers (the nominal visible range) but may be more intense at greater wavelengths. The wavelength of radiation is also influenced by the Doppler shift. For distant objects that are moving away from us rapidly, this can distort the wavelengths that are in the visible region. The bottom line is that your question is not easy to give a short simple answer because there are a lot of factors that come into play. Regards,

Vince Calder

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