Glossary: moon, stars and poster printing
Short explanations of the terms that keep coming up around personalized posters — astronomical as well as print-related.
- Moon phase
- The moon phase describes how much of the moon surface visible from Earth is currently lit by the sun. A full cycle from new moon to new moon averages 29.53 days and is called a synodic month. Because the sequence follows fixed laws, the phase can be calculated exactly for any date in the past or future.
- New moon
- At new moon the moon sits between Earth and sun. The side facing Earth lies in shadow, making the moon practically invisible. Illumination is close to zero percent.
- Full moon
- At full moon the Earth sits between sun and moon, so the side facing us is fully lit. Illumination reaches nearly 100 percent. A full moon occurs roughly every 29.5 days.
- Waxing and waning moon
- The moon is waxing between new moon and full moon, when the lit area grows, and waning between full moon and new moon, when it shrinks. In the northern hemisphere a waxing crescent opens to the left; in the southern hemisphere it is mirrored.
- Crescent and gibbous
- A crescent is any phase where less than half the disc is lit. When more than half but not all is lit, the phase is called gibbous.
- Star map
- A star map shows how the sky looked at a particular moment above a particular place: which stars stood above the horizon and in what arrangement. It follows unambiguously from date, time and geographic coordinates. Two different dates, or two different places, produce two different maps.
- Constellation
- A constellation is a human-defined grouping of stars that forms an imagined figure in the sky. The International Astronomical Union recognises 88 official constellations. The stars in one are usually not close to each other in space — they only appear together from our viewpoint.
- Geographic coordinates
- Coordinates identify a point on Earth: latitude gives the position north or south of the equator, longitude the position east or west of the prime meridian. They are written either as decimals (52.52° N / 13.405° E) or in degrees, minutes and seconds.
- 300 DPI
- DPI stands for dots per inch. In print production 300 DPI is the standard for sharp results, because the human eye can no longer separate individual dots at that density. Screens only need 72 to 96 DPI — which is why a screen-resolution file looks blurry once printed.
- 5:7 aspect ratio
- The ratio of width to height. 5:7 closely matches the ISO A paper sizes, so an image in this ratio fits standard A4, A3, A2 and A1 frames without cropping. An image in 2:3, by contrast, has to be cropped for A sizes.
- Passe-partout (mat)
- A passe-partout, or mat, is the card frame with a cut-out that sits between picture and frame. It creates distance from the glass, protects the paper and makes a motif look calmer. It is common for framed posters but not required.
- Digital download
- With a digital download the buyer receives a file rather than a physical object. Printing, packaging and shipping fall away; in return the buyer prints it. Under EU law the right of withdrawal for digital content lapses once execution has begun with express consent.
- Matte and glossy paper
- Matte paper scatters light and does not reflect behind glass; glossy paper makes colours appear stronger but reflects heavily. For posters behind glass, matte or satin is usually chosen. A paper weight from 200 gsm upwards is considered substantial and will not warp in a frame.
- SunCalc
- SunCalc is an open, freely available programming library for calculating sun and moon positions. For a given date and place it returns the moon phase, illumination, rise and set times and the position in the sky. Lumeries uses it to compute the astronomical data on its posters.