Pick a date and watch every planet move along its real orbit. Then compare the planets side by side, find your weight and age on each one, and test yourself with a quiz.
Planets do not move in perfect circles. Each one follows an ellipse with the Sun slightly off-center, moves faster when it is closer to the Sun, and the farther out it is, the longer its year — these are Kepler's three laws, and the simulator follows them exactly.
Speed
Distances
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Distance from the Sun
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Distance from Earth
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Orbital speed now
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One orbit (year)
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Eccentricity
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Closest – farthest from Sun
A rough guide: “too close” means less than 15° from the Sun. Uranus and Neptune need binoculars or a telescope; the rest are easy with the naked eye when well placed.
Kepler's third law: T² = a³
a is the average distance from the Sun in astronomical units (1 AU = Earth's average distance) and T the orbital period in Earth years. For every planet T² ÷ a³ comes out at almost exactly 1.
Sizes to scale
The Sun is 109 times wider than Earth — only its edge fits on the left. Distances between the planets are not to scale here.
Planet fact sheet
Values from NASA's planetary fact sheet. For the gas giants “surface” gravity is measured where the pressure equals Earth's sea-level air pressure. Venus spins backwards, so its day is shorter than one spin.
Your weight on other worlds
Your mass — how much of you there is — never changes. What changes is how hard each world pulls on it, so the bathroom scale would read differently.
Your age on other planets
A planet's year is one trip around the Sun, so your age there is your age in days divided by that planet's year.
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Accuracy
The Sun and its eight planets
The Sun holds 99.86% of all the mass in the solar system, and everything else circles it. In order outward come Mercury, Venus, Earth and Mars — the four small rocky planets with iron cores — then the asteroid belt, and then the four giants: Jupiter and Saturn, made mostly of hydrogen and helium, and Uranus and Neptune, rich in water, ammonia and methane ices. All eight orbit in the same direction and almost in the same flat plane, a leftover of the spinning disk of gas and dust they formed from 4.6 billion years ago.
Pluto was counted as the ninth planet from its discovery in 1930 until 2006, when astronomers defined a planet as a body that orbits the Sun, is massive enough to be round and has cleared the neighborhood around its orbit. Pluto passes the first two tests but shares its zone with thousands of other icy bodies in the Kuiper belt, so it is now a dwarf planet. Tick “Pluto and asteroid belt” in the simulator to add it: its tilted, stretched orbit even dips inside Neptune's for 20 of every 248 years.
Kepler's first law: orbits are ellipses
In 1609 Johannes Kepler, working from Tycho Brahe's careful measurements of Mars, showed that planets do not move on circles. Each orbit is an ellipse, and the Sun sits not at its center but at one of its two foci. How stretched an ellipse is gets measured by its eccentricity e: 0 is a perfect circle and values near 1 are long and thin. The closest point to the Sun is called perihelion and the farthest aphelion; their distances are a(1 − e) and a(1 + e), where a is the average distance.
Most planetary orbits are surprisingly round. Earth's eccentricity is only 0.017, so its distance swings between 147.1 and 152.1 million km — and the closest point comes in early January, in the northern winter. Mercury (0.206) and Pluto (0.249) are the clear exceptions. Select Mercury in the true-scale view and the off-center Sun and the perihelion and aphelion markers make the ellipse obvious.
Kepler's second law: equal areas in equal times
A line from the Sun to a planet sweeps out equal areas in equal times. Near perihelion that line is short, so to cover the same area the planet has to travel farther along its path in the same time — it speeds up. Near aphelion the line is long and the planet crawls. Turn on “Kepler's 2nd law” and the simulator colors twelve wedges, each swept in one twelfth of the planet's year: short and fat near the Sun, long and thin far away, yet every wedge has the same area.
The same law gives a neat shortcut: at perihelion and aphelion the speeds are in the inverse ratio of the distances, (1 + e) ÷ (1 − e). Mercury moves about 1.5 times faster at perihelion (59 km/s) than at aphelion (39 km/s). Watch the “orbital speed now” readout while the animation runs to see it change. Physically, the law is conservation of angular momentum — the same reason a spinning skater speeds up when pulling in their arms.
Kepler's third law: T² = a³
Ten years after the first two laws, Kepler found how the size of an orbit sets the length of the year. Measure the average distance a in astronomical units (Earth's average distance, 149.6 million km) and the period T in Earth years, and for every planet T² = a³. The table under the simulator works it out: T² ÷ a³ comes out between 0.98 and 1.00 for all eight planets and Pluto. Jupiter, 5.2 AU out, needs √(5.2³) ≈ 11.9 years for one lap.
The law hides a second effect: outer planets do not just have longer paths, they also move more slowly. Neptune's orbit is 30 times wider than Earth's, so its path is 30 times longer, but its year is 165 times longer because it travels at only 5.4 km/s against Earth's 29.8 km/s. Newton later showed why: the Sun's gravity weakens with the square of the distance, and T² = a³ falls straight out of that.
How big, how far: why the true scale looks empty
The solar system is mostly empty space. If the Sun were a ball 1.4 m across, Earth would be a 1.3 cm marble 150 m away and Neptune a 5 cm ball 4.5 km away. No screen can show both the sizes and the distances at once, so the simulator gives you two views. True scale keeps the distances honest and zooms to fit the planet you select — pick Mercury to see the inner planets, Neptune to see everything — while the planets themselves are drawn hundreds of times too big so you can find them.
The readable scale uses a logarithmic squeeze so all eight orbits fit at once: each planet's direction and date are exact, but the outer orbits are pulled in. The size panel on the Compare tab does the opposite, drawing the planets at their true relative sizes with the distances ignored. Jupiter is 11 times wider than Earth, and the Sun 109 times — only its edge fits on the left.
Days, years and the planets in tonight's sky
A planet's day and year are separate clocks. Jupiter spins in under 10 hours yet takes 11.9 years to go round the Sun. Venus turns backwards so slowly that one spin (243 Earth days) is longer than its year (225 days), and from its surface the Sun would rise in the west roughly every 117 days. Uranus lies on its side, so each pole gets 42 years of daylight followed by 42 years of night.
Because the simulator knows where every planet is, it also tells you where to look. The table under it measures each planet's angle from the Sun as seen from Earth. Less than about 15° and it is lost in the glare; east of the Sun it shows up in the evening, west of it before dawn. Mercury and Venus, inside Earth's orbit, never stray far from the Sun, while an outer planet near opposition — directly opposite the Sun — rises at sunset, stays up all night and is at its brightest.
Gravity, weight and your age on other planets
Your mass is the amount of matter in you and stays the same anywhere. Your weight is the force of a world's gravity on that mass, so it scales with surface gravity: multiply by g ÷ 9.8. On Mars (3.7 m/s²) a 60 kg person would read about 22.7 kg on a bathroom scale, on Jupiter (23.1 m/s²) about 141 kg. Saturn, Uranus and Venus all pull at close to Earth's strength even though Saturn is 95 times more massive — its gravity is spread over a much larger radius.
Ages work the same way with the year. Your age on a planet is the number of days you have lived divided by that planet's year: a 30-year-old has been around the Sun about 16 times in Mars years, 2.5 times in Jupiter years and 124 times in Mercury years. Enter your birthday on the Compare tab to get your age on every planet and the date of your next birthday there; it is saved only in your browser.
How the positions are calculated
Each planet's orbit is described by six numbers — its size, eccentricity, tilt, the direction of its perihelion and of its node, and where the planet was on 1 January 2000 — taken from the approximate orbital elements published by NASA's Jet Propulsion Laboratory, with their slow drift over the centuries. For any date the simulator works out the mean anomaly, solves Kepler's equation M = E − e sin E by Newton's method and turns the result into a position in the plane of Earth's orbit. Sizes, gravity, days and temperatures come from NASA's planetary fact sheet.
Between 1800 and 2050 these elements place the planets within a fraction of a degree of their true positions, plenty for a top-down view, finding planets in the sky or checking dates of oppositions; the model reproduces Earth's perihelion in early January and the 2025 oppositions of Mars, Saturn and Neptune. The picture is a view from above the plane of Earth's orbit, so the small tilts of other orbits are flattened, and the further you go beyond 2050 the more the positions drift. It is a learning tool, not a navigation-grade ephemeris.
Open the orbit simulator on today's date and look at the table under it. Planets more than about 15° from the Sun are visible: those east of the Sun in the evening after sunset, those west of it in the morning before sunrise, and outer planets near opposition all night. Venus, Jupiter, Mars and Saturn are easy with the naked eye; Mercury is low in twilight; Uranus and Neptune need binoculars or a telescope.
How accurate are the planet positions?
Between 1800 and 2050 they are within a fraction of a degree, using NASA JPL's approximate orbital elements and an exact solution of Kepler's equation. That is accurate enough to show Earth's perihelion in early January and the dates of oppositions. Dates outside that range still work but drift more, and the top-down view flattens the small tilts between orbits.
Why do planets sometimes seem to move backwards?
Because we watch from a moving Earth. When Earth overtakes an outer planet on the inside track, around opposition, that planet appears to drift backwards against the stars for a few weeks — retrograde motion — just as a slower car seems to slide back as you pass it. Run the simulator through a Mars opposition and draw an imaginary line from Earth to Mars to see the line swing back.
Would I really weigh more on Jupiter?
Yes — about 2.4 times as much, because Jupiter's gravity at the level of its cloud tops is 23.1 m/s² against Earth's 9.8. Jupiter has no solid surface, so the figure is for the height where the pressure equals sea level on Earth. Your mass would not change at all; only the pull on it would.
Why is the true-scale view so empty?
Because space really is that empty. Neptune is 30 times farther from the Sun than Earth, so when the whole orbit fits on screen the inner planets crowd into a few pixels around the Sun. The true-scale view therefore zooms to the planet you select; the readable scale squeezes distances logarithmically so you can see every planet's position at once.
Is my birthday sent anywhere?
No. Your birthday, weight and unit choice are calculated and stored only in your own browser so they are still there next time. Clear the fields or your browser data to remove them.