Answers to last issue's Do You Know?

1. I know that planets rotate on their own axes. Why does this happen? Are there any special forces that cause this?

Answer: If you strike a billiards ball with the cue (stick), then sometimes you only manage a glancing blow on the ball sending it into a spin rather than sending it across the table. Most scientists believe that planets probably acquired their spin in much the same way, when clumps of matter collided during the planets' formation about 4.5 billion years ago.
When our solar system was nothing but a cloud of gas and dust, what was likely a shock wave from a nearby supernova bounced up against it and caused it to collapse. As it collapsed, its own gravitational forces pulled it into a flat, spinning disk. And since everything in our solar system was formed from that same disk, its momentum sent nearly everything spinning in the same direction. (Almost everything: Uranus and Venus are different. Venus travels around the sun once every 225 Earth days but it rotates clockwise once every 243 days. This is perhaps due to subsequent collisions with asteroids.)
Why have they continued spinning? This is because of inertia. In the vacuum of space, spinning objects maintain their momentum and direction — and hence their spin, because no external forces have been applied to stop them. So, the planets in our solar system keep spinning.
The axis of rotation is different from the magnetic poles. This difference creates force for the earth to rotate on its axis. The same is the case with all planets in the solar system. Moon does not have a magnetic field; hence it does not rotate on its axis.
Thus we have the following picture. The sun itself rotates slowly, only once a month. The planets all revolve around the sun in the same direction and in virtually the same plane. In addition, they all rotate in the same general direction, with the exceptions of Venus and Uranus.

2. I heard somewhere: A human can survive 3 minutes without air, 3 days without water, and 3 weeks without food. How long can one really survive without food or water?

Answer: Some years ago, a Canadian couple got stuck in the mud in a van in Nevada in the middle of nowhere. They waited for help for 3 days, then the husband went looking for help. He was never found, but the woman, Rita Chretien, was found by a group of hunters after she had been in the van for 48 days ! She was nearly dead, and had lost 15 kilos. But she had survived, eating only some candy and drinking water from a stream nearby.
At the age of 74 and frail in body, Mahatma Gandhi survived 21 days of total starvation while only allowing himself sips of water. Gandhiji did not have much energy reserve before the fast either. (Note that Gandhiji performed a total of 14 hunger strikes !)
As one may expect, there is not enough authentic medical data to know exactly how long human beings can survive in near-total starvation conditions with continued hydration. There have been many reported instances, but we do not have exact information on caloric intake, so prediction is difficult. But what is clear is that the body can moderate metabolism to conserve energy and that individual survival varies markedly. The body's ability to alter its metabolism is poorly understood, but it occurs at least in part through changes in thyroid function. 
In fact, this is one reason scientists attribute to explain how genes causing diabetes have persisted in evolution, since these were perhaps helpful in the past to survive periods of starvation by enabling more economical use of energy.
Most important factor of all, however, appears to be hydration. Even little sips of water matter. Rita Chretien survived her 48 day ordeal in large part due to the availability to some melted snow for drinking. Indeed, had no water been available, she may not have survived. In examples of hospitalized individuals who are in a persistent vegetative state, who become cut off from artificial sustenance, death occurs within 10-14 days. Note that these people are in a coma and completely immobile, thereby consuming the lowest amount of energy possible. So it is likely that the same conditions (no food or water) in a person who is at least somewhat active, and who may perspire, would only lead to a much swifter end.

3. How does a flame look in zero gravity?
Answer: In space you cannot have fire since there is no oxygen to sustain combustion. Inside a spacecraft or in the International Space Station, you have the same air as on Earth, but because gravity is millions of times weaker, an open flame behaves very differently.
How does fire work, here on Earth? As fuel burns, it heats the air around it making it less dense. As gravity pulls down anything with a higher density, the hot air travels upwards and leaves the vicinity of the fire. With the hot air gone, fresh air is drawn into the gap providing a new source of oxygen-rich air. This is called buoyancy and it is what makes the flame shoot up and flicker. Thus, the cycle continues until all the fuel is used up. 
In microgravity, there is no updraft and oxygen is drawn into the flame through a completely different mechanism. The first such experiment was performed in 1997 aboard the Columbia shuttle. Scientists noticed that the flame was spherical, like a fireball. The flame is fed by diffusion, which is a much slower process. The flame occurs at a border between fuel and air; effectively the entire surface of the flame is the “bottom”, reacting with fresh air close enough to the fuel source to combust, resulting in a rough sphere. Because exhaust gases like carbon dioxide cannot leave the combustion area, by the same principle, the outward diffusion of combustion gases can limit the inward diffusion of oxygen to such an extent that the zero gravity flame will die a short time after ignition.
Fire also has a different color in microgravity. When a candle burns, it is being consumed molecule by molecule. Sometimes, the fuel, long strings of carbon, gets pushed upwards where it burns like charcoal, glowing yellow. Without gravity, the carbon strings do not get burned, and the flame is blue, cooler, and much dimmer.

4. Did travellers in the past actually depend on the North Star to guide them? How could they use its position?

Answer: Imagine setting up a camera near the North Pole in winter, pointing it at the sky and then taking pictures over one (long, dark) day. Here is what you would see in the time-lapse video: Polaris, (or the North Star, or the Pole Star) almost directly overhead, like a beacon. Over 24 hours, the rest of the stars would appear to slowly circle it.
The stars' circling is really an illusion, of course. It is the Earth that is spinning under the stars, taking 24 hours to turn once around. While the Earth turns, Polaris appears to stand still only because of its position in the sky: lined up almost perfectly with our planet's axis.
How does this work? Earth's North (celestial) pole traces a small circle over 24 hours as the planet turns. Since the pole happens to be pointed at Polaris, the medium-bright star is always directly overhead there. That makes Polaris the Earth's North Star. (There is no corresponding South Star, simply because the South celestial pole is not pointing at any easily visible star.)
Since Polaris stands above the North Pole like a glowing directional beacon, it is the star to steer by in the Northern Hemisphere. Sailors, hikers, and even birds have used it to find their way in the dark for many centuries.
While Polaris is directly overhead at the top of our planet, it sits on the horizon at the Earth's equator. Just south of the equator, Polaris disappears from view. Between the North Pole and the equator, Polaris is at an in-between position in the sky, corresponding to your latitude north of the equator. This can help you figure out where you are, even in the middle of the sea.

Sources: Discover, Space, Scientific American, ZMEScience