The sky, explained plainly
Every guide answers one question at a time, and every object opens in your real sky.
Guided tours on Earth
Meet the Sun
Watch one day go by and see where the Sun rises, peaks and sets. · 2 min
Meet the Moon
Find the Moon, see its shape, and learn which way its lit side points. · 2 min
Why the Moon changes shape
Step through a month and see how the Sun lights the Moon. · 3 min
The Sun through the seasons
Compare the Sun's path on the longest and shortest days of the year. · 2 min
Find the North Star
Use the Big Dipper to find Polaris, the star the whole sky turns around. · 2 min
The planets' road
See where to look for planets, and why they all follow the same line. · 2 min
On the Moon and Mars
The Earth from the Moon
Stand at Tranquility Base and watch the Earth hang in the sky, turn, and change its phase. · 3 min
A day that lasts a month
Sunrise over the Apennine mountains at the Apollo 15 site, and the long lunar morning. · 2 min
An eclipse, seen from the Moon
During a total lunar eclipse, stand on the Moon and watch the Earth hide the Sun. · 2 min
The Earth on the horizon
At the edge of the Moon's near side, watch the Earth rise and set over the weeks. · 2 min
The far side
Where the Earth never rises: a night sky with no Earth and no moonlight. · 1 min
Sunset on Mars
From Gale crater: a butterscotch sky that turns blue around the setting Sun. · 2 min
Home, seen from Mars
Find the Earth and the Moon as a pair of stars in the Martian twilight. · 2 min
A sol on Mars
One Martian day, from a butterscotch dawn to a blue sunset, at Jezero crater. · 2 min
The two moons of Mars
Phobos races across the sky from west to east twice a day; Deimos crawls the other way. · 2 min
Questions
Why do the Sun and stars move across the sky?
They don't — the Earth spins once a day, and we're carried along.
Why is the Sun higher in summer?
The Earth's axis is tilted 23.4°, so through the year our hemisphere leans towards and then away from the Sun.
Why doesn't it get dark right after sunset?
Sunlight still reaches the air high above us and scatters down, fading slowly as the Sun sinks further.
Why does the Moon change shape?
We see different amounts of its sunlit half as it circles the Earth.
Why does the Moon rise later every day?
While the Earth spins, the Moon also moves along its orbit, so the Earth needs about 50 extra minutes to catch up with it.
Why can I see the dark part of a crescent Moon?
Sunlight reflected off Earth lights up the Moon's night side — earthshine.
What is an eclipse?
When the Sun, Earth and Moon line up exactly, one casts a shadow on the other.
What is the ecliptic?
The yearly path of the Sun against the stars — and the road the Moon and planets travel along.
How are planets different from stars?
Planets are nearby worlds that reflect sunlight and slowly move against the fixed stars.
Why do planets sometimes go backwards?
When the Earth overtakes an outer planet, it seems to drift backwards for a few weeks.
Why do stars twinkle?
Moving pockets of air bend starlight back and forth on its way to your eye.
Why are stars different colours?
A star's colour tells its temperature: blue stars are hottest, red ones coolest.
Do stars live forever?
No — stars are born in clouds of gas, shine for millions to billions of years, and then die.
What is a light-year?
The distance light travels in a year: about 9.5 trillion km.
What is the Milky Way?
Our home galaxy, seen from the inside: a flat disc of hundreds of billions of stars.
Why can't I see many stars from the city?
Artificial light brightens the sky and drowns out faint stars.
What is a constellation?
A pattern of stars that people named, and today an official area of the sky.
Why do the stars circle the North Star?
Polaris sits almost exactly above Earth's axis, so as we spin, it stays put while the rest of the sky turns around it.
How do I find the North Star?
Follow the two end stars of the Big Dipper's bowl about five times their distance.
How can I measure the sky with my hand?
At arm's length, a fist covers about 10°, three fingers about 5°, a little finger about 1°.
Sun, Moon and planets
The Sun
Our star: a ball of hot gas so big that 1.3 million Earths would fit inside.
The Moon
Earth's only natural satellite, about 384,000 km away.
The Earth
Home, seen from outside: a blue and white world about four times wider than the Moon looks from here.
Phobos
The larger of Mars' two tiny moons: a dark, lumpy rock 27 km long, only 6,000 km above the ground.
Deimos
Mars' smaller, outer moon: from the ground, a bright star that drifts slowly across the sky.
Mercury
The smallest planet and the closest to the Sun — hard to catch, always near twilight.
Venus
The brightest planet — often called the Evening Star or Morning Star.
Mars
The Red Planet — its rusty dust gives it that orange glow.
Jupiter
The giant planet — more than twice as massive as all the others together.
Saturn
The ringed planet — even a small telescope shows its rings.
Uranus
An ice giant tipped on its side, at the edge of naked-eye visibility.
Neptune
The farthest planet — too faint for the eye, a blue dot in a telescope.
Bright stars
Sirius
The brightest star in the night sky, in the Great Dog.
Canopus
The second-brightest star — a southern-sky giant.
Arcturus
A bright orange giant — follow the arc of the Big Dipper's handle to find it.
Vega
A brilliant blue-white star, overhead on summer evenings.
Capella
A bright yellow star — actually two giant stars orbiting each other.
Rigel
Orion's bright blue-white foot — a supergiant.
Procyon
The Little Dog's bright star, close to us at 11 light-years.
Betelgeuse
Orion's red shoulder — a dying supergiant.
Altair
The bright star of the Eagle, part of the Summer Triangle.
Aldebaran
The orange eye of Taurus the Bull.
Antares
The red heart of the Scorpion — a rival of Mars.
Spica
The bright blue-white star of Virgo.
Pollux
The brighter of the Gemini twins.
Castor
One of the Gemini twins — really a family of six stars.
Deneb
The tail of the Swan — one of the most distant naked-eye stars.
Regulus
The heart of Leo the Lion.
Fomalhaut
The lonely bright star of the autumn sky, low in the south.
Polaris
The North Star — the sky seems to turn around it.
Mizar
The bend of the Big Dipper's handle — with a faint partner, Alcor.
Dubhe
One of the two Pointer stars of the Big Dipper.
Merak
The other Pointer star of the Big Dipper.
Alnilam
The middle star of Orion's Belt.
Bellatrix
Orion's left shoulder, the "female warrior".
Achernar
The end of the river Eridanus, deep in the southern sky.
Acrux
The brightest star of the Southern Cross.
Hadar
One of the two Pointers to the Southern Cross.
Rigil Kentaurus (Alpha Centauri)
The closest star system to the Sun — 4.4 light-years away.
Constellations
Orion
The most recognisable constellation: a hunter with a belt of three stars.
Ursa Major
Seven bright stars form the Big Dipper, the key to finding your way around the northern sky.
Ursa Minor
The Little Dipper, with the North Star at the tip of its handle.
Cassiopeia
A bright W (or M) shape on the opposite side of the North Star from the Big Dipper.
Cygnus
A swan flying down the Milky Way, also seen as a big cross.
Lyra
A small harp-shaped constellation with brilliant Vega.
Aquila
An eagle soaring along the Milky Way, marked by bright Altair.
Scorpius
One of the few constellations that really looks like its name.
Sagittarius
The Teapot: towards it lies the centre of our galaxy.
Leo
A crouching lion: a backwards question mark for its head and a triangle for its tail.
Taurus
A V-shaped face with an orange eye, and the Seven Sisters on its shoulder.
Gemini
Two bright stars, Castor and Pollux, mark the twins' heads.
Canis Major
Orion's hunting dog, home of Sirius, the brightest star in the sky.
Boötes
A kite shape with orange Arcturus at its tail.
Pegasus
The Great Square: four stars marking the horse's body.
Andromeda
Home of the Andromeda Galaxy, the farthest thing you can see with your eyes.
Perseus
The hero who rescued Andromeda — and the source of the August Perseid meteors.
Crux
The smallest constellation and the most famous southern landmark.
Centaurus
A large southern constellation holding the nearest stars to the Sun.
Virgo
The second-largest constellation, rich in distant galaxies.
Galaxies, nebulae and clusters
Andromeda Galaxy
Our giant neighbour galaxy — the farthest thing you can see with your eyes.
Orion Nebula
A stellar nursery: a glowing cloud where new stars are being born.
Pleiades (Seven Sisters)
A tiny dipper of bright blue stars — one of the prettiest sights in the sky.
Beehive Cluster
A swarm of stars in Cancer — a misty patch to the eye, a swarm in binoculars.
Hercules Globular Cluster
A ball of hundreds of thousands of ancient stars.
Ring Nebula
The glowing smoke ring of a dying star.
Crab Nebula
The wreck of a star that exploded in the year 1054.
Lagoon Nebula
A huge glowing cloud in Sagittarius, visible to the eye from a dark site.
Trifid Nebula
A nebula split into three by dark lanes of dust.
Dumbbell Nebula
A bright dying-star nebula shaped like an apple core.
Whirlpool Galaxy
A face-on spiral galaxy tugging a smaller companion.
Bode's Galaxy
A bright spiral galaxy near the Big Dipper.
Sombrero Galaxy
A galaxy seen edge-on, with a dark lane of dust across its bulge.
Triangulum Galaxy
The third-largest galaxy in our Local Group.
Ptolemy's Cluster
A bright open cluster by the Scorpion's tail, known since ancient times.