MPSC 2021 Prelims Paper-I · Question 30 of 100

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.

• (1)Venus, 8.8 crore — 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.
• (3)Mars, 2.8 crore — 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.
• (4)Saturn, 5.2 crore — 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.

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.

MPSC keeps a small block of basic astronomy in its general studies paper, and the questions are set on the fixed numbers — the order of the planets, the extremes among them, and the two or three figures that every textbook carries. This item is a fill-in-the-blank with two gaps, which is a form worth recognising because it can be answered from either gap. If the planet with an 88-day year is known, the distance follows; if the distance of 5.8 crore kilometres is known, the planet follows. That doubles the chance of a correct answer and it also supplies a check, since a wrong option will usually be internally inconsistent — two of the three here place a planet at a distance that would put it inside Mercury's orbit, which can be spotted without knowing any figure at all. The loose use of 'rotation' where 'revolution' is meant is the paper's own, and the card reproduces it; a candidate should read the quantity, note the ambiguity and answer the question that the numbers make plain.

• Mercury is the innermost planet, at an average distance of about 58 million kilometres from the Sun, and completes one orbit in 88 Earth days — the shortest year of any planet.
• Mercury rotates on its own axis once every 59 Earth days, and its solar day, from one sunrise to the next, lasts about 176 Earth days.
• Venus lies about 108 million kilometres from the Sun with a year of 225 Earth days, and rotates once in 243 Earth days — longer than its own year — in the retrograde direction.
• Mars lies about 228 million kilometres or 1.5 astronomical units from the Sun, with a year of 687 Earth days and a rotation of 24.6 hours, so its day is close to Earth's.
• Saturn lies about 1.4 billion kilometres or 9.5 astronomical units from the Sun, takes about 29.4 Earth years to complete one orbit and rotates in only 10.7 hours.

• Confusing revolution with rotation; Mercury revolves around the Sun in 88 days but rotates on its axis in 59, and the ENGLISH column of this paper uses the wrong word — the Marathi prints परिभ्रमण, revolution about another body, and is correct
• Accepting a distance that would place an outer planet inside Mercury's orbit; two of the wrong options here fail that test without any recall of the right figure
• Mixing the units in which distance is quoted — kilometres, crore kilometres and astronomical units all appear in this topic, and 5.2 belongs to Jupiter but as astronomical units, not crore kilometres
• Assuming a planet's day must be shorter than its year; Venus rotates in 243 Earth days against a year of 225

Basic astronomy in MPSC papers is set on fixed comparative facts: which planet is nearest or farthest, largest or smallest, hottest or coldest, which has the longest day or the shortest year, and which is known for a named feature. The fill-in-the-blank with two gaps, as here, is one favoured form; the other two are the single-line factual item and the statement cluster asking which of three claims about a planet is correct. A single table with the eight planets in order, carrying distance in both million kilometres and astronomical units, orbital period, rotation period and one distinguishing fact each, answers nearly all of them. Add Kepler's laws in one line, because the Commission occasionally asks for the reason rather than the number.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which planet of the solar system has a rotation period longer than its own period of revolution around the Sun?

  • (a)Mercury
  • (b)Venus
  • (c)Mars
  • (d)Saturn

  Answer(b) Venus — it rotates once on its axis in 243 Earth days while completing one orbit of the Sun in 225 Earth days, so its day is longer than its year, and it rotates in the retrograde direction. Mercury, by contrast, revolves in 88 Earth days and rotates in 59; Mars rotates in 24.6 hours and revolves in 687 Earth days; and Saturn rotates in 10.7 hours and revolves in about 29.4 Earth years.

• practice — not a real PYQ

  Arrange the following planets in increasing order of their distance from the Sun: 1. Mars 2. Mercury 3. Saturn 4. Venus

  • (a)2, 4, 1, 3
  • (b)2, 1, 4, 3
  • (c)4, 2, 3, 1
  • (d)1, 2, 4, 3

  Answer(a) 2, 4, 1, 3 — Mercury at about 58 million km, Venus at about 108 million, Mars at about 228 million and Saturn at about 1.4 billion kilometres, which is 9.5 astronomical units. The order of increasing distance is also the order of increasing year length, from Mercury's 88 Earth days through Venus's 225 and Mars's 687 to Saturn's 29.4 Earth years, as Kepler's third law requires.