Observing · August 20, 2026

Why the Full Moon Is the Worst Night to Look at the Moon

The Moon reached first quarter this morning at 04:46 Paris time. Tonight, at the centre of its disc, the Sun stands eight degrees above the lunar ground: every ridge there casts a shadow seven times longer than its own height. On 27 August, at full Moon, the Sun will be at 86 degrees, almost straight overhead, and the shadow will be a tenth of the relief. A factor of seventy in seven nights. That is the whole explanation of a paradox that disappoints every beginner: the night the Moon is brightest is the night it is least interesting to look at.

What grazing light does

The arithmetic fits on one line. At the centre of the lunar disc, the Sun's height above the local horizon is 90 degrees minus the phase angle, which is the Sun-Moon-Earth angle. A cast shadow then measures the height of the relief divided by the tangent of that solar altitude, which comes to the same thing as multiplying it by the tangent of the phase angle.

At full Moon the phase angle falls towards zero: the Sun is at the zenith of the point we are looking at, and shadows vanish underneath the objects casting them. At first quarter the angle is 90 degrees: the Sun is on the lunar horizon, and shadows become infinite. We checked the arithmetic against the exact instant of this morning's quarter: the engine gives a phase angle of 89.85 degrees, an illuminated fraction of 50.1 % and a solar altitude of 0.15 degrees at the centre of the disc. The terminator does pass through the middle, to within a tenth of a degree.

Read the shadow column as a column of visible detail. At 7.1, a thousand-metre scarp throws seven kilometres of black shadow: even plain binoculars show that the edge of the disc is ragged. At 0.1, the same scarp throws a hundred metres, which is less than atmospheric turbulence lets anyone resolve from the ground. All that is left then is the difference in soil colour, and the Moon becomes a flat map.

The terminator is the only line that matters

Every piece of relief you will see sits in a narrow band along the shadow line. That is where the Sun is rising, and therefore where the light is grazing. The rest of the disc, already in full lunar day, is flattened.

That band moves. The Sun takes a month to go round the Moon, about twelve degrees of lunar longitude a day: every evening the terminator uncovers fresh ground that the previous night left in darkness and the next will flatten. A crater that is spectacular one evening will be invisible three nights later. That is why looking at the Moon is not something you do once: it is something you do again, and the show is never the same.

The summer problem: it is low

The altitude column of the table is less flattering. At 22:00 the Moon never climbs above fourteen degrees over the Paris horizon, and on 20 August it is only at ten. The reason is seasonal: in summer the ecliptic, the road the Sun, the Moon and the planets follow, lies very flat against the evening horizon at our latitudes. The evening Moon takes the full consequences.

At ten degrees you are looking through roughly five times as much atmosphere as at the zenith, and the image boils. The practical advice is therefore awkward: the best lighting falls on the evening the Moon is lowest. You have to pick your night inside that trade-off, and 23 or 24 August, with shadows still equal to or longer than the relief and a Moon at thirteen or fourteen degrees that stays up until the middle of the night, probably offer the best balance of the period.

What to look through

• With the naked eye you see only the maria, those large dark patches. No crater is within reach: the eye's resolving power, about one arcminute, corresponds to a hundred kilometres on the Moon.
• A pair of 10×50 binoculars held firmly, or rested on something, already shows that the terminator is ragged rather than smooth. That is the first moment you physically understand you are looking at a landscape.
• A 60 mm refractor at 50× shows the large craters and their central peaks, and that is where the shadow length becomes spectacular.
• At full Moon the glare becomes uncomfortable in any instrument: a neutral filter, or simply a card with a hole in it over the objective, makes the view bearable. It will not bring the relief back.

A bonus: the Moon does not always show us the same face

It keeps the same side towards us, but it rocks. Tonight the libration in latitude reaches 6.8 degrees: we are looking well over its north pole, at ground that is normally hidden from us. On 28 August it will be the reverse in longitude, at minus 4.8 degrees, and the eastern limb will open up. Over the same period it comes closer: 403,224 kilometres tonight, 388,021 on the 28th, which grows its apparent diameter by almost four per cent.

What these figures do not tell you

The shadow length published here holds at the centre of the disc. Elsewhere the Sun's altitude differs, and near the terminator it tends to zero whatever the phase: there is always a line where shadows are enormous. What the phase changes is where that line runs, and therefore which ground it shows off.

The ratio takes no account of foreshortening. Near the edge of the disc you are looking at the ground at a slant, and a long shadow can end up squashed along the line of sight. The finest shadows are therefore seen halfway between the centre and the limb.

Finally, the altitudes and times are computed for Paris and corrected for standard refraction; subtract an hour for the UK and Ireland. For your exact position, the SkyChart sky map gives the Moon at the hour and the altitude that concern you. And if you are waiting for the real lunar event of the month, it is the eclipse of 28 August, and it falls on precisely the night the Moon is at its flattest.

This article was translated from the French edition by AI, then reviewed before publication. The figures are not translated: phase angles, solar altitudes, shadow ratios and times are computed by SkyChart and are identical in both editions.

• Beginners