Space Physics

The Earth, Sun and Moon

Three of the familiar rhythms — day and night, the seasons, the phases of the Moon — all come from simple motions: the Earth spinning, the Earth orbiting the Sun, and the Moon orbiting the Earth.

Three rhythms of the sky

The Earth rotates on its tilted axis once in about 24 hours, giving the apparent daily motion of the Sun across the sky and the cycle of day and night. It orbits the Sun once in about 365 days; because its axis is tilted, this produces the seasons. The Moon orbits the Earth in about one month, giving the Moon’s cycle of phases.

ExtendedOrbital speed

The orbital speed is the orbit’s circumference divided by the period: v = 2πr / T. Worked example: the Earth orbits the Sun at a radius of 1.5 × 10¹¹ m once every 3.15 × 10⁷ s, so v = 2π × 1.5 × 10¹¹ / 3.15 × 10⁷ ≈ 3.0 × 10⁴ m/s — about 30 km every second.

The Solar System

Our Solar System is one star and everything bound to it — eight planets, their moons, and a scattering of smaller bodies — all formed from the same slowly collapsing cloud of gas and dust.

What the Solar System contains

The Solar System contains one star, the Sun; the eight planets in order from it; minor planets such as the dwarf planet Pluto and the asteroids of the asteroid belt; moons orbiting the planets; and smaller bodies including comets. The four planets nearest the Sun are small and rocky; the four furthest are large and gaseous.

How it formed

The Solar System formed by accretion: an interstellar cloud of gas and dust, containing many elements, was pulled together by gravity. As the material rotated it flattened into an accretion disc, from which the Sun and planets gradually built up.

Gravity, light and orbits

Gravity rules the Solar System. The Sun holds nearly all its mass, and that gravity keeps every planet in orbit. Distances are so vast that even light takes minutes or hours to cross them.

Gravity and light-travel time

A planet’s gravitational field strength at its surface depends on the planet’s mass, and it weakens with distance from the planet. The Sun contains most of the Solar System’s mass, so its gravitational attraction holds the planets in orbit. Distances are measured by how long light takes to cross them.

Worked example: the Sun is 1.5 × 10¹¹ m from Earth and light travels at 3.0 × 10⁸ m/s, so sunlight takes t = 1.5 × 10¹¹ / 3.0 × 10⁸ = 500 s, a little over 8 minutes, to reach us.

ExtendedElliptical orbits

Planets, minor planets and comets follow elliptical orbits, with the Sun not at the centre. Further from the Sun, its gravitational field is weaker and the planet’s orbital speed is smaller. A body on an elliptical orbit travels faster when closer to the Sun — a consequence of the conservation of energy.

The Sun as a star

The Sun is not special — it is a fairly ordinary, medium-sized star. What makes it the centre of our lives is simply that it is so close.

An ordinary star

The Sun is a medium-sized star consisting mostly of hydrogen and helium. It radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum.

Stars are powered by nuclear reactions that release energy. In a stable star such as the Sun, these reactions are the fusion of hydrogen into helium.

Stars, galaxies and light-years

Beyond the Solar System the distances become so vast that ordinary units fail. Astronomers measure them by the time light takes to cross them — the light-year. This section is Extended.

ExtendedGalaxies and light-years

A galaxy is made up of many billions of stars. Our Sun is a star in the galaxy called the Milky Way, and every other star in it lies far further from Earth than the Sun does. Such distances are measured in light-years: one light-year is the distance light travels through space in one year, about 9.5 × 10¹⁵ m. The light reaching us from a distant star therefore left it years — sometimes millions of years — ago, so looking out into space is also looking back in time.

The life cycle of a star

This section is Extended. Every star is born from a cloud of gas, lives by balancing gravity against the heat of fusion, and dies when its hydrogen runs out. What happens at the end depends on one thing: its mass.

ExtendedFrom nebula to remnant

A star forms from an interstellar cloud of gas and dust containing hydrogen. Gravity collapses it into a protostar, which heats up until it becomes a stable star — gravity balanced by the outward force of its high temperature. When the central hydrogen is used up, the star expands into a red giant, or a red supergiant if it is more massive.

A less massive star then throws off a planetary nebula and is left as a white dwarf. A massive star explodes as a supernova, leaving behind a neutron star or, if massive enough, a black hole.

The expanding Universe

This section is Extended. The Milky Way is one of many billions of galaxies, about 100 000 light-years across. The light from the others carries a clue that changed our whole picture of the cosmos: it is stretched towards the red.

ExtendedRedshift and the Big Bang

Light from distant galaxies is redshifted — its observed wavelength is longer than the light emitted on Earth — because those galaxies are moving away from us. This is evidence that the Universe is expanding, which supports the Big Bang Theory.

ExtendedCosmic microwave background

Cosmic microwave background radiation (CMBR) is microwave radiation of a specific frequency observed at all points in space. It was produced shortly after the Universe formed and has been stretched into the microwave region of the spectrum as the Universe expanded.

Hubble’s law and the age of the Universe

This section is Extended. The further away a galaxy is, the faster it recedes. That simple relationship — Hubble’s law — lets us estimate something extraordinary: the age of the Universe itself.

ExtendedSpeed, distance and age

A galaxy’s recession speed v is found from the redshift of its light, and its distance d from the brightness of a supernova within it. The two are linked by the Hubble constant H₀: v = H₀d. Rearranged, 1/H₀ = d/v gives an estimate for the age of the Universe — evidence that all matter began at a single point.

Worked example: taking H₀ = 2.2 × 10⁻¹⁸ per second, age ≈ 1 / H₀ = 1 / (2.2 × 10⁻¹⁸) ≈ 4.5 × 10¹⁷ s — about 14 billion years.