Question Corner

Showing only one face

QUESTION: Why do we always see only one side of the moon?

Nagasubramanian, Chennai

ANSWER 1: The Moon revolves around the Earth in a period of about 27 days; it also rotates once on its axis in the same time and so it always keeps the same face towards the Earth. This phenomenon is known as captured rotation.

Inspite of the fact that the Moon's axial rotation is equal to its period of revolution round the Earth, we can actually examine more than half of the total surface. The reason is that Moon travels round the Earth in an ellipse, not in a circle since it takes elliptical path, the rate of axial spin remains constant, whereas orbital velocity changes and moves fastest when closest to us. We can thus see a little round alternate edges of the Moon. Also, the lunar orbit is tilted with reference to ours, so that the Moon is sometimes north and sometimes south of the mean plane, enabling us to see some way beyond alternate poles. These minor shifts, known as Librations, allow us to examine four- sevenths of the total surface. The remaining three-sevenths of the Moon is permanently hidden from our inquiring eyes.

Leo Christopher, Sirumugai, T.N.

ANSWER 2:The time taken for the moon to turn on its axis once and the time taken for it to revolve once around the earth are the same. Hence the moon shows us the same face every night. This is called synchronosis rotation or captured rotation.

N.Dharmeshwaran, Chennai

Cholesterol control

QUESTION: Can `total cholesterol' levels vary widely in different samples taken over a short period of time when a person is observing near staple food habits ?

R.Raghavan, Chennai.

ANSWER: Total blood cholesterol level should be less than 200 mg/dl. When a person is observing near stable food habits, life style, exercise levels, stress and strain and does not have any other disorders such as Diabetes Mellitus and under no medication there is no reason for marked fluctuation in the level of blood cholesterol. But cholesterol synthesis is a complex and energy expensive process, and it is clearly advantageous to be able to regulate the synthesis of cholesterol so as to complement the intake of cholesterol in the diet. In mammals, cholesterol production is regulated by intracellular cholesterol concentration and by the hormones glucagon and insulin.

Cholesterol, a white wax like substance is needed to build cells and make hormones. It is produced in the liver, intestines and other cells. Some cholesterol enters the body through foods. Lipids are carried through the blood by Lipoproteins. Lipoproteins are mainly of 3 kinds i.e. LDL, HDL & VLDL (containing Tg.) LDL - low density lipoproteins which carry cholesterol from the liver to the various parts of the body.

High density lipoproteins (HDL) pick up cholesterol, convert them into cholesterol esters (which cannot be deposited in arteries) and take it back to the liver for elimination. HDL is called good cholesterol because it takes cholesterol away from the arteries.

Present expert opinion is that the ratio of LDL to HDL should not be more than 5. For example, if the LDL cholesterol is 200 mg/100 ml one should have HDL of at least 40 mg/ 100 ml. If one has a LDL to HDL ratio below 5, then he is relatively safe. This simply means that HDL is the deciding factor for reducing the risk of heart attacks.

HDL level is increased by aerobic exercise and abstinence from alcohol and with diet control i.e., a healthy diet rich in fresh fruits, vegetables, cereals and poly-unsaturatical fatty acids (PUFA) rich oils.

R. Muhunthan, Coimbatore, T.N.

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