About The Day You Were Born
There are a hundred “what happened the day you were born” pages and most of them are inventing things — a lucky colour, a spirit animal, a compatibility score. This one has a rule: if it is not measured, it is not here. The fun comes from the facts, which turn out to be surprising all by themselves.
So the weather is the actual weather at your birthplace that day, taken from ERA5 — the reanalysis that reconstructs the atmosphere from every observation recorded at the time. The moon phase comes from the US Naval Observatory, exact to the minute, with the illuminated fraction computed from where your date fell in the lunar cycle. Whether it was a public holiday comes from a real calendar. What twenty dollars then is worth now comes from the Consumer Price Index, and the petrol price from the retail series, both published by the Federal Reserve Bank of St. Louis.
Where a source has no value for your date, the tool says so rather than filling the gap — petrol prices simply do not exist before 1976, and pretending otherwise would make everything else here less trustworthy. Nothing about you is stored. The share link carries your result so the preview image shows your day, not a stock photograph of a cake.
Add each person as you look them up. The list is kept in this browser only — nothing about your family is uploaded.
A clean, single-page version with the site furniture stripped out — print it, or save it as a PDF for a card.
How to Use The Day You Were Born
Pick your birth date, search for the town you were born in, and press the button. The place matters more than you would think: a birthday in Chicago and the same birthday in Phoenix are two completely different days of weather, and the archive is queried at your coordinates rather than for a country. If your exact town is not listed, the nearest larger town is close enough — ERA5 works on a grid of roughly 30 kilometres, so anywhere in the same region gives the same answer.
What a Reanalysis Is, and Why It Is Better Than an Old Weather Report
Historic weather is usually served one of two ways. Either somebody hands you the nearest weather station’s log — which may be forty miles away, may have a gap on the day you care about, and may have moved since — or somebody quietly interpolates and presents the result as fact. A reanalysis does something better. It takes every observation recorded at the time (surface stations, ships, weather balloons, aircraft and, from the late 1970s, satellites) and feeds them into a single modern weather model run backwards over the whole period, producing a physically consistent reconstruction of the entire atmosphere.
ERA5, the one behind this tool, covers 1940 to the present hour on a grid of about 31 kilometres, with 137 levels of atmosphere. Because the same model is used for every year, a 1950 reading and a 2020 reading are directly comparable — something raw station records can never promise, since instruments, siting and observing practice all changed underneath them. It is the reference dataset for climate work worldwide, which is a slightly absurd thing to be using to look up somebody’s birthday, and exactly why the answer is trustworthy.
The Moon on Your Birthday
The lunar cycle runs 29.53 days from one new moon to the next — the synodic month, which is slightly longer than the 27.3 days the moon actually takes to orbit Earth, because Earth has moved along its own orbit in the meantime and the moon has to catch up to line up with the sun again. Phase dates here come from the US Naval Observatory and are exact to the minute; the illuminated percentage is calculated from where your date fell between one new moon and the next.
A detail worth knowing: illumination is not a straight percentage of the way through the cycle. The moon is a sphere and you are seeing a projected disc, so the lit fraction follows a cosine curve. Halfway from new to full is not a quarter lit, it is exactly half lit — which is why the phase is called first quarter even though the disc looks half full. One in every twenty-nine or so people gets a full moon; if that is you, it is a genuine one-in-29 fact rather than a manufactured one.
What Money Was Doing
The price figures use the Consumer Price Index for All Urban Consumers, not seasonally adjusted, which the US Bureau of Labor Statistics has published monthly since 1913. The “what twenty dollars is worth now” number is a straight ratio of the index in your birth month to the most recent published month. It answers a narrow question — how many of today’s dollars buy the same basket — and it is worth knowing what it does not answer.
It does not say things felt more or less affordable, because the basket itself changed beyond recognition. A 1985 basket contains no mobile phone, no broadband and no streaming subscription, while a car in it had no airbags and half the lifespan. Statisticians adjust for quality changes, but there is no honest way to compare a 1985 television with a 2026 one. Treat the multiplier as a good measure of the price level and a rough one for living standards.
Why Days Alive Beats Age in Years
Age in years is compressed to the point of being uninformative — everyone knows roughly what forty means. The day count is not. Most people are surprised to find they passed 10,000 days at 27, and that 20,000 days lands at about 54. Those are worth marking, and unlike a birthday almost nobody sees them coming, which is why the tool tells you when the next one falls.
Want the same treatment for the climate rather than a single day? Your Personal Climate Stripes draws one bar per year of your life, coloured by that year’s temperature in your own town. Browse every Fun & Novelty tool for more.
Frequently Asked Questions
Where does the weather for my birthday actually come from?
From ERA5, the European Centre for Medium-Range Weather Forecasts reanalysis, served through the Open-Meteo archive. A reanalysis takes every observation recorded at the time — weather stations, ships, balloons, later satellites — and runs a modern weather model over it to reconstruct conditions everywhere on a consistent grid. It is not a guess and it is not a nearby station reading; it is the best physical reconstruction we have of what the atmosphere was doing at your coordinates that day.
How far back can I look up a birthday?
To 1 January 1940. That is the start of the ERA5 archive. Earlier dates would mean using scattered station records with large gaps, which would produce something that looks authoritative while quietly being a guess, so the tool declines instead.
How accurate is the moon phase?
The phase dates come from the US Naval Observatory, the same source almanacs use, and are exact to the minute. The illuminated percentage is computed from how far through the lunar cycle your date fell, which is accurate to within a percent or so. The one thing it does not account for is your latitude affecting which way round the crescent appeared.
Why does the price comparison only work for some dates?
Consumer price data runs from 1913, so the "what twenty dollars is worth now" figure works for any date the tool accepts. The retail petrol price series starts in 1976, so births before then show no fuel price rather than an invented one. Both come from the Federal Reserve Bank of St. Louis, and both are United States figures — the comparison is a US one even when your birthplace is not.
Is my birth date stored anywhere?
No. The date and place are sent to our server to look up the archives, the result is cached against the date and coordinates rather than against you, and nothing is written to an account unless you press Save. If you share the link, the date is in the URL you are choosing to share — which is the point of it.
