The Earth spins (rotates 自转) on its axis 轴 once in about $24$ hours. The axis is tilted 倾斜 (not straight up). This spin gives us day and night, and makes the Sun 太阳 appear to move across the sky each day.
The Earth also moves around the Sun in an orbit 轨道, once in about $365$ days (one year). Because the axis is tilted, different parts of the Earth lean towards the Sun at different times of the year. This gives the seasons 季节: it is summer in the half of the Earth that leans towards the Sun.
The axis keeps the same tilt all year, so each hemisphere leans toward the Sun for one half of the orbit and away for the other
The Moon 月球 orbits the Earth once in about one month. As it goes round, we see different amounts of its lit side, which gives the phases 月相 of the Moon (new moon, half moon, full moon).
The Moon's lit half always faces the Sun, so as it orbits we see different amounts of it: a new moon between us and the Sun, a full moon opposite the Sun, and half moons in between
So, from shortest to longest: one day (Earth's spin) < one month (Moon's orbit) < one year (Earth's orbit).
For an object going round a circle, the distance for one full orbit is the circumference 周长$2\pi r$, where $r$ is the radius of the orbit. The orbital speed 轨道速率 is this distance divided by the period 周期$T$ (the time for one orbit):
$$v = \frac{2\pi r}{T}$$
The same equation works for planets, moons and satellites.
Worked example. A space station orbits the Earth at a radius of $r = 7000\ \text{km}$ and takes $T = 5800\ \text{s}$ for one orbit. Find its orbital speed.
First convert the radius to metres: $7000\ \text{km} = 7.0 \times 10^6\ \text{m}$. Then
The Solar System 太阳系 has one star, the Sun, at its centre. Around it move the eight planets 行星. In order from the Sun they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The four planets nearest the Sun are small and rocky 岩石.
The four planets furthest from the Sun are large and gaseous 气态 in the syllabus comparison: they have deep atmospheres of gas 气体 rather than an Earth-like solid surface.
The two nearer giants, Jupiter and Saturn, are gas giants 气态巨行星. The two outer giants, Uranus and Neptune, are ice giants 冰巨行星: their composition differs from that of Jupiter and Saturn. The name does not mean that their interiors are solid frozen ice.
The Sun and its eight planets in order: four small rocky planets, then the asteroid belt, then four large gaseous planets — the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune (not to scale)Mars, the red planet, one of the rocky inner planetsJupiter, the largest planet, with its swirling bands of gasSaturn and its rings, photographed by the Cassini spacecraft
The Solar System also contains:
minor planets, such as dwarf planets 矮行星 like Pluto, and asteroids 小行星 (most lie in the asteroid belt between Mars and Jupiter);
moons (natural satellites 卫星) that orbit the planets;
comets 彗星 and other small bodies.
Worked classification. The Sun is a star. Mars and Jupiter are planets. Pluto is a dwarf planet, so it is not one of the eight named planets. An asteroid or a comet orbits the Sun; the Moon is Earth's natural satellite and orbits Earth while Earth orbits the Sun.
Orbits and gravity
The Sun holds most of the mass of the Solar System. Its gravity 引力 (gravitational pull) reaches out and keeps the planets in their orbits. This force is the gravitational attraction of the Sun.
The gravitational field strength 重力场强度 tells you how strong gravity is:
at the surface of a planet it depends on both the mass and the radius of the planet;
around a planet it gets weaker as you move further away.
In the same way, the Sun's gravity gets weaker further out, so the outer planets move more slowly (smaller orbital speed) than the inner ones.
Planetary data
Exam questions often give a table like this one and ask you to read a pattern from it.
Planet
Distance from Sun / million km
Orbital period
Density / g/cm³
Temperature / °C
Surface $g$ / N/kg
Mercury
58
88 days
5.4
170
3.7
Venus
108
225 days
5.2
460
8.9
Earth
150
365 days
5.5
15
9.8
Mars
228
687 days
3.9
−60
3.7
Jupiter
779
11.9 years
1.3
−110
23
Saturn
1430
29.5 years
0.7
−140
9.0
Uranus
2870
84 years
1.3
−195
8.7
Neptune
4500
165 years
1.6
−200
11
For the giant planets, temperature values refer to atmospheric levels, not a solid surface.
Patterns to notice: the further a planet is from the Sun, the longer its orbital period. The outer planets are colder, but temperature does not fall for every successive planet: Venus is hotter than Mercury. The four inner planets are dense (rock and metal), while the four outer planets have low densities (gas and ice). The gravitational field strength at the surface depends on the planet's mass and size, not on its distance from the Sun.
Worked interpretation. Mercury has density $5.4\ \text{g/cm}^3$ and Jupiter has density $1.3\ \text{g/cm}^3$. These values support the inner/outer density comparison. But a claim that every more distant planet is colder fails: Venus is farther from the Sun than Mercury (108 versus 58 million km) yet has a higher listed temperature (460°C versus 170°C). Venus's thick atmosphere traps heat. Use the data to support a pattern, then state exceptions; distance alone does not determine temperature.
Elliptical orbits
Most orbits are not perfect circles but ellipses 椭圆 (a stretched circle), and the Sun is not at the centre of the ellipse. A comet has a very stretched orbit. An object in an elliptical orbit moves faster when it is closer to the Sun. We explain this with the conservation of energy 能量守恒 — the total energy 能量 stays the same, so as the object's gravitational potential energy 重力势能 falls (coming closer), its kinetic energy 动能 rises (it speeds up).
The Sun sits at one focus of the ellipse; the planet speeds up as it comes closer and slows down as it moves away
Light travel time
Distances in space are huge, so we often work out how long light takes to cross them. Light travels at $3.0 \times 10^8\ \text{m/s}$. Using $\text{time} = \dfrac{\text{distance}}{\text{speed}}$, light from the Sun (about $1.5 \times 10^{11}\ \text{m}$ away) takes about $500\ \text{s}$, which is roughly $8$ minutes, to reach the Earth.
Worked example. (a) How far is one light-year in metres? (b) The star Sirius is $8.6$ light-years away. How far is that in metres? (One year $= 3.2 \times 10^7\ \text{s}$.)
(a) Distance = speed × time $= 3.0 \times 10^8 \times 3.2 \times 10^7 = 9.6 \times 10^{15}\ \text{m}$. (With a more exact year, $3.15 \times 10^7\ \text{s}$, it is $9.5 \times 10^{15}\ \text{m}$.)
(b) $8.6 \times 9.5 \times 10^{15} = 8.2 \times 10^{16}\ \text{m}$. The light we see from Sirius tonight left it $8.6$ years ago.
The planets formed from a giant cloud of gas and dust 尘埃 in space, which contained many chemical elements 元素. This is the accretion 吸积 model:
gravity pulled the cloud together;
the cloud was spinning, so it flattened into a spinning accretion disc 吸积盘;
most of the matter fell to the centre and became the Sun, while the planets grew from the leftover material in the disc.
Near the hot young Sun, only rock and metal could stay solid, so the inner planets are rocky. Far out, where it was cold, gases could also collect, so the outer planets grew large and gaseous.
A real accretion disc around the young star HL Tauri: the dark gaps are where new planets are gathering up the dust — our Solar System began the same way
The Sun is a star 恒星 of medium size, made mostly of hydrogen 氢 and helium 氦. It radiates most of its energy in the infrared, visible light and ultraviolet 紫外线 parts of the electromagnetic spectrum 电磁波谱.
A star is powered by nuclear reactions 核反应 in its core. In a stable star, the reaction is nuclear fusion 核聚变: hydrogen nuclei join to make helium, releasing huge amounts of energy.
The Sun, our nearest star, seen in ultraviolet light
A galaxy 星系 is a group of many billions of stars held together by gravity. Our Sun is one star in the galaxy called the Milky Way 银河系. All the other stars in the Milky Way are very much further from the Earth than the Sun is.
A spiral galaxy: billions of stars held together by gravity
Distances between stars are so big that we measure them in light-years 光年. One light-year is the distance light travels in one year (in the vacuum of space), which is about $9.5 \times 10^{15}\ \text{m}$.
The Milky Way is about $100\,000$ light-years across (its diameter 直径). It is just one of many billions of galaxies. All of these galaxies together make up the Universe 宇宙.
A giant telescope studies the night sky; the laser helps it make sharp images of distant stars
A star is born, lives and dies over a very long time:
A star forms from an interstellar cloud 星际云 of gas and dust that contains hydrogen.
Gravity pulls the cloud inwards, and it heats up. This collapsing, heating cloud is a protostar 原恒星.
The protostar becomes a stable 稳定 star when the inward pull of gravity is balanced by an outward pressure force. Its hot interior provides the pressure needed for this balance.
In time, every star runs out of hydrogen fuel 燃料 for fusion.
What happens next depends on the mass of the star:
A medium star (like the Sun) swells into a red giant 红巨星. It then throws off its outer layers as a glowing cloud called a planetary nebula 星云, leaving a small, hot white dwarf 白矮星 at the centre.
A much heavier star swells into a red supergiant 红超巨星 and then explodes as a supernova 超新星. This blast spreads out a nebula containing hydrogen and new, heavier elements, and leaves behind a neutron star 中子星 or a black hole 黑洞.
Every star begins in a nebula; how it ends depends on its mass
The nebula from a supernova can later form new stars, with planets orbiting them — so our own Solar System came from earlier stars.
When a star or galaxy moves away from us (it is receding 退行), the light we receive from it has a longer wavelength 波长 than normal — it is shifted towards the red end of the spectrum. This is redshift 红移.
The dark lines in a distant galaxy's spectrum are shifted toward the red end — this is redshift
The light from distant galaxies is redshifted, and the further away a galaxy is, the bigger its redshift. This tells us that the galaxies are moving apart: the Universe is expanding 膨胀. Running the expansion backwards points to an earlier, hotter and denser Universe, as described by the Big Bang 大爆炸 theory.
Every galaxy is moving away from us, and the farther ones move away faster — the Universe is expanding
More evidence and the age of the Universe
Faint microwave 微波radiation 辐射 is found coming from every direction in space. This is the cosmic microwave background radiation 宇宙微波背景辐射 (CMBR). It was made as high-energy radiation soon after the Big Bang, and has been stretched out into the microwave part of the spectrum as the Universe expanded.
The whole sky mapped in microwaves: this faint glow reaches us from every direction as the leftover heat of the young Universe — strong evidence for the Big Bang
For a distant galaxy:
its speed $v$ of moving away can be found from the redshift of its starlight;
its distance $d$ can be found from the brightness of a supernova seen in it.
The Hubble constant 哈勃常数$H_0$ links these two:
$$H_0 = \frac{v}{d}$$
Its value today is about $2.2 \times 10^{-18}$ per second. Turning this around gives an estimate for the age of the Universe:
$$\frac{d}{v} = \frac{1}{H_0}$$
Worked example. The Hubble constant is about $H_0 = 2.2 \times 10^{-18}$ per second. Estimate the age of the Universe.
That is about $1.0 \times 10^{25} \div 9.5 \times 10^{15} \approx 1.1 \times 10^{9}$ light-years: about a billion light-years.
Worked example (Hubble constant from a graph). Astronomers measure the distance and the speed of recession of six galaxies and plot speed against distance. The best-fit line passes through the origin and through $(5.0 \times 10^{24}\ \text{m},\ 1.1 \times 10^{7}\ \text{m/s})$.
Read the scale factors on the axis labels: a reading of $1.1$ on an axis labelled "speed / $10^7$ m/s" means $1.1 \times 10^{7}$ m/s. Use two points far apart on the line, not two data points, because the points scatter around the line. A straight line through the origin shows that speed is proportional to distance.
The gradient of the speed–distance line is the Hubble constant
In $v = \dfrac{2\pi r}{T}$, $r$ is the orbit radius and $T$ is the time for one full orbit in seconds — convert km to m and hours to seconds first.
Planets closer to the Sun feel stronger gravity, so they orbit faster and take less time. In an elliptical orbit a body speeds up as it nears the Sun (gravitational potential energy turns into kinetic energy).
A light-year is a distance, not a time — how far light travels in one year.
A star is stable when the inward pull of gravity is balanced by the outward push from the energy released by fusion in its core. What it becomes when its fuel runs out depends on its mass.
Redshift — light from distant galaxies shifted to longer (redder) wavelengths, with a bigger shift the further away — is the evidence that the Universe is expanding, which supports the Big Bang.
Common mistakes
Treating a light-year as a time. It is the distance light travels in one year.
Using the diameter, or the height above the surface, as $r$ in $v = 2\pi r/T$. Use the orbit radius, measured from the centre of the planet or star.
Leaving $T$ in hours or days. Change it to seconds when $v$ is wanted in m/s.
Saying redshift means the galaxy looks red. It means the lines in its spectrum are shifted to longer wavelengths.
Saying the Sun is at the centre of an elliptical orbit. It is at one focus, off-centre, so the distance to the Sun changes around the orbit.
