The Earth, the solar system and the universe

Earth is about 4.5 billion years old, and its story is one of slow change punctuated by dramatic events. The planet formed from dust and gas around the young Sun, heated and differentiated into a dense iron core, a rocky mantle, and a thin crust. That crust is broken into tectonic plates that move a few centimetres a year, carrying continents, building mountains where they collide, and opening oceans where they separate. Earthquakes and volcanoes mostly occur at plate boundaries, which is why their geography is not random: the Pacific Ring of Fire, for instance, follows the edges of the Pacific plate.

The seasons illustrate how a simple geometric fact shapes daily life. Earth orbits the Sun once a year, but seasons are caused mainly by the tilt of Earth's axis, about 23.5 degrees from vertical, not by distance from the Sun. When the northern hemisphere tilts toward the Sun, it receives more direct sunlight and longer days — summer — while the southern hemisphere has winter, and vice versa. The Moon, Earth's only natural satellite, raises tides through its gravity, and its phases reflect how sunlight falls on it as seen from Earth, not Earth's shadow, except during a lunar eclipse.

Beyond Earth, the solar system contains eight planets: four small rocky ones close to the Sun (Mercury, Venus, Earth, Mars) and four giants further out (Jupiter, Saturn, Uranus, Neptune). Pluto, once counted as a planet, was reclassified as a dwarf planet by the International Astronomical Union in 2006. The Sun, a middle-aged star, holds more than 99 percent of the system's mass and powers Earth's climate. Its light takes about eight minutes to reach us, so we always see the Sun as it was eight minutes ago.

The universe is far older and larger. Current evidence indicates it began about 13.8 billion years ago in a hot, dense state — the Big Bang — and has been expanding and cooling ever since. Galaxies, including our Milky Way, formed from that cooling matter, and stars within them forge heavier elements in their cores: the carbon in our bodies and the iron in our blood were made inside stars that died before the Sun was born. Astronomy thus connects the everyday to the cosmic: we are, quite literally, made of stardust, and studying the sky is also studying our own origins.