Absence of tides
PHOTO K. GAJENDRAN
What is the reason for ocean tides? Why is it that no tides are observed in rivers?
B.N. ARJUN
Visakhapatnam, Andhra Pradesh
Tides are huge shallow water waves, caused by a combination of the gravitational force of the moon and sun and the motion of earth. Two phenomena contribute to make tides. One, gravity and inertia force cause the ocean surface to bulge. Two, tides occur as earth rotates beneath the bulges.
Gravity tends to pull Earth and the moon toward each other, but inertia — the tendency of moving objects to continue in a straight line — keeps them apart. Earth and moon don’t smash into each other because they are in a stable orbit: their mutual gravitational attraction is exactly offset by their inertia.
Earth-moon system revolves once a month (27.3 days) around the systems’ centre of mass, Because Earth mass is 81 times that of the moon, this common centre of mass is located not in space but 1650 km inside earth. The moon’s gravity attracts the ocean surface toward the moon. Earth’s motion around the centre of mass of the Earth – moon system throws up a bulge on the opposite side of Earth. Two lunar tidal bulges result.
Sun influence is only 46 per cent that of moon, since it s 387 times farther away from moon despite being 27 million times more massive than the moon. Thus solar tides result by gravitational and inertial interaction of the sun.
When Earth, sun and moon are in line lunar and solar tides will be additive, resulting in spring tides. When earth, moon and sun form a right angle solar tide will diminish, lunar tide and neap tides result.
Tides occur in small bodies of water like rivers, lakes even in a small glass of water, but they are too small to detect. Each molecule of water in a river responds to the same planetary forces that affect molecules in the ocean.
L. Dayaanandu
Engineer, VSSC, ISRO
Thumba, Thiruvananthapuram
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