The occurrence of seasons on the Earth is caused mainly by which of the following?

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 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.