Why does the moon turn red during a lunar eclipse?

You can credit Earth's atmosphere with providing an orangish color to the moon during an eclipse. The atmosphere acts like a filtered lens. It bends red sunlight into our planet's shadow and scatters out blue light. It's the same reason why sunrises and sunsets appear reddish. If Earth were an airless planet, its shadow would be pitch black and the eclipsed moon would be invisible.

If you were an astronaut standing on the moon during totality, you would see Earth eclipsing the sun. Earth would appear as a dark disk surrounded by a brilliant red ring?our atmosphere glowing with the light of all the planet's sunsets and sunrises. It's this light that we see bathing the moon during totality.

What causes the Northern Lights?

The Northern Lights, or aurora borealis, arise when charged subatomic particles (protons and electrons) from the sun react with gases in Earth's upper atmosphere. The sun continually releases these particles in a discharge called the solar wind. Earth's magnetic field captures some of the particles, collecting them in doughnut-shaped magnetic regions called the Van Allen belts. The solar wind normally sweeps around Earth's outer atmosphere. As it does, it builds up a charge of more than 100,000 volts between the magnetic field and the atmosphere. This forces some of the charged particles out of the Van Allen belts and funnels them into the atmosphere at the north and south magnetic poles. The ions spiral downward, exciting the gases they encounter into a glowing frenzy.

Because this stream of solar particles is continual, people living near the north and south magnetic poles can see an aurora on nearly every clear night. To see these at lower latitudes, we need a large solar storm, or flare. When the sun has a major eruption it releases huge quantities of charged particles, creating a "gust" in the solar wind. When the particles reach Earth, they swamp the Van Allen belts and pour into the atmosphere. The huge influx of particles creates an awe-inspiring display that can pulsate and shimmer along Earth's magnetic field lines.

The most common type of aurora is a curtain of green caused by charged particles hitting oxygen atoms some 60 miles high. Slower particles that don't travel deeply into the atmosphere cause red aurorae by hitting atoms 120 miles up. Green aurorae with red fringes are a mixture of the two. Highly energetic particles can travel deeper into the atmosphere to create a red base to the common green curtain. These particles are reacting with nitrogen some 45 miles up.

If there is a black hole in the middle of our galaxy, approximately how long will it be until Earth is pulled in?

Surprisingly, not even our distant ancestors will have to worry about this less-than-appealing fate. According to Newton's law of gravity, the pull of a black hole depends only on its mass and its distance from us. A black hole at the center of our galaxy would have no more effect on us than a huge star cluster having the same mass.

In the same way, if a black hole with the sun's mass replaced our sun, Earth would continue to orbit the black-hole sun just as it does now. (Although it would quickly get a lot colder!) A black hole's reputation for immense gravity comes from the fact that you can get much closer to it. A black hole with the sun's mass would be only a couple of miles across?compared with the sun's 864,000-mile diameter?and that factor of about 100,000 translates into a 10 billion-fold increase in the gravitational pull near the surface of such a black hole.

How can I find out how far away a particular star is?

Tables listing distances (among other data) for bright and nearby stars are readily available in The Observer's Handbook, published each year by the Royal Astronomical Society of Canada. Distances for many thousands of stars, bright and faint, can be found in volume 1 of Sky Catalogue 2000.0.

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