Gate 2011 1.1

1.1 The increase in the length of a day on the earth at a rate of 2.4 milliseconds/100 years is due to

(A) prolate tidal bulge

(C) spring tide

(B) tidal friction

(D) bodily earth tide

Prolate Tidal Bulge: For large astronomical bodies that are nearly spherical due to self gravitation the tidal distortion produces a slightly prolate spheroid i.e. an axially symmetric ellipsoid that is elongated along its major axis.

Tidal friction: The tidal friction of the earth is manifested in a gradual increase in the length of the day. The effect is very small. Tidal theory predicts an increase in the length of the day of only 2.4 millisecond per century.

Spring Tide: The unusually high tides at opposition (when the full moon is on the opposite side of the earth from the sun) and conjunction (The new Moon is on the same side of the earth from the sun) are called spring tides.

Bodily Earth tide: The tidal deformation corresponds to a redistribution of mass, which modifies the gravitational potential of the Earth and augments the elevation

of the free surface called bodily earth tides

  So why is Earth's rotation slowing down in the first place? A large part of it is due to something called tidal braking, which basically means that the Moon's tidal pull is acting as a drag on our planet. "The heaping up of water drags on Earth as it spins underneath,“

    Incidentally, as our spin slows down, the Moon's orbit is also growing by around 4 cm a year. But that's not the only factor involved - the world's sea levels, as well as electromagnetic forces between Earth's core and its mantle, have an effect on Earth's spin too, which is why it's so unpredictable.

    One example of this is that ice ages slow the planet's rotation down by freezing all the water in the poles, causing the planet to squish slightly and spin more languidly, just like an ice skater when she puts her arms wide.

What two motions do all planets have?

    That the Moon keeps the same part of its surface always turned toward Earth is attributed to the past effects of tidal friction in the Moon. The theory of tidal friction was first developed mathematically after 1879 by the English astronomer George Darwin (1845–1912), son of the naturalist Charles Darwin.

    Tidal friction, in astronomy, the strain produced in a celestial body (such as the Earth or Moon) that undergoes cyclic variations in gravitational attraction as it orbits, or is orbited by, a second body. Friction occurs between water tides and sea bottoms, particularly where the sea is relatively shallow, or between parts of the solid crust of planet or satellite that move against each other. Tidal friction on the Earth prevents the tidal bulge, which is raised in Earth’s seas and crust by the Moon’s pull, from staying directly under the Moon. Instead, the bulge is carried out from directly under the Moon by the rotation of the Earth, which spins almost 30 times for every time the Moon revolves in its orbit. The mutual attraction between the Moon and the material in the bulge tends to accelerate the Moon in its orbit, thereby moving the Moon farther from Earth by about three centimeters (1.2 inches) per year, and to slow Earth’s daily rotation by a small fraction of a second per year. Millions of years from now these effects may cause the Earth to keep the same face always turned to a distant Moon and to rotate once in a day about 50 times longer than the present one and equal to the month of that time. This condition probably will not be stable, due to the tidal effects of the Sun on the Earth-Moon system.