# What Do You Observe (Part 2)

Bert | 10:13 Wed 19th Aug 2015 | Science
Further to Matheous-2's question. We are told that the 'shooting stars' that we may have seen recently (the Perseid Meteor Shower) are meteors, and that they can be as small as a grain of sand. So how can we see them if they are so small?
They may be 60 miles away. I know we are not seeing a grain of sand at a distance of 60 miles, we are seeing the light emitted as it heats up colliding with molecules in the atmosphere, but I still don't see how we can see anything that small that far away. How can something the size of a grain of sand emit enough light - in all directions - for us to be able to see the tiny, tiny amount that is fortunate enough to go in the direction of our 5mm (a guess) pupils?

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Contrast that with seeing light from suns billions of miles away - we dont normally have a problem accepting that - against a black backgroung its amazing how little of a glow the human eye can detect.
Question Author
But the sun is not billions of miles away. It's 93 million miles away. Also it is somewhat larger than a grain of sand – it's big enough for us to see that it has a perceptible diameter, even with the naked eye. You could extend the argument to stars that ARE billions of miles away, and which do not have a perceptible diameter, but, again, in reality, they are massive objects putting out vast amounts of energy. I cannot deny that we see meteors, but I struggle to believe that they are the size of grains of sand. And not only do we see the light, but that light is emitted while the meteor is travelling, I don't know, hundreds of feet at least to make that streak in the sky.
I think I did which was why I said suns. If we can pick up light that far away a small item that much closer should be a doddle.
We can only see the light at night , during the day there are just as many shooting stars but we can't see them as Sunlight is far brighter, and 'drowns out' the light from the shooting star. It is actually the meteor burning up in the atmosphere that we see.
The meteor is being vaporised and heated to temperatures that make it glow.
Bert, if you are in the UK or can get access to BBC content by other means, you can do no worse than download the most recent Sky At Night episode where the presenter demonstrated how compressing air rapidly causes it to heat up. He uses a glass tube and plunger and infra red camera because of the limits of the demo equipment but the principle is the same.

At speeds of miles per second, the dust grains heat air molecules to temperatures which make them glow. Excitation of electrons by magnetism creates auraroae and heating atoms does the same thing by another route - like metal glowing white hot.

If your observing site is free of light pollution you should see a persistent glowing stripe, 5 times wider than the initial bright streak. This is either heat spreading and heating more air or vapourised bits of sand grain or a bit of both.

What fascinates me is that you would think the comet debris should be floating in space and the earth should have swept a hole through the cloud or mopped them up within a few passes. The fact we see more meteors every year means that they are moving around and entering the swept zone.

Nothing should shift its orbit around the sun without gravitation perturbation so it must be other orbiting rock chunks doing that.

Comets are not 'dead' they are continuously spewing clouds of dust and gas , this gets more intense as they get near the Sun. So there is a constantly renewing source of the dust and particles that form meteors / shooting stars.
It will never be all 'swept up' as more is always replacing it.
@EDDIE51

Ah, but what is the return frequency of the source comet? (I didn't think to write down its name, when he said it but don't worry, I can google it).

Some comets are on a parabolic orbit - one pass around the sun and then flung out of the solar system, completely.

The Perseid meteor shower is caused by the debris left by the comet Swift Tuttle on its last 'pass' of the Earth's orbit . It will be back again in 2026 and leave a new batch of dust to prolong the 'fireworks'
http://www.space.com/23066-perseids.html
One milligram at a velocity of 20km/s has a kinetic energy of 200 joules.
@Bert

If you google the two words: leonids engraving

and then click the images tab, you can get an idea of why they call these events "showers". What we have, these days is a pale imitation of what they witnessed a century or two ago.

Fair play, they didn't have light pollution, in 1833. We do, so many of the very faintest meteors just aren't visible to the modern-day observer. I presume they rarely went outdoors in the dark either so I guess there must have been a serious glow from behind the curtains, to draw people into stepping out and witnessing the event.

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