Solar System and the Earth: Planets, Moon and Motions

The correct answer is B, Venus. Venus is wrapped in a very thick atmosphere of carbon dioxide, which lets sunlight in but does not let the heat escape. This runaway greenhouse effect keeps its surface hotter than that of any other planet, even at night and even on the side turned away from the Sun.

Option A, Mercury, is nearer the Sun and is therefore the usual wrong choice, but it has almost no atmosphere to hold heat, so its nights are bitterly cold. Option C, Mars, is farther away, has a thin atmosphere and is a cold desert planet. Option D, Jupiter, is a gas giant far beyond the asteroid belt and is colder still at its cloud tops. Remember Venus for three records at once: hottest planet, brightest planet in our sky, and the twin of the Earth in size.

The correct answer is A, 8 minutes 20 seconds. The Sun is about 149.6 million kilometres from the Earth, a distance called one astronomical unit, and light travels about three lakh kilometres in a second. Dividing one by the other gives roughly 500 seconds, that is eight minutes and twenty seconds, so we always see the Sun as it was a little over eight minutes ago.

Option B, 1.3 seconds, is the time moonlight takes to reach us from a distance of about 384,400 kilometres, and examiners often place the two figures in the same question. Option C, 4.2 years, is the time light takes from Proxima Centauri, the nearest star after the Sun. Option D, 24 hours, is the time the Earth takes for one rotation on its axis and has nothing to do with the travel of light.

The correct answer is C, Jupiter. Jupiter is the fifth planet from the Sun and the largest of all by both diameter and mass; it is a gas giant of hydrogen and helium. It also has the fastest rotation of any planet, completing a spin in about ten hours, carries the storm called the Great Red Spot, and holds Ganymede, the largest satellite in the solar system.

Option A, Saturn, is the second largest and is known instead for its bright rings and for having the lowest density of any planet. Options B and D, Neptune and Uranus, are the two outermost planets and are much smaller than Jupiter and Saturn; Neptune is the farthest from the Sun and Uranus is the one that rolls on its side. Keep the size order in mind: Jupiter, Saturn, Uranus, Neptune, then the Earth.

The correct answer is B, Venus. Almost every planet, the Earth included, rotates from west to east, which is why the Sun appears to rise in the east. Venus and Uranus are the two exceptions and rotate from east to west, so on Venus the Sun would appear to rise in the west. Venus also turns extremely slowly, taking longer to complete one rotation than to complete one revolution around the Sun.

Option A, Mercury, rotates in the usual direction and is remembered instead as the smallest planet and the fastest to complete a revolution, in about 88 days. Option C, Mars, also rotates west to east and has a day close in length to ours. Option D, Saturn, is a fast-spinning gas giant rotating in the normal direction. The pair to remember for this question is simply Venus and Uranus.

The correct answer is C. The axis of the Earth is tilted about 23.5 degrees from the vertical and keeps pointing in the same direction throughout the year. As the Earth revolves, each hemisphere is therefore turned towards the Sun for part of the year, receiving more direct rays and longer days, and away from it for the other part, which gives summer and winter.

Option A is the commonest trap. The Earth is actually closest to the Sun in early January, when the northern hemisphere has winter, so distance cannot explain the seasons. Option B, rotation, causes day and night, not the seasons. Option D, the pull of the Moon, is the main cause of tides in the oceans. A useful way to remember it: rotation gives day and night, revolution with tilt gives the seasons and the year.

The correct answer is B, Saturn. Saturn is a huge ball of hydrogen and helium spread over an enormous volume, and its average density is less than that of water. This makes it the least dense planet of the solar system, and the picture of Saturn floating in water is the standard way the fact is taught. Saturn is also famous for its system of bright rings and for Titan, its largest moon.

Option A, Jupiter, is the largest and heaviest planet, but it is more tightly packed than Saturn and denser than water. Option C, Neptune, is a distant ice giant and denser still. Option D, Mercury, is a small rocky planet with a large iron core and is among the densest of all. Pair the two records in your memory: Jupiter the largest, Saturn the lightest for its size.

The correct answer is B, Mars and Jupiter. The asteroid belt is a broad band of rocky bodies, from dust-sized grains to bodies hundreds of kilometres across, going round the Sun between the orbit of Mars and that of Jupiter. They are generally taken to be material that never came together into a planet, because the powerful gravity of Jupiter kept disturbing it.

Option A is wrong because nothing of the kind lies between the Earth and Mars. Options C and D place the belt among the outer planets, where the Sun is instead circled by the giant planets and their many moons; far beyond Neptune lies a different zone, the Kuiper belt, which is where Pluto belongs. The asteroid belt is also the boundary used to divide the inner terrestrial planets from the outer giant planets.

The correct answer is A, 1.3 seconds. The Moon is on average about 384,400 kilometres from the Earth. Light covers about three lakh kilometres in a second, so it crosses that distance in a little over a second. This is the standard figure asked in one-line questions about the Moon, along with its distance.

Option B, eight minutes and twenty seconds, is the time sunlight takes to reach us, and the two figures are deliberately placed together in such questions. Option C, 27.3 days, is the period the Moon takes both to rotate once on its axis and to revolve once around the Earth; because the two are equal, the same face of the Moon is always turned towards us. Option D is far too long and matches nothing in the Earth and Moon system.

The correct answer is C, 2006. In that year the International Astronomical Union adopted a formal definition of a planet: a body that orbits the Sun, is massive enough for its own gravity to pull it into a nearly round shape, and has cleared the neighbourhood around its orbit. Pluto, discovered in 1930, satisfies the first two conditions but shares its region with many other bodies of the Kuiper belt, so it was moved to the new class of dwarf planets.

Options A, B and D name years in which no such decision was taken. Since 2006 the solar system has been described as having eight planets, and questions that ask for the number of planets expect the answer eight. Remember also that Ceres, the largest body of the asteroid belt, is counted among the dwarf planets under the same definition.

The correct answer is B, the Tropic of Cancer. On 21 June the northern hemisphere is tilted towards the Sun, whose rays fall vertically on the Tropic of Cancer at 23.5 degrees north. This is the summer solstice: the longest day and the shortest night of the year in the northern hemisphere, and the day on which the Arctic Circle has continuous daylight.

Option A, the equator, receives vertical rays on 21 March and 23 September, the two equinoxes, when day and night are equal everywhere on the Earth. Option C, the Tropic of Capricorn at 23.5 degrees south, gets vertical rays on 22 December, the winter solstice for the northern hemisphere. Option D, the Arctic Circle at 66.5 degrees north, never receives vertical rays, since the Sun can be overhead only between the two tropics.

The correct answer is C, Jupiter. Ganymede is one of the four large moons of Jupiter first seen by Galileo, the others being Io, Europa and Callisto, and it is larger than the planet Mercury, which makes it the biggest satellite in the solar system. Jupiter, the largest planet, has by far the most striking family of moons among the giant planets.

Option A, Saturn, has Titan, the second largest satellite and the only moon with a thick atmosphere. Option B, Mars, has only two tiny moons, Phobos and Deimos, which are captured bodies shaped like potatoes. Option D, Neptune, has Triton, remembered because it moves around its planet in the direction opposite to the planet's rotation. Note also that Mercury and Venus have no satellite at all.