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Basic Astronomy Questions


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That's a noble suggestion, Oliver. Good on ya!

Maybe you could also post up little pointers, stuff you think we should keep in mind or know. Even if you think some will already know the stuff, it is always good for everyone to revise and a great way for you to solidify your own studies and passion.

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  • 4 weeks later...

I have spent 2 days wrestling with True North/Magnetic North/Grid North/Celestial North/Geographical North etc. If you could summarise that it might save people going through what I did!

Also I cold check that I have in fact understood it.

Cheers!

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True north is the direction of the north pole, so it is straight up a line of longitude passing through where you are. If you turn and face the north pole, you are looking true north.

Magnetic north is the direction of the magnetic north pole, which is somewhere in northern Canada. Its a few degrees off from true north, depending where you are, and typically you add in an offset for your location as a quick fix. It changes over time, whereas the north pole doesn't. Its called magnetic declination. If you're calculating a course at sea or in the air, you often work in true coordinates, then apply the relevant declination so you can navigate by a magnetic compass (or vice versa - work in magnetic coords and adjust to true).

Grid north is just a distortion on the map. As they are printed square, with square grid lines, the ones out to the side are not exactly north, because we live on a sphere - but for most small maps the difference is negligible.

Celestial North is just the true north projected out. So imagine the Earths globe spinning, then a skewer through the north to south pole extended out into space - that is the celestial north.

Geographical north is the same as the north pole, or the terrestrial north pole.

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  • 1 month later...

Has anyone ever observed gravitational lensing?.

I remember seeing a picture somewhere but not sure how or where/what it was taken from.

I`m guessing it was from Hubble.

Could/was the effect ever used to reconstruct an image of a distant object?.

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Has anyone ever observed gravitational lensing?.

I remember seeing a picture somewhere but not sure how or where/what it was taken from.

I`m guessing it was from Hubble.

Could/was the effect ever used to reconstruct an image of a distant object?.

There are hundreds and thousands probably of gravitational lenses. So yes lots have been observed, and one of the common things is to invert the distorted image using the known mass doing the distortion to reconstruct the original.

In fact I believe there are surveys looking just specifically for lensing objects, and thats before you even consider microlensing and similar.

So yes - there are lots of lenses, from things like the classic Abell 2218 cluster, to symetric lenses known as Einstein crosses.

Its a very active area of research.

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I think I know the answer but I'll ask the question anyway.

Most lunar eclipses (total) show a usual pale ruddy-orange Moon from the Sun's light refracting through our atmosphere and then illuminating the shadowed Moon.

OK, what causes a 'blue' Moon during a total lunar eclipse? No, not the 'once in a blue Moon' thing - the shadowed Moon has a very noticable pale blue color instead of the usual eclipse color.

I have observed one such eclipse and the color was an awesome pale blue but these 'blue Moon' eclipses are rare. The one I saw was fairly close to my horizon, as a hint to what I think is the answer. :)

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It is because if the light is close to the horizon, it has to go through a thicker layer of atmosphere than it would do if it was perpendicular to the ground. Electrons are absorbed and scattered due to Raleigh scattering, and with the atmosphere this preferentially scatters the blue light, so we see it as blue for the same reason we see the sky to be blue.

This may not be the astronomically correct answer. It I think it is somewhere along these lines.

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It is because if the light is close to the horizon, it has to go through a thicker layer of atmosphere than it would do if it was perpendicular to the ground. Electrons are absorbed and scattered due to Raleigh scattering, and with the atmosphere this preferentially scatters the blue light, so we see it as blue for the same reason we see the sky to be blue.

This may not be the astronomically correct answer. It I think it is somewhere along these lines.

This would be a reason for seeing a bright object near the horizon as red, not blue (as in red sunsets etc). I can't think of a reason for a blue lunar eclipse, so please tell us your answer Mr Q!

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Thats a really handy chart - thanks for that.

The answer however is a bit of a surprise - I know some are unpredictable but aren't things like Halleys Comet on a predicatable orbit - so should we not be able to at least hazard an informed guess when next they will appear in the night sky in relative proximity or am I completey over simplifying things?

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Thats a really handy chart - thanks for that.

The answer however is a bit of a surprise - I know some are unpredictable but aren't things like Halleys Comet on a predicatable orbit - so should we not be able to at least hazard an informed guess when next they will appear in the night sky in relative proximity or am I completey over simplifying things?

We can track the next time comets like Halley's Comet are visible (2062 in that case) as we know there orbits. Most of the long period comets come form the oort cloud 1 light year away from the sun, so we have no way of knowing which is going to be disturbed from it and come our way next.

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This would be a reason for seeing a bright object near the horizon as red, not blue (as in red sunsets etc). I can't think of a reason for a blue lunar eclipse, so please tell us your answer Mr Q!

Sorry about that, seems I was leading you up the wrong path. Thanks for the clarification acey.

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We can track the next time comets like Halley's Comet are visible (2062 in that case) as we know there orbits. Most of the long period comets come form the oort cloud 1 light year away from the sun, so we have no way of knowing which is going to be disturbed from it and come our way next.

Additionally, new long-period comets tend to be detected when they are still faint and a long way off: predicting whether they will become naked-eye objects is very difficult, and there are often high expectations of comets that turn out to be damp squibs. Comets sometimes fragment as they move closer to the sun, or else just fail to live up to prediction (e.g .Comet Kohoutek). Equally, there can be comets that suddenly brighten unexpectedly due to an eruption of material (e.g. Comet Holmes). "Great comets" appear about once a decade on average.

http://en.wikipedia.org/wiki/Great_Comet

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Just wondering, does anyone know if anyone has ever come up with a definition for energy itself, as I was thinking that I didn't know any way that you could define it as it is so varied.

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Ok, I have a question.

I was discussing this last night with my most excellent OH.

Orbits, now circular or close to circular orbits I understand, I read a lot and Hawking/Newton/Einstein make sense. So the Sun acts to 'pull' objects towards it and if they travel with enough velocity they will be pulled towards the Sun but miss (basically). So if you can climb a high enough ladder and throw a tennis ball with enough force, it will orbit the earth (I know impossible, but in theory).

Now my problem is orbits that are exaggerated ellipses, lets take Halley's comet for an example.

post-34005-133877779325_thumb.gif

What causes the sudden (relatively) turn (marked with the red circle) back into the solar system? Is it simply that the comet loses so much momentum that the pull from the Sun makes it slow until it almost stops and then starts to 'Fall' back towards the Sun where it gets its sling shot back to the red circle again?

I know this seems simple, its just I have a problem with a comet not having any or very little momentum, so thought there might be a much better explantion :)

I think, by writing this out I have answered my own question, but I will post anyway so that those more knowledgeable can add/change/completely refute my possible answer :(

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Just wondering, does anyone know if anyone has ever come up with a definition for energy itself, as I was thinking that I didn't know any way that you could define it as it is so varied.

Classical definition: "capacity to do work". There are other more modern definitions too.

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What causes the sudden (relatively) turn (marked with the red circle) back into the solar system? Is it simply that the comet loses so much momentum that the pull from the Sun makes it slow until it almost stops and then starts to 'Fall' back towards the Sun where it gets its sling shot back to the red circle again?

That's pretty much it. A bit like what happens when you throw something in the air, except that for orbits you have to take into account the weakening of the sun's gravitational force as you move further away (hence Kepler's laws etc).

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That's pretty much it. A bit like what happens when you throw something in the air, except that for orbits you have to take into account the weakening of the sun's gravitational force as you move further away (hence Kepler's laws etc).

Excellent, thats what I love about science, you think something is confusing, sit and think about it for awhile and suddenly its all clear.

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