How Big Is the Sun? A Size Comparison of the Sun and Planets

The Sun is about 1.39 million kilometres across. That number is too big to mean anything on its own, which is the whole problem with space: every figure is enormous, so they all blur into the same kind of enormous. The way through is ratios. Measured in Earths, it is about 109 across. Jupiter is about 11. The Moon is a little over a quarter of one.

Space rounds in Scale of Things work exactly that way. Planets, moons, asteroids and stars are measured across, by their diameter, and set beside each other on one ground line. Here is how the main bodies of the solar system compare, and a few stars beyond it.

Our star is about 109 times as wide as Earth and about 10 times as wide as Jupiter. By volume it would hold around 1.3 million Earths, and it carries about 99.8% of all the mass in the solar system. Everything else, the planets, moons, asteroids and comets together, is the rounding error.

It is also a fairly ordinary star. It looks special only because it is close: as you will see below, plenty of stars are hundreds of times wider.

Diameters are mean figures from NASA, rounded, with each planet's width compared with Earth's.

• Jupiter, 139,820 km: about 11 Earths across, and roomy enough to hold more than 1,300 Earths.
• Saturn, 116,460 km: about 9 Earths across. That is the planet alone, which is how the game draws it. Its bright main rings span about 270,000 km, well over twice the planet's width.
• Uranus, 50,724 km: about 4 Earths.
• Neptune, 49,244 km: just narrower than Uranus, though it is the heavier of the two.
• Earth, 12,742 km.
• Venus, 12,104 km: 95% of Earth's width, very nearly a twin.
• Mars, 6,779 km: just over half of Earth.
• Mercury, 4,879 km: under two fifths of Earth, and smaller than two moons.

The jump in the middle of that list is the thing to remember. The four giant planets are all nearly four times Earth's width or more, and the four rocky ones are all Earth's size or less. There is nothing in between.

The largest moons are worlds in their own right. Jupiter's Ganymede, at 5,268 km, and Saturn's Titan, at 5,150 km, are both wider than the planet Mercury, though Mercury, being dense rock and metal, is more than twice as massive as either. Jupiter's Callisto, at 4,821 km, falls just short of Mercury.

Then come Io at 3,643 km and our own Moon at 3,475 km, making ours the fifth largest moon in the solar system, ahead of Jupiter's Europa at 3,122 km and Neptune's Triton at 2,707 km.

What makes the Moon unusual is not its size but its size beside its planet: it is about 27% of Earth's width, while Ganymede is under 4% of Jupiter's. Only Pluto's companion Charon, about half Pluto's width, outdoes it.

Pluto is 2,377 km across, smaller than the Moon and narrower than the United States is wide. Eris, found in 2005, is almost exactly the same width at 2,326 km and about a quarter more massive, which is part of why both ended up classed as dwarf planets. Further down:

• Makemake, a dwarf planet beyond Neptune, about 1,430 km.
• Ceres, the largest object in the main asteroid belt, about 939 km.
• Vesta, about 525 km.
• Mars's two moons, Phobos at about 22 km and Deimos at about 12 km, small enough to be shaped like potatoes.
• The asteroids Ryugu and Bennu, both visited and sampled by spacecraft, at about 900 m and 490 m.

The game's space catalog runs this whole range, from Bennu, about five and a half American football fields across, to the supergiant stars. That is a factor of more than two billion in a single category, which is why space rounds so often need the zoom keys.

Star sizes are much less certain than planet sizes. A star has no hard surface, the biggest ones pulse, and their distances are hard to pin down, so published radii for the same supergiant can differ by a third or more. These are the figures the game uses, in round numbers.

• Proxima Centauri, the nearest star, is a small red dwarf about 1.5 Jupiters across, only about 15% of the Sun's width.
• Sirius A, the brightest star in the night sky, is about 1.7 Suns across, and Vega about 2.4.
• Arcturus is a red giant about 25 Suns across, and Polaris, the North Star, about 38.
• Aldebaran, the red eye of Taurus, is about 45 Suns, and the blue supergiant Rigel about 79.
• Antares is about 680 Suns across, and Betelgeuse, in the shoulder of Orion, about 760.

Put Betelgeuse in the Sun's place and, on that figure, its surface would lie beyond the orbit of Mars, out in the main asteroid belt. Mercury, Venus, Earth and Mars would all be inside the star.

Scale everything down so that a million kilometres becomes one metre. Then:

1. The Sun is a ball 1.39 m across, about the height of a ten-year-old.
2. Earth is a blueberry 1.3 cm wide, 150 m away: more than one and a half American football fields.
3. The Moon is a peppercorn 38 cm from the blueberry.
4. Jupiter is the size of a grapefruit, about 780 m from the big ball.
5. Neptune is a little bigger than a golf ball, 4.5 km away.
6. Proxima Centauri, the nearest star, is about 40,000 km away, roughly once around the real Earth.

That last line is the one worth sitting with. The solar system is mostly empty, and the space between stars is emptier by far.

It is a popular claim, and it is almost true. Add up the diameters of the seven other planets and you get about 380,000 km. The average distance from the centre of the Earth to the centre of the Moon is about 384,400 km, so on paper they fit. Measured surface to surface, though, the average gap is only about 376,000 km, a little too short. The planets fit with room to spare only when the Moon is near the far end of its orbit, about 405,000 km from Earth's centre.

A few things to know before your next space round:

• Everything is measured across, through the middle. For Saturn that means the planet, not the rings.
• Places and space share rounds, so a country can stand beside a moon. A good bridge between the two: Australia is about 4,000 km from east to west, wider than the Moon. Our piece on how big countries really are has more of those.
• Near-twins are traps. Earth and Venus differ by about 5%, so if they share a round, leaving the target level with the reference already earns the full 100. Resist the urge to adjust.
• Big gaps are common. Neptune is nearly four times Earth's width, and beside it Earth looks like a coin. Trust the ratio, not the picture, and use the zoom.

Ratios this large are easier to hold if you think in factors of ten, which is the subject of our piece on powers of ten and the scale of the universe. Then try a few runs of Scale of Things and see whether the giant planets still come up short.