Supermoon Explained: Perigee, Apogee, Size and Brightness

What makes a full Moon a supermoon?

The important question behind a percentage is “compared with what?” NASA's widely quoted figures of up to about 14% larger apparent diameter and 30% brighter compare a close full Moon with a far full Moon. They are not a promise that every named supermoon exceeds an average full Moon by those amounts.

Separate distance, phase and local visibility

Distance describes how far away the Moon is. Phase describes the illuminated appearance seen from Earth. Moonrise describes when it crosses a local horizon under the chosen calculation. These are different quantities; knowing one does not supply the other two.

Perigee and the exact full-Moon instant need not coincide. A date labelled “supermoon” can include a closest approach and full phase occurring at different hours. A calendar date also depends on the time zone. Read event times and their definitions before assuming one timestamp describes everything.

This article contains no event forecast. The distances below are deliberately chosen teaching values, not a month's ephemeris, record extremes or a rule for deciding which future full Moons qualify.

Why size and brightness have different percentages

For the small angle occupied by the Moon, apparent angular diameter is approximately proportional to physical diameter divided by distance. If physical diameter stays the same, the near-to-far angular-diameter ratio is far distance / near distance.

For a distance-only comparison with the same illumination and viewing conditions, total received light follows the inverse-square relation. Its near-to-far ratio is approximately (far distance / near distance)². This model concerns the light received from the whole lunar disk, not an assertion that every patch of its surface becomes intrinsically more luminous.

Thus a diameter ratio of 1.14 corresponds to a light ratio of about 1.2996 in this simplified model. The associated increases are 14% and 29.96%, not 14% and 14%. A ratio includes the original amount; subtract 1 before multiplying by 100 to obtain the percentage increase.

A distance table you can calculate yourself

At 360,000 km relative to the 400,000 km baseline, the diameter ratio is 400,000 / 360,000 = 1.111111… . That gives about 11.11% more angular diameter. Squaring the unrounded ratio gives 1.234567… , or about 23.46% more received light.

For 384,000 km, the same calculation gives about 4.17% more angular diameter and 8.51% more light relative to 400,000 km. The table prints extra decimals so readers can check arithmetic; those decimals do not claim equivalent accuracy for a real observation.

Keep unrounded values during calculation. Squaring a ratio already rounded to 1.11 produces a slightly different result from squaring 400,000 / 360,000. That is a rounding effect, not a change in the orbit.

Changing the baseline changes the headline

The same near Moon gives a smaller percentage when compared with the nearer baseline. Both rows are consistent. Here 384,000 km is just a selected comparison distance; it is not presented as an exact average for all full Moons.

The reverse comparison also uses a different denominator. In this model, the disk at 400,000 km has 90% of the angular diameter at 360,000 km, so it is 10% smaller. Its received light is 81%, so it is 19% lower. “11.11% larger” in one direction and “10% smaller” in the other are not contradictory.

When reading a graphic, identify whether its claim concerns diameter, apparent area or total light. A circle drawn 30% wider would exaggerate a claim of 30% more light. The two percentages describe different measurements.

How to make a fair photographic comparison

Use the same camera, focal length and image scale, and record any crop or resizing. A tighter crop displayed at the same screen size can make the Moon look larger without measuring any astronomical difference. For a diameter comparison, count pixels across the disk in images with a consistent scale.

In an invented pair, disks spanning 1,000 and 1,111 pixels have a measured ratio of 1.111. That observation is usable only if the scale is comparable. It does not by itself establish the cause or prove those photos were taken at perigee and apogee.

JPL's measuring activity also stresses exposure: a blown-out edge is difficult to measure. Comparing image brightness needs more control than comparing disk size, because exposure, processing and atmospheric conditions can all change pixel values. Two attractive photographs are not automatically a calibrated light measurement.

Why a huge-looking horizon Moon is a separate question

The Moon illusion is the perception that the Moon looks unusually large near the horizon. NASA distinguishes this visual effect from its measured angular size. A memorable view between buildings therefore does not, by itself, identify a supermoon.

Use local moonrise and moonset to plan a horizon view. That tool gives local rising and setting information; it is not a perigee-distance calculator. For calendar context, read phase versus tithi. A Purnima date alone does not specify the Moon's distance.

Likewise, closeness and fullness do not by themselves guarantee an eclipse. The eclipse-season guide explains the additional alignment. Keeping these questions separate makes an observing plan and a calendar explanation more precise.

Frequently asked questions