◐ Which side of the Moon is lit tonight? Draw it first, then see the real one

Umbriel's free Draw the Moon tells you where the Moon is tonight and not what it looks like. You tap where you think the lit edge runs and say which side is bright, and then the true shadow is laid over your guess, with both in plain numbers. Over a year your drawings become your own Moon atlas, one row for each lunation.

• What you'll needA guess; no chart and no account
• What you getTonight's Moon sign and degree, your drawn edge laid against the real phase with both lit fractions stated, and a printable atlas of every Moon you have drawn
• CostFree, no account required

Most people could not say, without looking, whether tonight's Moon is lit on the left or the right, or how much of it. That is not a failing; it is what happens when a thing is always there. Draw the Moon turns it into a small nightly question.

The page tells you where the Moon is, its sign and degree, and nothing about its shape. You tap the disc where you think the lit edge crosses the Moon's middle, or use the slider beneath it, and choose which side is lit. Then the real Moon is drawn over your guess: its shadow in engraved hatching, the lit part clear, and your edge as a dashed line across it.

The reveal is two facts and nothing more: you drew it 30% lit; it is 44% lit. There is no "off by", no closeness score, no best guess and no streak. The drawing is not a test and can be tapped anywhere. It is only you and the sky.

The Moon is drawn the way it looks from the Northern Hemisphere, where a waxing Moon is lit on the right and a waning Moon on the left. From the Southern Hemisphere the picture is mirrored, and near the equator the lit side tips towards up or down instead.

Each Moon you draw is kept, and together they make a Moon atlas: a row for each lunation, a column for each day of the Moon's age, each drawn day a small disc of the real Moon with your guessed edge across it in sinopia. A day you did not draw is blank paper. An atlas of two discs is as finished as one of thirteen rows, and the whole thing prints as an engraved plate.

What you draw stays private, on your device, and if you sign in it syncs end to end encrypted, the same way the journal does. Nothing is ever posted.

The Moon's phase, its lit fraction, which side is lit and its position all come from the engine at the moment you draw, and nothing astronomical is typed in. The lit fraction is the engine's own, from the angle between the Moon and the Sun: a quarter Moon is half lit and a Full Moon fully lit. On the disc, the lit edge crosses the Moon's middle at a distance of one minus twice the lit fraction from the centre, measured in radii, on the right for a waxing Moon and the left for a waning one. Your guess is stored as the point where you put that edge and the side you chose, and turned back into a lit fraction by the same geometry, so the two numbers can be set side by side. One Moon is drawn a day; the day's Moon, once shown, stays shown. The engine's Moon longitude is measured against JPL's ephemeris at a mean of 1.3 arcminutes and a worst of 5; five arcminutes moves the lit fraction by less than a tenth of a percent, and the page says the true Moon is good to that. The disc is drawn the way the whole app draws the Moon: hatching for the shadow, the lit part left clear, a waxing Moon lit on the right and a waning one on the left, as seen from the Northern Hemisphere. The atlas files each drawing by the Moon's age in days since the New Moon, and copies the true phase into your entry, so an old page of the atlas never moves.