Gravity holds the Solar System together, and the Earth's daily spin and tilted orbit explain day, night and the seasons. Beyond that, the distances are so large that ordinary units stop being useful.
Gravity is a non-contact force of attraction between any two masses. It is only noticeable when at least one of them is very large, so we feel the Earth's pull but not each other's.
Weight is the force of gravity on an object: \(\text{weight} = \text{mass} \times \text{gravitational field strength}\). On Earth \(g\) is about 10 N/kg, so a 5 kg bag weighs about 50 N.
Mass and weight are not the same. Mass, in kilograms, is the amount of matter and never changes. Weight, in newtons, depends on where you are. On the Moon \(g\) is about 1.6 N/kg, so the same object weighs about a sixth as much. Gravity between the Earth, Moon and Sun is what keeps the Moon orbiting us and the Earth orbiting the Sun, and the gravitational pulls of the Moon and the Sun produce tides in the oceans, with the Moon having the larger effect.
Our Sun is a star: a huge ball of hot gas releasing energy. It only looks different from the other stars because it is so much closer.
Billions of stars make up our galaxy, the Milky Way, held together by gravity, and there are billions of other galaxies beyond it. The planets do not produce visible light in the way stars do. We see them mainly because they reflect sunlight.
Day and night are caused by the Earth spinning on its axis once every 24 hours: the side facing the Sun has day, the side facing away has night. The Earth takes one year to orbit the Sun.
The seasons are caused by the Earth's axis being tilted. When your hemisphere is tilted towards the Sun it is summer there: the days are longer and the sunlight strikes the ground more directly, so it is more concentrated and warmer. Six months later the same hemisphere is tilted away and it is winter.
The two hemispheres therefore have opposite seasons. And the seasons are not caused by the Earth being closer to the Sun. That is a common misconception, and the opposite seasons in the two hemispheres disprove it.
Distances between stars are so vast that kilometres become unwieldy, so astronomers use the light year, the distance light travels in one year, about \(9.5\times10^{15}\) m.
Despite the name it is a distance, not a time. It also means we see distant objects as they were: light from a star 100 light years away left it a century ago.
Back to The Solar System practice ยท View all The Solar System questions
