Why your age is a different number everywhere else
Your age is not a measurement of you. It is a count of laps - the number of times one particular planet has gone round one particular star since you turned up on it. Nothing about that number is a property of the person holding it.
Which is why it changes so completely the moment you count in anything else. A thirty-year-old has been round the Sun thirty times, and in the same stretch Mercury has gone round about a hundred and twenty-four times, and Neptune has not finished once. The person did not change. The lap did.
Even here the count is approximate. A year is not a whole number of days, which is the entire reason February occasionally grows one, and why your birthday drifts by a few hours around the calendar date without anybody noticing. If you like the feeling of a number turning out to be much larger or much stranger than expected, there is a trillion dollars to spend on this site that works on the same principle.
How long a year actually is on each planet
A year is one orbit, so a planet further out has a longer one. The figures below are NASA's, quoted at the precision NASA prints them.
Mercury goes round in 88 days, so it manages about four years to our one. Venus takes 225 days. Mars, the one people actually search for, takes 687 Earth days - NASA also gives that as 669.6 sols, a sol being one Martian day of about 24.6 hours. So a Mars year is a bit under two of ours, and your age on Mars is a bit over half what it is here.
Then it stops being relatable. Jupiter takes 4,333 Earth days, about twelve years. Saturn takes 10,756, about 29.4 years, so your first Saturn birthday arrives somewhere near your thirtieth. Uranus takes 30,687 days, about 84 years - a full Uranian year is a long human life. Neptune takes 60,190 days, about 165 years, which is longer than anybody has ever lived. Neptune was discovered in 1846 and finished its first full lap since then around 2011.
The days are stranger than the years. A day on Jupiter is 9.9 hours and on Saturn 10.7, so the two largest planets have the shortest days. Mercury's full day-night cycle runs 176 Earth days. And Venus turns so slowly that its day, 243 Earth days, is longer than its year.
Which numbers here are measured and which are invented
Every card on this page carries a label saying what kind of number it is, and a line saying where it came from. There are four kinds, and the difference between them is the difference between a fact and a joke.
Measured means a physical constant that somebody went and measured, with the source printed on the card. All the planetary figures are this, and so are the seconds, minutes and hours, which are measured in the sense of being defined exactly.
Convention means real, but decided by people rather than found in nature. A week is seven days because it has been seven days for a very long time, not because anything in the sky takes seven days to do anything. Olympiads, school terms and decades are the same: genuine, useful, and entirely agreed.
Folklore means something people repeat that is not true. There is one of these, and it is dog years. It gets its own kind rather than being filed under convention, because those are not the same thing at all - a convention is something we agreed on, and folklore is something we got wrong and kept saying.
Made up means invented for this page. The socks, the kettles, the loaves of bread. Those cards say so in the same place the Mars card names NASA.
The label sits on the card itself rather than in a heading above a group, because a card gets screenshotted on its own and a heading does not survive the crop. A page that mixed measured constants with invented ones and left you to work out which was which would be misinformation with jokes on top, and it would be much easier to write.
Why "dog years" is not a real unit
Multiply by seven. Everybody knows it, it is the most recognisable conversion on this page, and it is wrong.
Dogs do not age at a constant rate, so no single multiplier can work. The American Veterinary Medical Association's rough guide is about fifteen human years for a dog's first year, another nine for the second, and roughly five a year after that. Size and breed matter enormously on top of that, and larger dogs age faster - which a single number cannot express at all.
Where did seven come from? Nobody appears to know, and that is worth saying plainly rather than inventing a tidy history. The American Kennel Club reports a plausible-sounding explanation - that it came from people living to about seventy and dogs to about ten - and also passes on a vet's suggestion that it was a marketing ploy to get owners bringing dogs in for annual check-ups. Both are offered as speculation. Neither is documented, and anyone who tells you the origin confidently is guessing.
The conversion stays on this page anyway, labelled folklore, because it is the number people arrive looking for. Correcting it where somebody is already thinking about it is more useful than leaving it out and letting them find it somewhere less careful.
What a birthday is, astronomically
A birthday is the moment Earth returns to roughly the same place in its orbit as it occupied when you were born. Roughly is carrying weight there: the orbit does not take a whole number of days, so the calendar and the sky drift apart and have to be reconciled with a leap day every four years.
That means your real orbital anniversary wanders by a few hours either side of the date on the cake. This page divides by 365.25 days, which is the figure NASA publishes for Earth. You may meet two others: the tropical year the calendar tracks, 365.2422 days, and the sidereal year at about 365.256. They differ by less than a thousandth of a percent, so nothing here changes visibly whichever you prefer - but if you check this arithmetic against another source and get a slightly different answer, that is why.
Entering your birthday makes the numbers exact rather than rounded to whole years. It is worked out in your browser, it is never put in the address bar or sent anywhere, and it is only kept on your device if you tick the box asking for it.
Questions people ask
How long is a year on Mars?
A year on Mars is 687 Earth days, which NASA also gives as 669.6 sols - a sol being one Martian day of about 24.6 hours. So a Mars year is a bit under two Earth years, and your age in Mars years is a bit over half your age here.
Are these numbers real?
Some of them. Every card says which kind it is: measured means a physical constant with the source printed on the card, convention means real but decided by people rather than measured, and made up means exactly that. The planetary figures come from NASA and are quoted at the precision NASA prints them.
Is the dog years conversion accurate?
No, and the card says so. The seven-to-one rule is folklore with no research behind it. Dogs age fastest in their first couple of years and the rate varies a lot by breed and size. It is on the page because it is the conversion everyone knows, and pointing out that it is a myth is more useful than quietly leaving it out.
Which length of year does this use?
It divides by 365.25 days, which is what NASA publishes for Earth - the same source as every planetary figure on this page. You may have met two other years: the tropical year the calendar tracks is 365.2422 days, and the sidereal year is about 365.256. All three differ by well under a thousandth of a percent, so nothing here changes visibly whichever one you prefer.
Does entering my birthday send it anywhere?
No. Everything is worked out in your browser. Your birthday is never put in the address bar and never sent anywhere, and it is only saved on your device if you tick the box asking for it, which is off unless you turn it on.
Why is my age in Neptune years less than one?
Neptune takes about 165 Earth years to go around the Sun once, which is longer than anybody has ever lived, so no human has completed a Neptune year. Neptune itself was discovered in 1846 and finished its first full lap since then around 2011.