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Comment by netcraft | Hacker News Reader

> The beam of a blazar is only visible along a narrow line of sight. If the Earth is not within that line of sight, they are easily recognizable by astronomers. Thus, detecting objects can be extremely difficult (and fortuitous).

I don't really understand the "is not within" in that second sentence. They're easier to see if we aren't in the line of sight?

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This a class of objects that look differently depending on the direction from which you look at it. Its shape is easily recognizable when looked at from the side, but when looking directly at the bean is just a point source of radiation.

Thus, while it is more luminous when looking at the bean, it may s easier than identify when looked at from the side.

Imagine looking at flashlight at nighttime. The flashlight is bright when looking at it directly but then it is difficult to tell what kind of light source it is exactly. Tilt it a little and it becomes less bright but more recognizable.

Blazars are just quasars with the jets pointed towards us. Quasars are basically galaxies with active super massive black hole in its centre. Luckily for us, our galaxy has a quiet black hole (quiet meaning it's not feeding like crazy). This picture might help https://ned.ipac.caltech.edu/level5/March04/Torres/Figures/f...

BL Lac in the picture is a blazar. So same object is classed differently depending on is orientation with respect to us.

I had trouble with it as well and concluded that it was an error that was not caught in editing. The correct sentence must be "If the Earth is within..."

Yes, I also noticed that. I think it should be either "not within -> not easily recognizable" or "within -> easily recognizable".

It's correct (but misleading) as is, actually. It's the kind of object that sends a beam out, and if you're being hit by the beam, it's hard to recognize, but by looking at it from the side, it becomes easier.

I think the "beam of light" coming from the blazar is not always facing towards us- meaning it is harder to see when it is not in a particular orientation. "Not within that line of sight" is an awfully confusing way to say that, yah

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