Astronomy | Kumaran Kumarathurai

The Solar System

The eight planets in order from the Sun.

Planet	Distance from the Sun (AU)	Radius (km)	Moons	Notable feature
Mercury	0.39	2,440	0	The smallest and fastest planet, with virtually no atmosphere
Venus	0.72	6,052	0	The hottest planet due to a thick CO₂ atmosphere and greenhouse effect
Earth	1.00	6,371	1	The only known planet with life
Mars	1.52	3,390	2	The "Red Planet" – iron oxide on the surface gives it its colour
Jupiter	5.20	69,911	95+	The largest planet in the solar system, a gas giant with the Great Red Spot
Saturn	9.58	58,232	145+	Known for its prominent ring system of ice and rock
Uranus	19.22	25,362	27+	An ice giant that rotates "on its side"
Neptune	30.05	24,622	14+	The outermost planet, with the fastest winds in the solar system

Pluto was reclassified from a planet to a dwarf planet in 2006 by the International Astronomical Union (IAU).

Celestial objects

The fundamental types of objects found in the universe.

Star

A giant ball of hot plasma that emits light and heat through nuclear fusion in its core.

Planet

A body that orbits a star, is roughly spherical, and has cleared its orbit of other objects.

Moon

A natural body that orbits a planet or another smaller body.

Asteroid

A smaller, rocky object orbiting the Sun, often found in the asteroid belt between Mars and Jupiter.

Comet

An object of ice and dust that forms a visible tail of gas and dust as it approaches the Sun.

Nebula

A giant cloud of gas and dust in space – often the birthplace of new stars.

Black hole

A region of space with gravity so strong that nothing, not even light, can escape.

Galaxy

An enormous collection of stars, gas, dust and dark matter held together by gravity – e.g. the Milky Way.

Distances, units and basic formulas

The measures astronomers use to describe enormous distances, plus a couple of central formulas.

Unit	Definition
Astronomical unit (AU)	The average distance from Earth to the Sun, about 150 million km
Light-year	The distance light travels in one year, about 9.46 trillion km
Parsec	About 3.26 light-years – often used to measure distances between stars

Basic formulas

Name	Formula
Kepler's 3rd law	T² ∝ a³
Escape velocity	v = √(2GM / r)
Brightness and distance (inverse-square law)	I ∝ 1 / d²

History of astronomy

From the geocentric model to the space age.

c. 150 CE

The geocentric model

Ptolemy compiles the astronomy of his day in the "Almagest", placing Earth at the centre of the universe – a model that holds for over 1300 years.

1543

The heliocentric model

Nicolaus Copernicus proposes that the Sun, not Earth, is the centre of the solar system.

1609

Galileo and the telescope

Galileo Galilei points a telescope at the sky and discovers Jupiter's moons among other things, supporting the heliocentric model.

1609–1619

Kepler's laws

Johannes Kepler describes the elliptical orbits of the planets and formulates his three laws of planetary motion.

1687

Newton's law of gravitation

Isaac Newton mathematically explains planetary motion with his universal law of gravitation.

1929

The expanding universe

Edwin Hubble observes that distant galaxies are moving away from us, showing that the universe is expanding.

1957–today

The space age

From Sputnik (1957) and the Moon landing (1969) to the James Webb Space Telescope (2021), humanity explores space directly with satellites, probes and telescopes.

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