How space works
Light-years, astronomical units, and the scale of space
The right distance unit makes a map readable. It also tells you how old the arriving light is—and how far that is from a realistic travel plan.
Updated
Start with the Earth–Sun distance
An astronomical unit, or AU, is exactly 149,597,870.7 kilometers. It is based on the scale of Earth's orbit, although Earth's actual distance from the Sun changes during the year. Using AU keeps planetary distances manageable: a planet at 5 AU has an orbital scale about five times Earth's, rather than a number hundreds of millions of kilometers long.
An orbital semimajor axis is the size of an ellipse, measured as half its longest diameter. It is not the planet's distance from its star at every moment. For a noticeably eccentric orbit, the nearest and farthest distances can differ substantially. Compare like quantities when reading a fact table: a semimajor axis, a current separation, and an Earth-to-planet distance answer different questions.
A light-year measures length
A light-year is the distance light travels in one Julian year in a vacuum, about 9.46 trillion kilometers. A star ten light-years away is seen by light that traveled roughly ten years before reaching us. That delay applies to the message, photograph, or spectrum, not just to what the naked eye sees.
The word year can make the unit sound like a journey time. A spacecraft needs a speed before a journey time can be calculated. Crossing ten light-years would take ten years only for light traveling in vacuum. Real spacecraft move much more slowly, and a useful mission must also consider acceleration, navigation, power, communication, and arrival.
Why research tables use parsecs
A parsec is about 3.26156 light-years. It comes from parallax: the apparent change in a nearby star's position as Earth moves around the Sun. An annual parallax angle of one arcsecond corresponds to a distance of one parsec. Smaller angles usually mean greater distance, but measurement uncertainty becomes especially important when the angle is tiny.
Space Pixel converts the archive's parsec distances to light-years for the profile summaries. The original parsec value remains in the table. Multiplying both the value and its uncertainty by the same conversion factor preserves their relationship. Extra decimal places in a conversion do not make the underlying observation more precise.
Read a universe map in layers
The Solar System, nearby stars, and galaxies occupy vastly different distance scales. Drawing planet diameters and every separation to the same scale would leave most objects invisible on an ordinary screen. The 3D explorer enlarges bodies and compresses distances so visitors can select them. Its arrangement is a navigation aid.
For a numerical comparison, use the tables. For the direction of a target in the sky, use actual sky coordinates and an observing chart. For a distant galaxy, distinguish light-travel time from other cosmological distance definitions; the universe has expanded while the light was traveling. A single unlabeled distance number can hide that difference.