Answers to last issue's Do You Know?

1. What is the difference between petrol and diesel?
Fossil fuels are fuels containing carbon - coal, oil and gas - that were formed over millions of years.
Oil (sometimes called petroleum) is formed from the decayed remains of animals and plants. Under the influence of heat and pressure, the decayed matter breaks down first into liquids and into gases. Both the liquid (petroleum) and gas phases (natural gas) collect in pools under the earth's surface. After a drilling and pumping process to extract it, oil is refined and turned into a variety of petroleum-based products.
Crude oil is the mixture of petroleum liquids and gases (including impurities such as sulphur) that is pumped out of the ground by oil wells. Crude Oil is extracted from the ground in several processes, the most common being beam pumps.
Production of petrol or gasoline is achieved by distillation of crude oil. Petroleum is another word for oil (see "Oil"). After being pumped up from the earth, petroleum is refined and turned into many products, including kerosene, benzene, gasoline (petrol), paraffin wax, and asphalt. Other materials that we use every day, like plastic and nylon, are also petroleum-based products.
The alkanes, also known as paraffins, are long chains of saturated hydrocarbons which contain only carbon and hydrogen and have the general formula C_n H_2n+2. They generally have from 5 to 40 carbon atoms per molecule, although trace amounts of shorter or longer molecules may be present in the mixture.
The type of fuel obtained depends on the type of hydrogcarbon. The alkanes from pentane with 5 carbon atoms (C_5 H_12 ) to octane with 8 carbon atoms (C_8 H_18 ) are refined into petrol (gasoline), the ones with 9 to 16 carbon atoms are turned into into diesel fuel and kerosene (primary component of many types of jet fuel) and heavier ones into fuel oil and lubricating oil.
At the heavier end of the range, paraffin wax is an alkane with approximately 25 carbon atoms, while asphalt used to surface roads has 35 and up. The heavier ones are usually cracked by modern refineries into more valuable products. Any shorter hydrocarbons are considered natural gas.
Hence petrol is a fossil fuel refined from crude oil. It evaporates easily and is mixed with air before it enters the engine to burn more cleanly. Diesel is also a fossil fuel that is refined from crude oil. It does not evaporate easily and is ideally suited for burning in diesel engines.
Diesel is a fuel, which like gasoline, is refined from oil. It is heavier (about 15% more dense) and oilier than gasoline. It is generally cheaper, too, because it needs less refining than gasoline.
The exhaust emissions from diesel engines contain low levels of carbon monoxide and hydrocarbons but they emit more particulates and oxides of nitrogen (Nox) than petrol engines.

2. Why don't you have an eclipse on every full moon day?
On every full moon day, the Moon is "behind" the Earth, as seen from the Sun. That is why an observer on the night-side of the Earth, facing in the direction opposite from the Sun, sees the full half of the Moon which is lighted up by the Sun.
If the Moon were directly behind the Earth, then the Earth would block the Sun's light from reaching it and the observer would see a lunar eclipse. This is why lunar eclipses only happen on full moon days.
But lunar eclipses do not happen on all full moon days. This is because the Moon, which is "behind" the Earth, is not directly behind. This can happen because the orbit of the Moon around the Earth is at angle of about 5 degrees with respect to the orbit of the Earth around the Sun. So the angle "Sun-Moon-Earth" on a full moon day can vary from 175 degrees to 180 degrees.
When this angle is between 179.5 degrees and 180 degrees, we have a lunar eclipse. But when it is between 175 degrees and 179.5 degrees, as is the case on most full moon days, the Earth's shadow does not fall on the Moon, we do not have an eclipse, and we see the full moon lighted up. You can almost write a poem about it!
Fig caption: Schematic diagram of the shadow cast by the Earth. Within the central umbra shadow, the Moon is totally shielded from direct illumination by the Sun. In contrast, within the penumbra shadow, only a portion of the sunlight is blocked.

3. Why is too much salt bad for your body?
Refined salt is mostly sodium chloride. Sodium is one of the primary electrolytes in the body. All four cationic electrolytes (sodium, potassium, magnesium, and calcium) are available in unrefined salt, as are other vital minerals needed for optimal bodily function.
Sodium plays an important role in the regulation of muscle contraction, fluid balance and nerve impulses in the human body and it is essential for overall good health. All our body fluids including blood, sweat, tears, etc contain sodium. It is essential to maintain proper balance of sodium in these fluids. Sodium in the body is mainly found in fluids that surround the body's cells, such as blood and lymph fluid. When sodium intake exceeds the amount the body can handle it builds up in the interstitial areas and the kidneys have to work extra hard to excrete it.
Too much or too little salt in the diet can lead to muscle cramps, dizziness, or even an electrolyte disturbance, which can cause severe, even fatal, neurological problems. Drinking too much water, with insufficient salt intake, puts a person at risk of water intoxication.
The risk for disease due to insufficient or excessive salt intake varies because of biochemical individuality. Some have asserted that while the risks of consuming too much salt are real, the risks have been exaggerated for most people, or that the studies done on the consumption of salt can be interpreted in many different ways.
Excess salt consumption has been linked to: exercise-induced asthma, heartburn, osteoporosis, especially in women, and hypertension (high blood pressure).

4. Where is the continent called Pangaea?
The continent Pangaea does not exist today. But, according to the theory of continental drift of the German geologist Alfred Wegener, it existed 25 crore years ago. It was a "supercontinent" which included all of our continents of today. See the figure on the front inside cover of this issue of {Jantar Mantar}.
One of the reasons why we believe this is that fossils of some shrubs, small dinosaurs and small reptiles which lived in that age are today found spread over several continents, which can be explained if these areas were once close together.
The breaking up of Pangaea happened with the creation of a new ocean, today's Atlantic Ocean, which split up Africa and North America; then with the separation of Africa, India, Australia and Antarctica, by the southern part of today's Indian Ocean; and finally by the opening of today's Norwegian Sea which split Greenland from Europe.
The breaking up is not yet over. It is believed that the Great Rift Valley of Africa might be covered with a new ocean which will separate its eastern part (today's Somalia, Kenya, Tanzania and Mozambique) from the rest of the continent.

5. What is a solstice and why is it special?
The answer to this question will appear in the next issue of JM. --Ed.