The Universe and the Solar System
The universe is the totality of space, time, matter and energy. Modern cosmology dates its origin to the Big Bang, an explosive expansion of an extremely hot, dense state roughly 13.8 billion years ago. From that moment, space itself has been stretching, carrying galaxies apart and cooling the cosmos to its present background temperature of about 2.7 K.
The distance that light, travelling in a vacuum at roughly 300,000 kilometres per second, covers in one year — approximately 9.46 trillion kilometres. It is a measure of distance, not time.
Structure of the universe
Galaxies are the basic building blocks of the cosmos. Our galaxy, the Milky Way, is a barred spiral containing an estimated 100–400 billion stars. The nearest large galaxy, Andromeda (M31), is about 2.5 million light-years away and is on a slow collision course with the Milky Way. Together with smaller neighbours, both belong to the Local Group, which in turn lies inside the Virgo Supercluster.
- Age of the universe: ~13.8 billion years.
- Observable radius: ~46.5 billion light-years (larger than 13.8 b ly because space has expanded).
- Galaxies: estimated 100 billion to 2 trillion in the observable universe.
- Dark matter and dark energy together account for roughly 95% of the universe's mass-energy; only ~5% is ordinary matter.
The Solar System
The Solar System formed about 4.6 billion years ago from a collapsing cloud of gas and dust. At its centre is the Sun, a yellow dwarf star that contains over 99.8% of the system's mass. Eight planets orbit the Sun, divided into two families:
- Terrestrial (inner) planets — Mercury, Venus, Earth and Mars. Small, rocky, with thin or no atmospheres compared to the outer giants.
- Jovian (outer) planets — Jupiter, Saturn, Uranus and Neptune. Large, gaseous or icy, with extensive ring or moon systems.
Between Mars and Jupiter lies the asteroid belt; beyond Neptune lies the Kuiper Belt, home to Pluto and other dwarf planets, and farther still the spherical Oort Cloud of long-period comets.
Quick planetary facts
| Planet | Distance from Sun (AU) | Day length | Year length |
|---|---|---|---|
| Mercury | 0.39 | 59 Earth days | 88 days |
| Venus | 0.72 | 243 Earth days (retrograde) | 225 days |
| Earth | 1.00 | 24 hours | 365.25 days |
| Mars | 1.52 | ~24 h 37 m | 687 days |
| Jupiter | 5.20 | ~10 hours | 11.86 years |
| Saturn | 9.58 | ~10.7 hours | 29.46 years |
| Uranus | 19.18 | ~17 hours (retrograde) | 84 years |
| Neptune | 30.07 | ~16 hours | 164.8 years |
Memorise the order using the mnemonic "My Very Educated Mother Just Served Us Noodles" (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune). Pluto was reclassified as a dwarf planet in 2006 by the International Astronomical Union.
Eclipses
An eclipse occurs when one celestial body passes into the shadow of another, or directly between an observer and a light source.
- Solar eclipse: the Moon passes between the Sun and the Earth, casting its shadow on a narrow strip of Earth's surface. Occurs only at the new moon phase.
- Lunar eclipse: the Earth passes between the Sun and the Moon, so Earth's shadow falls on the Moon. Occurs only at the full moon phase.
A solar eclipse is briefer (a few minutes of totality) and visible only along a narrow path, while a lunar eclipse can be observed from the entire night-side hemisphere of Earth and may last a few hours.
Kepler's laws — the physics behind the motion
Johannes Kepler (1571–1630) deduced three laws of planetary motion from Tycho Brahe's observations:
- Law of orbits — every planet moves in an ellipse with the Sun at one focus.
- Law of areas — a line joining a planet to the Sun sweeps out equal areas in equal times, so planets travel faster when closer to the Sun (perihelion) and slower when farther (aphelion).
- Law of periods — the square of the orbital period is proportional to the cube of the semi-major axis: T² ∝ a³.
Newton later showed that all three laws follow from his law of universal gravitation.