A distance experiment

How long does light take to cross the Solar System?

Distances from Mercury to Neptune are hard to picture. Light travel time makes them easier to compare. Choose a planet, then travel on to the nearest star.

35 projects on space and the sky

From the Sun01—09

The logarithmic axis starts at one minute. Planet values are mean orbital distances; Proxima is shown at its distance from the Sun.

SelectedEarth

Mean distance1 au

Light travel time8 min 19 sec

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01 / Light

When we look at stars, we see their past.

Light left each star at a different time. Astronomers use measured relationships between some stars’ brightness and pulsation period to estimate distances too great to cross.

The light that left How old is sunlight: 8 minutes across space, 170,000 years inside the Sun Who measured the universe? Why is the night sky dark? Olbers' paradox and the light that has not reached us yet

02 / Worlds

A planet can be followed.

Orbits, eclipses and craters leave traces that can be calculated or measured. These projects show what motion and terrain tell us, and where the picture remains incomplete.

Halley’s Comet: where it is now, and its whole 75-year orbit from 1986 to 2061 We Never See the Planets The other side of the Moon: why it looks so different, and who has landed there A Hundred Times Rarer

03 / Your sky

The same sky changes with place and date.

An eclipse, aurora or sunrise behind a mountain does not appear the same everywhere. Choose a place or time in these projects and see what a sky map can calculate.

When will the sky go dark at noon over your town? The northern lights have been seen from Romania. How many times since records began? When the sun really reaches you: sunrise and sunset behind the mountains for any place in Romania How many stars can still be seen from Romania's cities

04 / The journey

From an atlas page to space you can walk through.

Some projects bring the Solar System’s scale to your street. Others follow real satellites and spacecraft, or ask how travel changes a clock.

The solar system at 1:10 billion: Neptune would be 450 m from the Sun The space station flies over your house. Here is when to look up The farthest thing people have ever made is a day away, even for light Travelling near the speed of light: fly to Andromeda in one lifetime

The astronomical unit (au) is defined exactly as 149,597,870,700 m and approximates the mean Earth–Sun distance. Planet light times use mean orbital distances and the speed of light in vacuum; Proxima is shown at its distance of 4.24 light-years. No marker shows a position calculated for today.

Space · 35 projects

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35 projects

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Distances and journeys
Halley’s Comet: where it is now, and its whole 75-year orbit from 1986 to 2061
Halley’s Comet passed the far point of its orbit on 8 December 2023 and is heading back to the Sun, which it rounds on 28 July 2061. The page follows the whole orbit by scroll, 1986 to 2061, on NASA JPL positions: of 27,563 days, the comet spends 78 closer to the Sun than Earth is and 35 years farther out than Neptune. With a year of birth it shows where the comet was then and how old you will be when it returns.
Stars and light
Falling into a black hole: what the sky looks like on the way in
A fall into a black hole seen through the faller’s own eyes. Each point of the picture is a ray of light followed through curved space with the equations of general relativity, over NASA’s map of 1.7 billion stars. At the horizon the black disc is 84° across and covers 13% of the sky; the rest is still stars. At Sagittarius A* the fall from the horizon to the centre takes 28 seconds.
The sky above us
The birthday sky: the Moon, planets and the Sun's constellation
What was in the sky on the day you were born? Pick a date and a place and get the sky chart: Moon phase, planets above the horizon, sunrise and sunset, plus the constellation the Sun was really in — not the magazine sign, because of precession. All computed in your browser.
Distances and journeys
The solar system at 1:10 billion: Neptune would be 450 m from the Sun
If the Sun were 14 cm across, Earth would be a 1.3 mm bead 15 m away, Jupiter at 78 m, Neptune at 450 m. Pin the Sun on your street and walk to each planet, with Voyager 1 on the same map and a printable class sheet. No textbook picture fits to scale.
Distances and journeys
How fast are you moving? Earth's spin, orbit, galaxy and the CMB
You sit still, yet Earth carries you along: spin, orbit, galaxy and cosmic background. Pick a place and see all four speeds on a single ruler.
Distances and journeys
How far are you from the sea? Distance to the coast vs space (100 km)
Space begins 100 km above your head. For 80% of the world's land, the sea is farther than that. Type your town and see which is nearer, measured along the globe.
Worlds and celestial bodies
Drag the sky back 640 million years and watch the Moon change
The Moon moves away 38.3 mm a year, almost as fast as your fingernails grow. Drag the sky back through time and see the Sun and Moon at their size then, with a shorter day and different eclipses; every number is marked measured, counted in rock or calculated.
Stars and light
Older Than Earth: which stars made you?
Nearly every atom in your body is older than the Earth: about 63 in 100 are primordial hydrogen, older than the stars. Follow one gold atom from the Big Bang to you, weigh your own stardust, and find where gold comes from. In Romanian and English.
Stars and light
Most of your atoms are 13.8 billion years old: where each part of your body was made
Six in ten of your atoms are hydrogen from the Big Bang. Your oxygen came from exploding giant stars, much of your carbon from dying stars, half the calcium in your bones from exploding white dwarfs, your iodine from colliding neutron stars. Set your weight and watch your body come apart.
Stars and light
Why is the night sky dark? Olbers' paradox and the light that has not reached us yet
If the stars went on for ever, the sky would blaze like the Sun. A forest where every line of sight hits a tree, a sky filling with suns, three centuries of wrong answers, and the 13.8-billion-year-old light that covers the whole sky, invisibly.
The sky above us
When the sun really reaches you: sunrise and sunset behind the mountains for any place in Romania
Almanac sunrise assumes a flat horizon. Calculated from the terrain for every day of the year: at Băile Herculane on 21 December the sun only clears the mountains at 10:54; 6.7 million people live where the terrain takes at least half an hour of sun on the shortest day.
Worlds and celestial bodies
Why Romanian weekdays still carry five Roman gods
Luni = Moon, marți = Mars, miercuri = Mercury, joi = Jupiter, vineri = Venus. Why the gods stand scrambled, plus Saturday, Sunday and the 321 law.
Distances and journeys
The space station flies over your house. Here is when to look up
You can see it with the naked eye, like a very bright star. Pick your town for the next passes, to the minute.
The sky above us
The northern lights have been seen from Romania. How many times since records began?
Three times in three years: November 2023, May 2024, January 2026. Since 1932, 23 storms have reached Kp 9.
Stars and light
The Plough didn't look like this 100,000 years ago
100,000 years ago the Plough's bowl gaped almost twice as wide. Drag time across the sky, watch Polaris lose the pole, and meet the people who looked up then.
Distances and journeys
The farthest thing people have ever made is a day away, even for light
Romanian is one of the 55 languages on Voyager 1's golden record. On 18 November 2026 the craft reaches one light-day from Earth, a moment with three different dates depending on how you measure it. The story, from the tour that was never made to the instruments going out.
The sky above us
When will the sky go dark at noon over your town?
Pick the nearest town and see every solar eclipse it gets from 1900 to 2100: which were total, how much Sun was left each time, and when the next shadow comes.
Worlds and celestial bodies
The Mocs meteorite (Mociu, 1882): the stone shower on Koch's map and the 35.70 kg stone
On 3 February 1882, after a fireball crossed the sky near Cluj, thousands of stones fell: small ones at one end, giants at the other. Watch them fall again on the map drawn at the time, and weigh the famous “56 kg stone” back into 102 stones. English and Romanian.
Stars and light
How old is sunlight: 8 minutes across space, 170,000 years inside the Sun
Light flies eight minutes to reach you, but its energy wandered inside the Sun for 170,000 years. A random-walk simulation, a race with a neutrino, and eight true-or-false sentences.
Worlds and celestial bodies
The water bear survived open space. A 37-degree day kills it
Dried out, the water bear spent ten days in the vacuum of space and woke after over 30 years frozen. Awake, it dies at 37 degrees. Measure yourself against it, to scale, and play with its limits.
Worlds and celestial bodies
The other side of the Moon: why it looks so different, and who has landed there
Turn a globe of the Moon and watch the dark seas almost disappear: they cover 30% of the near side and 2% of the far side. With a map of crust thickness, the South Pole–Aitken basin, the places where Chang’e 4 and Chang’e 6 landed, and what the first samples brought back from there say.
Worlds and celestial bodies
We Never See the Planets
A planet becomes visible through the light it removes. Move a planet across a star and see the signal, then choose a real world from NASA's archive.
Stars and light
How many stars can still be seen from Romania's cities
The sky over 3,181 places in Romania, from Globe at Night observations and a satellite-light estimate: Bucharest observers' median is 34 stars, against about 3,000 under a natural sky.
The sky above us
Sixteen Minutes
The sun reaches its true noon up to sixteen minutes before or after twelve o'clock on your clock. Only four days a year do the two match exactly.
Worlds and celestial bodies
A Hundred Times Rarer
The Moon has 5,185 confirmed impact craters wider than 20 kilometres. Earth, despite taking more hits per square kilometre, has only 52. An explorer with 70 real craters, from 200 years old to 2.229 billion years old.
The sky above us
When the Sun Disappears: An eclipse atlas
A multidisciplinary atlas of what a total solar eclipse reveals about nature, minds, societies, institutions and knowledge.
The sky above us
The 1961 eclipse in Bucharest: two minutes under cloud
The eclipse was total in Bucharest on 15 February 1961, but cloud hid totality from the ground. Newspapers, scientific reports and Romanian archives preserve observations made on the ground and from the aircraft.
Stars and light
The light that left
Choose a year and see the stars whose light set out for Earth then. 8,714 stars from HYG 4.1 form an interactive map of the sky.
The sky above us
A plough, an auger and three saints
The same stars, cut differently.
The sky above us
The Light of Your Days
A century of daylight, city by city.
Distances and journeys
Travelling near the speed of light: fly to Andromeda in one lifetime
Accelerate at 1 g, the pull you feel standing on Earth, and in 28 years and 7 months aboard you reach the Andromeda galaxy. Fly there through a sky of 85,611 catalogued stars and watch two clocks for home: the one that runs in Earth’s frame, and the latest light from Earth that has caught up with you.
Worlds and celestial bodies
What was the Moon on the night you were born? Find your birth Moon
On the night you arrived, the Moon had a shape. Put in your birthday and the sky turns back to it, then find out when it returns.
The sky above us
After sunset, the sky is blue for a different reason
A sky computed from the physics of air, for your place and evening. Scroll and the Sun sinks to the stars; take out the ozone and the blue hour turns yellow.
The sky above us
The evenings the sun sits on Ceaușescu's palace
Seen from Alba Iulia Square, the setting sun comes down exactly onto the top of Bucharest's Palace of the Parliament on 29 March and 13 September. Look up your street and see the evenings the sun sets or rises at its end, in a view drawn from the city's real buildings.
Stars and light
Who measured the universe?
Henrietta Leavitt found the relation between a Cepheid’s period and brightness, but did not measure distances. Plot the 25 stars in Table I and follow how Hertzsprung and Hubble turned the relation into a cosmic distance scale.

About the collection

This collection brings together relevant projects from the site’s library. Each entry has a title, a description and a link to its available edition. The complete projects carry their sources and evidence limits.

Reviewed 2026-10-15 · Collection as text

Sources

Cosmic distances · NASA Science
Planetary mean distances are rounded; their real distances change along their orbits.
Chapter 1: The Solar System · NASA Science
What is a light-year? · NASA Science