Moon phase on your baby's birth date: how to look it up
To find the moon phase on your baby's birth date, enter the date and your location in the US Naval Observatory's Complete Sun and Moon Data for One Day tool. It works for any year between 1700 and 2100 and returns the phase and the fraction of the moon that was lit.
What is a moon phase?
A moon phase is how much of the moon's sunlit half you can see from Earth. NASA explains that the sun always lights one half of the moon, and the part of that half facing us changes as the moon travels along its orbit. The whole cycle repeats about every 29.5 days.
NASA names eight phases. The US Naval Observatory (USNO) defines each one by how much of the disc is lit and whether that share is growing or shrinking:
| Phase | How much is lit | Lit share is... |
|---|---|---|
| New moon | 0% (the unlit side faces Earth) | - |
| Waxing crescent | less than half | increasing |
| First quarter | 50% | increasing |
| Waxing gibbous | more than half, not all | increasing |
| Full moon | 100% | - |
| Waning gibbous | more than half, not all | decreasing |
| Last quarter (NASA: third quarter) | 50% | decreasing |
| Waning crescent | less than half | decreasing |
So "waxing" and "waning" are the two words that matter on a birth date. They tell you whether your baby arrived as the moon was filling up or emptying out.
How do you look up the moon phase for a birth date?
Use the USNO's one-day tool for the exact date. You type the date, pick a location and a time zone, and it returns the lunar phase information for that day next to the rise and set times of the sun and moon. The phase and the fraction lit are worked out for noon on that date, in the time zone you chose.
Use the USNO's list of primary phases for the nearest milestone. Its Dates of Primary Phases of the Moon service covers the same 1700 to 2100 range and lists every new moon, first quarter, full moon and last quarter with a date and time. Those times are in Universal Time, so convert them to your own clock before you read off the date. Close to midnight, that conversion can move a phase onto the previous or the next day.
Does the time of birth change the moon phase?
On a day when one of the four primary phases falls, the time of birth can change the phase's name. The USNO defines the four primary phases as instants: a full moon is the moment the moon's ecliptic longitude is 180 degrees past the sun's, and a new moon the moment the two are equal. For practical purposes, the USNO says, those moments happen at the same time wherever you are on Earth.
Take a birth on a full-moon day. Born before the full-moon instant, your baby arrived under a moon whose lit share was still increasing, which the USNO's definitions call waxing gibbous. Born after it, the lit share was already decreasing: waning gibbous. The same goes for a new-moon day, where a crescent is waning before the instant and waxing after it.
A full moon is a moment, and your baby's birth time says which side of it they landed on.
If you know the time of birth, compare it with the USNO's list of primary phases for that month, converted to your time zone.
Does the moon look different in the southern hemisphere?
The phase is the same everywhere, but the picture flips. NASA's answer is that someone in the other hemisphere sees the moon in the same phase, upside down compared with what they are used to. Its example is March 8, 2021: the waning crescent sat on the left side of the moon seen from the northern hemisphere and on the right seen from the southern one.
So for a baby born in Sydney or Cape Town, a moon drawn the northern way round shows the right phase, turned upside down compared with the moon over their home.
Putting the birth moon on a poster
Our Luna poster style prints the real moon phase on the birth date as a single disc, captioned "the moon on the day you were born". It sits in the Moon & stars group of birth poster designs and you can order it on the framed poster or the canvas. It is not one of the card-pack designs.
Two choices are built in, and you should know them before you order. The moon is calculated for noon UTC on the birth date, the same moment for every order, so the time of birth does not change the picture. And a waxing moon is lit on the right, the way it looks from the northern hemisphere.
If you have a date, you can see the moon it gives straight away: open Luna in the poster editor and type it in.