________ planet in solar system found ________ k.m. from sun and takes 88 days to complete one rotation around sun.

Question

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Correct — option (2), Mercury, 5.8 crore. The stem carries two blanks and a single clue that fixes both at once: the planet that goes round the Sun in 88 days. Mercury is the innermost planet of the solar system and orbits the Sun once every 88 Earth days, the shortest year of any planet, and it does so from an average distance of about 58 million kilometres — that is 5.8 crore kilometres, which is exactly what the second blank in this option supplies. The two halves of the option agree with each other and with the clue, and no other option achieves that. The relationship behind the numbers is worth holding rather than merely memorising: by Kepler's third law the square of a planet's orbital period is proportional to the cube of its mean distance from the Sun, so the nearer a planet is, the faster it moves and the shorter its year. Mercury is nearest and therefore quickest. One printing point deserves attention. The stem says the planet 'takes 88 days to complete one rotation around sun', but going round the Sun is a revolution, not a rotation; rotation is the spin of a body on its own axis, and Mercury's rotation takes about 59 Earth days. The looseness is the English column's alone: the Marathi prints सूर्याभोवती एक परिभ्रमण, and परिभ्रमण is revolution about another body as against परिवलन for axial spin, so that sentence is exactly right. The fact being tested is plainly the orbital period either way, since 88 days is Mercury's year. A candidate should read the number rather than the word here, and should keep the two motions separate in their own notes.

Why the other options are wrong — (a) Venus is not the planet with an 88-day year, and 8.8 crore kilometres is not its distance. Venus orbits the Sun from an average distance of about 108 million kilometres — 10.8 crore — and completes one orbit in 225 Earth days. The option's number looks like Mercury's 88 with a decimal point moved, which is the trap: a candidate who remembers the figure 88 in connection with this question but not what it measures may accept 8.8 crore as the distance and Venus as the planet. Venus is worth knowing for the opposite oddity, that its rotation takes 243 Earth days, longer than its own year of 225, and that it spins in the reverse direction to most planets.  (c) Mars is neither the fastest orbiting planet nor 2.8 crore kilometres from the Sun. It lies at an average distance of about 228 million kilometres, that is 22.8 crore, which is one and a half astronomical units, and its year is 687 Earth days, nearly twice the length of ours. The distance printed in this option is not merely wrong but impossible: 2.8 crore kilometres would place Mars less than half as far from the Sun as Mercury, which is the innermost planet. That kind of internal impossibility is worth watching for in a two-blank question, because it can be caught without recalling the correct figure at all — the order of the planets outward from the Sun is enough.  (d) Saturn is the sixth planet from the Sun, at an average distance of about 1.4 billion kilometres or 9.5 astronomical units, and its year runs to about 29.4 Earth years — the very opposite of an 88-day orbit. The figure of 5.2 crore kilometres offered here would put Saturn closer to the Sun than Mercury, which is impossible for a planet whose orbit lies beyond Jupiter's. Note also that the number 5.2 belongs to the solar system in a different place: Jupiter's mean distance from the Sun is about 5.2 astronomical units, and this option appears to be trading on the familiarity of that figure while attaching it to the wrong planet and the wrong unit.

Concept: The eight planets of the solar system fall into two groups. The four inner or terrestrial planets — Mercury, Venus, Earth and Mars — are small, rocky and close in, at 58, 108, 150 and 228 million kilometres from the Sun respectively. Beyond the asteroid belt lie the four giants: Jupiter at about 5.2 astronomical units, Saturn at 9.5, then Uranus and Neptune. Orbital period lengthens rapidly with distance, from Mercury's 88 Earth days through Venus's 225 and Earth's 365 to Mars's 687 and Saturn's 29.4 years, and Kepler's third law states the rule exactly: the square of the period is proportional to the cube of the mean distance. Two distinct motions have to be kept apart. Revolution is the journey around the Sun and gives the year; rotation is the spin about the body's own axis and gives the day. The two are unrelated in length — Mercury revolves in 88 days but rotates in 59, and Venus rotates in 243 days, longer than its own year of 225, and does so in the retrograde direction. Mercury is also the smallest planet, has almost no atmosphere and shows the greatest temperature range between its day and night sides.

State PCS 2021 Q30 — Geography PYQ with Answer

State PCS 2021 Q30 — Geography PYQ with Answer