Fourteen kinds of object, from the Moon to the edge of the observable universe, and what to look for in each.
Cosmodle's catalogue covers several hundred objects. They are not all measured the same way: it makes sense to ask how many moons a planet has, but not a galaxy, and a constellation has no single distance at all. So each kind of object is compared on its own set of properties. Knowing which set applies is half the game.
The eight planets and a handful of dwarf planets are compared on distance from the Sun, temperature, diameter, mass, surface gravity, number of moons, rings, orbital period, rotation period and when they were first recorded. The naked-eye planets have been known since prehistory, while Uranus, Neptune and the dwarf planets each have a discovery year. Rings are a strong clue: only the four giant planets and the dwarf planet Haumea have them.
Moons are compared on the planet they orbit, how far from it they orbit, temperature, diameter, mass, gravity and orbital period. The parent planet is usually the fastest way in. Jupiter and Saturn between them hold most of the moons in the game, so once you know the parent, orbital distance separates the inner moons, which circle in hours or days, from the distant captured ones.
These small bodies share a profile: distance from the Sun, diameter, mass, temperature and orbital period. Most asteroids stay in the main belt between Mars and Jupiter, within a few astronomical units of the Sun. Comets swing much wider. Their average distances run from a couple of astronomical units to thousands, and their orbital periods from a few years to tens of thousands, so a single distance guess usually tells you which of the two you are dealing with.
Stars are the largest group in the catalogue. They are compared on distance from Earth, surface temperature, diameter, mass and when they were first recorded. Temperature is a good early lever: red giants like Betelgeuse and Antares sit around 3,500 kelvin, the Sun at about 5,800, and hot blue stars like Rigel and Spica well above 10,000. Stars you can see with the naked eye show the date of the earliest catalogue that lists them rather than a discovery year.
Planets around other stars are compared on their host star, distance from Earth, equilibrium temperature, diameter, mass, orbital period and discovery year. The nearest are a little over four light years away. Many of the famous ones are "hot Jupiters" that orbit their star in a few days, so a very short orbital period is a useful hint.
The 88 constellations recognised by the International Astronomical Union are compared on their hemisphere, the area of sky they cover, the brightness of their brightest star, whether they belong to the zodiac, and when they were first recorded. Hydra is the largest and Crux the smallest. The forty-eight constellations described by Ptolemy around AD 150 show that date; the rest were added by later astronomers, mostly for the southern sky.
Nebulae are compared on distance, diameter, nebula type, apparent brightness and discovery year. Star clusters use a similar set with cluster type in place of nebula type. The type tile is what separates them within their group: an emission nebula from a planetary nebula, an open cluster of young stars from a globular cluster of very old ones. Most of both sit within tens of thousands of light years, inside the Milky Way or in its immediate neighbourhood.
Galaxies are compared on distance, diameter, estimated mass, galaxy type and discovery year. Distance runs from our own Milky Way, at zero, to hundreds of millions of light years. Galaxy type, spiral, elliptical or irregular, is an exact match and narrows the field quickly.
Black holes are compared on distance, the diameter of their event horizon, mass, type and discovery year. Type splits them into stellar-mass black holes a few times heavier than the Sun, the rare intermediate class, and the supermassive ones at the centres of galaxies, which can be billions of times heavier. Mass and event-horizon size go up together, so one arrow tells you about the other.
Quasars are the brightest objects in the catalogue and the most distant: hundreds of millions to more than thirteen billion light years away. They are compared on distance, the mass of their central black hole, redshift, apparent brightness and discovery year. Redshift rises with distance, so the two arrows usually agree.
A few entries are not lasting objects at all but explosions: supernovae, gamma-ray bursts and a kilonova from two neutron stars colliding. They are compared on distance, redshift, peak brightness, event type and discovery year. The historical supernovae are nearby and were seen by eye; the gamma-ray bursts happened billions of light years away.
Figures are drawn from NASA and JPL, the NASA Exoplanet Archive, SIMBAD and Wikipedia, and rounded for readability. Catalogues disagree and values get revised, so if something looks wrong, the About page says how to report it.
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