Moon Illumination, Age and Distance: What the Numbers Mean

The percentages and kilometres on a Moon page are not decoration — they are three different measures: how much light, where in the cycle and how far away. Illumination and age are related to phase geometry; distance is related to the orbit. Together they do not fully describe how the Moon looks. Let us examine each one.

Illumination: 0–100%

Illumination is the share of the Moon's disc visible from Earth that is lit by direct sunlight:

Illumination changes non-linearly: near new and full moon it changes slowly, while near quarters it changes faster — up to about 10 percentage points per day. That is why a "nearly full" Moon stays visible for several nights in a row; the exact rate depends on the Moon's orbital position.

Important: 50% means half the disc, not "a quarter". The confusion comes from the word "quarter", which refers to about one quarter of the synodic cycle and a roughly 90° difference in ecliptic longitude, not a quarter of the disc's area.

Moon age: 0–29.5 days

Age is the astronomical cycle counter since the last new moon:

Whether illumination is increasing or decreasing, not one age threshold, shows whether the phase is waxing or waning. An astronomical age near 14.8 days corresponds to full moon, but it does not replace the local lunar-day number.

Distance: 356–407 thousand km

The Moon moves along an ellipse, so its distance from Earth changes:

The angular diameter varies from ~29.4′ to ~33.5′. The difference is barely visible to the naked eye, but obvious in a telescope or photos taken with the same focal length. A full moon at perigee gives a supermoon; at apogee, a micromoon. Distance by itself does not determine illumination or the brightness of the night.

How to read a day card on lunar.day

On moon/phase/[date] for your city:

• Phase — one of the 8 states (for example, "waning gibbous");
• Illumination — percentage of the visible disc;
• Lunar day — the local lunar-calendar day number, not astronomical age;
• Moonrise and moonset — local times that depend on coordinates and the horizon;
• Distance — kilometres at the selected moment, when available.

Example: "waning gibbous, 78%, lunar day 18, distance 404,000 km" means after full moon, illumination is decreasing and the Moon is farther than average — but it is not automatically a micromoon, which requires full moon near apogee. Local rise time also cannot be inferred from distance alone.

Accuracy and limits

Phase, illumination and lunar events are calculated from astronomical ephemerides. City coordinates primarily affect local moonrise, moonset and culmination times; the real horizon and elevation can shift observed times. This is astronomy rather than astrology: the numbers describe geometry and light, not the "energy of the day".

Summing up

Illumination answers "what share of the disc is lit", astronomical age answers "how long since new moon", and distance answers "what is the angular size". They describe different aspects of lunar geometry and do not fully describe visibility: local horizon, weather, observing time and the observer's location also matter.