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Solar system scale wiki
18 terms that explain how big the solar system really is and how things move in it, each in two sentences and each linked to a live page that shows it. Figures are standard published values (IAU, NASA).
All terms
- Astronomical unit (AU)
- Light time (light-minute)
- Light year
- Parsec
- Perihelion and aphelion
- Opposition
- Conjunction
- Ecliptic
- Sidereal and synodic period
- Kepler’s laws
- Barycentre
- N-body problem
- Lagrange points
- Asteroid belt
- Kuiper belt
- Heliopause
- Apparent magnitude
- Escape velocity
Astronomical unit (AU)
The average distance from Earth to the Sun, fixed by the IAU at exactly 149,597,870.7 km. It is the yardstick for distances inside the solar system: Mars is about 1.5 AU from the Sun, Neptune about 30 AU.
Light time (light-minute)
How long light takes to cross a distance. Light moves at 299,792.458 km/s, so sunlight reaches Earth in about 8 minutes 20 seconds, and a radio signal to Voyager 1 takes well over 20 hours one way.
Light year
The distance light travels in one Julian year, about 9.46 trillion km. It is used for stars and galaxies: the nearest star system, Alpha Centauri, is about 4.4 light years away.
Parsec
A distance of about 3.26 light years, defined from the tiny apparent shift of a star as Earth orbits the Sun. Astronomers use it for the distances between stars.
Perihelion and aphelion
The closest and farthest points of an orbit around the Sun. Earth is at perihelion in early January, about 147.1 million km from the Sun, and at aphelion in early July, about 152.1 million km away.
Opposition
When an outer planet lies opposite the Sun in our sky, rising at sunset and visible all night. It is also close to its nearest point to Earth, so it looks biggest and brightest then.
Conjunction
When two bodies appear close together in the sky, or when a planet lines up with the Sun. A planet near conjunction with the Sun is lost in its glare and cannot be observed.
Ecliptic
The plane of Earth’s orbit around the Sun. The planets orbit close to this plane, which is why they all appear along the same line across the sky.
Sidereal and synodic period
A sidereal period is the time a planet takes to orbit the Sun once against the stars (Earth: 365.256 days). A synodic period is the time between repeats of the same alignment seen from Earth, such as one opposition to the next (Mars: about 780 days).
Kepler’s laws
Three rules for orbits: planets move on ellipses with the Sun at one focus, they sweep equal areas in equal times (so they move fastest at perihelion), and the square of the period grows with the cube of the orbit size.
Barycentre
The centre of mass of two or more bodies, the point they actually orbit. For the Sun and Jupiter it lies just outside the Sun’s surface, so the Sun wobbles as Jupiter orbits.
N-body problem
Predicting the motion of three or more bodies pulling on each other. There is no general formula, so it is solved step by step on a computer. GalaxySim 3D switches to an N-body integration when you change a mass.
Lagrange points
Five places near two large bodies where a small object can hold a steady position relative to them. The James Webb Space Telescope orbits the Sun–Earth L2 point, about 1.5 million km from Earth, and SOHO sits at L1.
Asteroid belt
A broad ring of rocky bodies between Mars and Jupiter, roughly 2.2 to 3.2 AU from the Sun. Despite the films, it is mostly empty space: spacecraft cross it without close encounters.
Kuiper belt
A region of icy bodies beyond Neptune, from about 30 to 50 AU, that includes Pluto and other dwarf planets. New Horizons flew through it after passing Pluto in 2015.
Heliopause
The boundary where the Sun’s wind gives way to interstellar space, about 120 AU out. Voyager 1 crossed it in 2012 and Voyager 2 in 2018, the only spacecraft to have done so.
Apparent magnitude
A scale for how bright an object looks from Earth. Lower numbers are brighter and each step is about 2.5 times in brightness: Venus can reach about −4.6, the faintest stars seen by eye about +6.
Escape velocity
The speed needed to leave a body’s gravity without further thrust: about 11.2 km/s from Earth, 2.4 km/s from the Moon and 617.5 km/s from the Sun’s surface.
Want to see these in motion? Open the 3D simulator. Cite or link to this page from the press kit.