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Brightness published for this comet in the article is in "magnitude" and it's a logarithmic value of total brightness as used in astronomy. Not brightness per u

by z92 14y ago

Brightness published for this comet in the article is in "magnitude" and it's a logarithmic value of total brightness as used in astronomy. Not brightness per unit area.

wtvanhest 14y ago
So if the comet is much smaller than the moon, is it reasonable to assume that it wouldn't reflect enough light to light the earth's surface like a full moon?
- Dylan16807 14y ago
  Why did you just ask the same question again? I guess you couldn't tell what z92 meant? Anyway the answer to your question is no. The article is talking about total light, in Watts. Area is not a factor.
  - wlesieutre 14y ago
    Watts is not a useful measure of visibility for humans, I would hope they're using lumens. Unless it's safe to assume that it reflects very similar amounts of visible light to the moon, but I don't know if that's the case.
    - bostonpete 14y ago
      Isn't light a synonym for visible light...?
      - montecarl 14y ago
        Only colloquially. Light in general spans the entire electromagnetic spectrum.
      - wlesieutre 14y ago
        In most contexts. But it's used frequently in "infrared light" too. Watts include radiation across the entire EM spectrum, from radio up to gamma rays, while lumens are normalized to human perception. You basically take the number of watts at each wavelength and multiply it by a factor that represents the eye's sensitivity at that wavelength. So 1 watt of IR is zero lumens, and 1 watt of blue light is fewer than 1 watt of yellow. Take a look at the spectral distribution of daylight [1]. You'll notice the area under the curve in IR is pretty similar to the amount of visible light. If the IR reflectivity of a comet were high and the visible were low (relative to the moon), knowing that they were reflecting about the same wattage wouldn't tell you much about relative brightness. [1] https://upload.wikimedia.org/wikipedia/commons/4/4c/Solar_Spectrum.png https://upload.wikimedia.org/wikipedia/commons/4/4c/Solar_Sp...
    - Dylan16807 14y ago
      Yes the scale is closer to lumens than watts, but more people probably know what a watt is so I used that. Anyway a lumen is just a way of weighting different wavelengths, and the magnitude scale uses a different way of weighting, so it's not lumens.
- z92 14y ago
  If the comet is much smaller than the full moon, then its surface will be brighter than the surface of the moon to still be of higher magnitude in brightness than moon. Magnitude is total visible luminosity of the body. Think of it as a logarithmic value of lumina. Total amount of visible light from that whole body that is visible to an observer on earth.
- rhizome 14y ago
  Comets don't reflect light, they generate it moving against solar wind and radiation. After those things are left behind it becomes a just a big ol' flying rock again.
  - danparsonson 14y ago
    To expand on this a little - the solar wind vapourises part of a comet, forming an atmosphere of sorts called a coma (and the characteristic tail) and this atmosphere glows both by reflecting sunlight and by ionisation. So yes, the cometary nucleus hardly reflects any light, but some light is reflected by the coma and the tail. (see en.wikipedia.org/wiki/Comet#Coma_and_tail, and just above that for details of light reflection from a cometary nucleus)

by z92 14y ago

Brightness published for this comet in the article is in "magnitude" and it's a logarithmic value of total brightness as used in astronomy. Not brightness per unit area.

wtvanhest 14y ago

So if the comet is much smaller than the moon, is it reasonable to assume that it wouldn't reflect enough light to light the earth's surface like a full moon?

Dylan16807 14y ago
Why did you just ask the same question again? I guess you couldn't tell what z92 meant? Anyway the answer to your question is no. The article is talking about total light, in Watts. Area is not a factor.
- wlesieutre 14y ago
  Watts is not a useful measure of visibility for humans, I would hope they're using lumens. Unless it's safe to assume that it reflects very similar amounts of visible light to the moon, but I don't know if that's the case.
  - bostonpete 14y ago
    Isn't light a synonym for visible light...?
    - montecarl 14y ago
      Only colloquially. Light in general spans the entire electromagnetic spectrum.
    - wlesieutre 14y ago
      In most contexts. But it's used frequently in "infrared light" too. Watts include radiation across the entire EM spectrum, from radio up to gamma rays, while lumens are normalized to human perception. You basically take the number of watts at each wavelength and multiply it by a factor that represents the eye's sensitivity at that wavelength. So 1 watt of IR is zero lumens, and 1 watt of blue light is fewer than 1 watt of yellow. Take a look at the spectral distribution of daylight [1]. You'll notice the area under the curve in IR is pretty similar to the amount of visible light. If the IR reflectivity of a comet were high and the visible were low (relative to the moon), knowing that they were reflecting about the same wattage wouldn't tell you much about relative brightness. [1] https://upload.wikimedia.org/wikipedia/commons/4/4c/Solar_Spectrum.png https://upload.wikimedia.org/wikipedia/commons/4/4c/Solar_Sp...
  - Dylan16807 14y ago
    Yes the scale is closer to lumens than watts, but more people probably know what a watt is so I used that. Anyway a lumen is just a way of weighting different wavelengths, and the magnitude scale uses a different way of weighting, so it's not lumens.
z92 14y ago
If the comet is much smaller than the full moon, then its surface will be brighter than the surface of the moon to still be of higher magnitude in brightness than moon. Magnitude is total visible luminosity of the body. Think of it as a logarithmic value of lumina. Total amount of visible light from that whole body that is visible to an observer on earth.
rhizome 14y ago
Comets don't reflect light, they generate it moving against solar wind and radiation. After those things are left behind it becomes a just a big ol' flying rock again.
- danparsonson 14y ago
  To expand on this a little - the solar wind vapourises part of a comet, forming an atmosphere of sorts called a coma (and the characteristic tail) and this atmosphere glows both by reflecting sunlight and by ionisation. So yes, the cometary nucleus hardly reflects any light, but some light is reflected by the coma and the tail. (see en.wikipedia.org/wiki/Comet#Coma_and_tail, and just above that for details of light reflection from a cometary nucleus)