Cool Sun Facts · Science & history

Sun facts

32 short explanations of our star, each with its sources: what it is made of, how it moves across your sky, why the seasons and the clocks disagree, and what its storms do. Where a value changes from day to day, the page works it out for today.

Your sky today, explained →

SunOur starThe Sun is a star, and an ordinary oneA G2V yellow dwarf: the nearest star, not the biggest or brightest.

SunOur starHow big is the Sun?About 1.4 million km across, and more than 330,000 Earths in mass.

SunOur starHow old is the Sun, and how long will it shine?About 4.6 billion years old, with roughly 5 billion years of hydrogen to go.

SunOur starHow far away is the Sun today?About 150 million km on average; today's figure is worked out for you.

SunInside and aboveWhat powers the Sun?Hydrogen fusing into helium in a core at about 15 million °C.

SunInside and aboveSunlight is old before it leaves the SunThousands of years to reach the surface, eight minutes to reach you.

SunInside and aboveThe surface you see is not solidThe photosphere is a layer of gas about 400 km thick, at about 5,500 °C.

SunInside and aboveAbove the surface: the chromosphere and its spiculesJets of gas thousands of kilometres tall, rising at up to 100 km/s.

SunInside and aboveWhy is the corona hotter than the surface?Up to 2 million °C above a 5,500 °C surface: still an open question.

SunLight and energyHow much light does the Sun give out?3.828 × 10²⁶ watts in all; about 1361 watts per square metre at Earth.

SunLight and energyWhat colour is the Sun?White. It looks yellow or red from the ground because of our air.

SunLight and energyUV is not the same as brightnessThe UV Index forecasts ultraviolet risk; it is not a light meter.

SunLight and energySunlight is about 7% stronger in January than in JulyEarth is closest to the Sun in early January, whatever the season where you are.

SunHow the Sun moves across your skySolar noon is rarely 12:00The Sun is highest at solar noon, which can be an hour or more from clock noon.

SunHow the Sun moves across your skyThe Sun rises due east only near the equinoxesSunrise swings north and south along the horizon over the year.

SunHow the Sun moves across your skyWhen exactly is sunset?When the top edge of the Sun meets the horizon, with the air's bending allowed for.

SunSeasons and clocksSeasons come from tilt, not distanceEarth is closest to the Sun in January, in the middle of northern winter.

SunSeasons and clocksSundials and clocks disagree by up to 16 minutesThe equation of time, and the figure-eight called the analemma.

SunSeasons and clocksSolstices and equinoxes are moments, not daysEach is an instant, the same everywhere, falling on different dates by time zone.

SunSeasons and clocksDay length changes fastest near the equinoxesMinutes a day in March and September; seconds near the solstices.

SunAn active starSunspots are cooler, magnetic regionsDark because they are cooler than their surroundings, not because they are holes.

SunAn active starThe Sun spins faster at its equatorAbout 25 days at the equator, about 36 near the poles.

SunAn active starThe Sun's eleven-year cycleSpots come and go, and the magnetic poles flip, about every 11 years.

SunAn active starWhat an X-class flare meansLetters for powers of ten in X-ray brightness, measured by a satellite.

SunAn active starCMEs and the solar windMaterial, not light: hours to days on the way to Earth.

SunLooking at the SunSolar images are colour-codedEach colour stands for a wavelength, and so for a temperature and a layer.

SunLooking at the SunNever look at the Sun; project itSunglasses are not enough. A pinhole and a piece of card are.

SunLooking at the SunFrom spots to the solar wind: four centuries of solar scienceGalileo's spots, Fraunhofer's lines, Carrington's flare, Parker's wind.

SunThe Sun and Earth's skyWhy the sky is blue and sunsets are redThe same scattering does both; the low Sun simply shines through more air.

SunThe Sun and Earth's skyCivil, nautical and astronomical twilightThree bands of the Sun's depth below the horizon: 6°, 12° and 18°.

SunThe Sun and Earth's skyWhat the Sun has to do with the auroraThe Sun supplies the storm; seeing it needs darkness, clear sky and the right latitude.

SunThe Sun and Earth's skyThe Sun's geometry cannot forecast cloudsSunrise, twilight and night are calculated; the weather is not.

How the live values are worked out: how these pages calculate.

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