Moon Phase Poster for Any Date

How the phase and the percentage are worked out

The page turns your date and clock time into a moment in Universal Time using the time zone you pick (daylight saving included for named zones), then asks the Polyatic star map's astronomy engine where the Moon and the Sun were at that moment. That engine follows Jean Meeus' Astronomical Algorithms: the Moon from the truncated ELP-2000/82 lunar theory (Meeus chapter 47), the Sun from his chapter 25 solar series, good to about 0.01°, with ΔT (the drift between Earth's rotation and atomic time) applied for past centuries.

From the two positions comes the phase angle i, the angle Sun–Moon–Earth. The lit fraction of the disc is k = (1 + cos i) / 2: 0 at new moon, 1 at full, 0.5 at the quarters. The Moon gains about 12.2° on the Sun each day and goes from new to new in 29.53 days on average, so the lit part changes fastest near the quarters, about 10.6 percentage points a day (0.45 an hour), and barely at all near new and full. That is why the hour matters a little and the date matters a lot. Worked example: on 3 May 1990 at 21:00 in London the poster shows a waxing gibbous moon, 70% lit; the Apollo 11 landing, 20 July 1969 at 20:17 UTC, happened under a waxing crescent 33% lit.

The drawing is geometric: the bright edge is a half circle on the side facing the Sun, and the shadow line (the terminator) is half an ellipse whose width is R × (1 − 2k). A crescent's terminator bulges towards the bright edge; a gibbous moon's bulges away from it.

Why the moon looks tilted from your town

The lit side always points along the sky towards the Sun, but which way that is on your horizon depends on where you stand. From Britain or the northern United States an evening crescent leans over to the lower right; near the equator it lies flat like a boat; from Sydney or Buenos Aires the whole moon is upside down compared with the northern view, so a waxing moon is lit on the left. When you give a place, the poster computes the direction of the bright edge (Meeus' position angle of the bright limb) and subtracts the parallactic angle for your latitude and that hour, which turns the disc to the way it actually hung. If the Moon was below your horizon at that time, it is drawn as it rose next and the poster says so in one line. Without a place it uses the textbook northern view: lit on the right while waxing, on the left while waning.

The Town box searches the same list as the Polyatic star map: 171,041 places of 1,000 or more people from GeoNames (CC BY 4.0), loaded from this site only when you start typing. Accents fold the way people type, so Aarhus finds Århus, Denmark and malmo finds Malmö; a town picked from the list fills its latitude and longitude, sets the clock to its own time zone (Århus reads 21:00 on Europe/Copenhagen time, daylight saving included) and prints the town under the date. There is no fuzzy matching: a misspelling finds nothing rather than the wrong town, and names in non-Latin scripts are found only in that script. A farm or a village too small for the list can still be placed by its coordinates under Or enter coordinates.

What the phase names mean

The four principal phases are instants: new moon when the Moon passes the Sun, first quarter at 90° east of it, full at 180°, last quarter at 270°. This poster uses each of those names within 7° of the exact angle, which is roughly 14 hours either side. In between are the four spans:

• Waxing crescent, less than half lit and growing, seen in the west after sunset.
• Waxing gibbous, more than half lit and growing, up in the evening and most of the night.
• Waning gibbous, shrinking from full, rising later each night.
• Waning crescent, a thin morning moon before dawn, just before the next new moon.

A "new moon" poster is a dark disc with a faint rim, because that is what was there: the Moon was in the Sun's glare. The percentage is rounded to a whole number, so a moon a few hours from full can read 100%.

How to print it

Every size is a single image at 300 dots per inch, the usual standard for photo and poster prints. The A sizes share the 1 : √2 shape, so the same layout fits A4, A3 and A2 frames; 50 × 70 cm is a common ready-made poster frame. 30 × 40 cm and the inch frames sold in US and UK shops (8 × 10, 11 × 14, 16 × 20 and 18 × 24 in) are squatter, 3 : 4 or 4 : 5, so on them the disc and the text block are scaled down together (to 94 % of the A-size layout on 3 : 4, 90 % on 11 × 14 and 88 % on 4 : 5) and centred, leaving a wider border at the sides instead of cropping the moon or a line of text. Each is printed edge to edge at its exact size: an 8 × 10 in PDF page is 576 × 720 points, 20.32 × 25.4 cm. Choose PDF for a print shop (the page is exactly the paper size), PNG for photo labs and online print services, and SVG if you want to edit the text in a vector program. In the print dialog choose Actual size or 100%, not Fit. A home printer cannot print to the edge, so the dark-sky style comes out with a thin white margin; trim it, or use the light-paper style, which also saves ink. On some phones the browser cannot build a full 300 dpi canvas for the large sizes; the page then saves the largest image that works, tells you the dpi it used, and a computer will give you the full size. On an iPhone, Download puts the PNG in the Files app; where the browser can hand files to the system share sheet (most phones), a Save to Photos or send… link under Download gives it the same print-size PNG, so Save Image puts it in Photos and any chat app can send it. If drawing the poster takes longer than the browser allows for one tap, the link changes to say the image is ready; tap it once more to open the sheet.

Honest limits: the face is always the near side as seen square-on; the small monthly wobble called libration (which tips up to roughly 10° more of one edge into view) is not drawn, so features right at the rim can be a little off. The NASA map is a mosaic taken with the Sun high overhead, so it shows how bright the ground is, not shadows: craters along the shadow line do not cast the long shadows a telescope shows there. The poster says nothing about the weather that night.

Where the Moon's face comes from

The face is a real map, not a drawing: the colour mosaic from the Wide Angle Camera on NASA's Lunar Reconnaissance Orbiter, as published (public domain) in the CGI Moon Kit by NASA's Scientific Visualization Studio (svs.gsfc.nasa.gov, visualization 4720), credited to NASA SVS and the LRO LROC (Arizona State University) and LOLA teams. The page turns it to grey, projects the near side onto the disc the way you see a ball from far away, and tints it in the poster's colours, so the dark seas (Mare Imbrium, Tranquillitatis, Oceanus Procellarum) and the bright rayed craters Tycho and Copernicus sit where you would find them with binoculars. Only the sunlit part is shown at full strength; the rest of the disc shows the face faintly, the way earthshine lights the Moon's night side.

On screen the face comes from the star map's 1536 × 768 map, which puts about 490 map pixels across the disc (one pixel is about 7 km on the Moon). That is fine on a phone but soft on paper, so when you save a PNG or PDF the page also fetches an 8192 × 4096 version of the same NASA map, cropped to the near side (about 1 MB, from this site). It gives about 2,600 pixels across the disc: more than an A4 (1,560 px) or A3 (2,210 px) disc can use at 300 dpi, and only 1.2× and 1.4× short of the A2 (3,125 px) and 50 × 70 cm (3,720 px) discs. The SVG keeps the on-screen face (1024 px), so for a large print choose the PDF or PNG.

Questions people ask

What phase was the moon on my birthday?

Type your birth date into the Date field, and the hour if you know it, and the poster redraws with the phase name and the percentage of the disc that was lit. On 3 May 1990 at 21:00 in London, for example, it was a waxing gibbous moon, 70% illuminated. The lit part changes by at most about 0.45 percentage points an hour, so if you do not know the hour the 21:00 default is never more than about 5 points off for that day.

Is the moon phase on the poster accurate?

The positions of the Moon and the Sun come from the same engine as the Polyatic star map, which follows Jean Meeus' Astronomical Algorithms. The illuminated percentage is good to well under one percentage point and the phase name only changes at a boundary. The face on the disc is NASA's Lunar Reconnaissance Orbiter colour map of the near side (from NASA SVS's CGI Moon Kit), turned the way it hung in your sky; the small monthly wobble called libration is not drawn, so craters near the edge can sit up to roughly 10° of lunar arc from where they were that night.

What size should I print the moon poster?

Every size is drawn at 300 dots per inch: A4 (2480 × 3508 px), A3 (3508 × 4961 px), A2 (4961 × 7016 px), 50 × 70 cm (5906 × 8268 px), 30 × 40 cm (3543 × 4724 px), and the inch frame sizes 8 × 10 in (2400 × 3000 px), 11 × 14 in (3300 × 4200 px), 16 × 20 in (4800 × 6000 px) and 18 × 24 in (5400 × 7200 px). A4 and A3 suit a home printer or a desk frame; A2 and 50 × 70 cm match standard poster frames and are best sent to a print shop as the PDF. Print at 100% or Actual size, not Fit to page.

Is the moon poster free?

Yes. There is no price, no account and no watermark, and the PNG, PDF and SVG files are yours to print, frame or give as a gift.

Is anything I type uploaded?

No. The date, the place and the words you type are used only inside this page in your browser; the moon is computed and the poster and its download are drawn on your own device. The address bar holds your choices so you can bookmark or share the poster, which means anyone you send that link to sees the same date and text.