Supermoon and micromoon: how much does the Moon’s size change?

An elliptical and changing orbit

The Moon does not keep a fixed distance from Earth. Its approximately elliptical orbit has a near point, **perigee, and a far point, apogee**. Successive perigees average about 27.55 days apart, rather than the roughly 29.53 days between full moons.

Solar gravity and other perturbations keep changing the orbit. Not every perigee reaches the same distance, and not every apogee is equally remote. Broad extremes of the centre-to-centre distance are around 356,000 to 407,000 kilometres, not a pair of fixed values for each month.

Because the orbital-distance cycle differs from the phase cycle, their close coincidences move through the year. An especially close full-moon/perigee alignment returns on a timescale of about fourteen months. Depending on the chosen definition, several successive full moons may qualify as supermoons.

An observer’s surface position also changes the distance slightly. A geocentric value measures between the centres of Earth and Moon; a topocentric distance refers to a particular observing point.

What “fourteen per cent larger” actually compares

Closer objects occupy larger angles. Across the broad lunar extremes, the angular diameter changes from roughly 29.4 to 33.5 arcminutes. That is measurable in comparable photographs, although less dramatic than many illustrations suggest.

The familiar fourteen per cent larger comparison is between a close extreme and a distant extreme. It is not fourteen per cent above an average full moon. Against an average reference, the increase is nearer seven or eight per cent.

An increase in diameter gives a larger increase in disc area. Between extremes, the full disc can occupy around thirty per cent more angular area and supply correspondingly more total light under comparable illumination. Actual observed brightness also depends strongly on altitude, haze and cloud.

The Moon has not physically grown. Its real surface is being viewed from a different distance. Keep physical diameter, angular diameter and brightness separate when interpreting the claim.

Can you recognise one by eye?

Without a reference disc beside it, judging a modest size change is difficult. The Moon illusion makes a low Moon near a skyline or canyon rim look large whether it is a supermoon or not.

For a useful comparison, photograph a supermoon and a micromoon with the same camera, focal length and image scale. Different zoom settings or unequal crops destroy the comparison. Similar altitudes reduce atmospheric differences.

A brighter landscape can be noticeable at a dark site, but transparency and visual adaptation affect the impression. A supermoon is not a promise of a radically different sky.

Tidal forces are somewhat stronger when a syzygy occurs near perigee. Local coastal water levels also depend on coastline, sea-floor shape, weather and timing. The label is not a flood forecast or a prediction of a natural disaster. It should not replace local coastal information for practical decisions.

A popular term without one universal boundary

“Supermoon” is not a single formal astronomical classification. Some conventions use a percentage of the distance range; others choose a fixed threshold. Different published lists can therefore count different numbers in the same year.

Perigean full moon states the geometry more directly. Perigean syzygy also includes a new moon near perigee, even though that does not produce a bright full disc at night. Micromoon is the corresponding popular name near the distant end.

The label can spark interest, but altitude, direction and timing are more useful for planning. An ordinary full moon clearing a well-chosen foreground can make a better observation than a supermoon hidden in haze.