The Moon

Which moon stands tonight, when it rises and sets, how far off it is, and what day of the biblical month this is. Drag the ribbon below to either side and you walk through the months, back as far as you like and forward as far as you care to look.

The moon leans, and that is not decoration

What you see is a photograph of the moon with a computed shadow laid over it. The moon keeps one face towards us, so the markings stay put and only the shadow travels. Two turns go into the drawing, and they are not the same one. Inside, the shadow lies across the face at the angle the sunlight actually makes with the moon’s own north. Outside, that whole face is tipped by the parallactic angle — what your latitude and the hour do to it. A drawing that used only their sum would put Mare Crisium in the wrong place.

The tilt is not for effect: it is the angle the moon truly makes at your latitude at the moment it crosses the meridian that day — the hour it stands highest and is best seen. Noon would be simpler to reckon, but at noon a waning moon is usually below the horizon, where a tilt says nothing. In the Netherlands a young moon lies on its side with the horns up and to the left; from the equator the same moon is a bowl; from the southern hemisphere it is upside down compared with what a northern reader expects.

The bright limb always points at the sun, even when the sun is below the horizon. That is what makes a tilted crescent worth reading: it tells you where the sun has gone.

Two places, and why they differ

The moon is drawn and timed for where you are. Rising, meridian and setting hold there and nowhere else; fifty kilometres away already makes half a minute’s difference.

The biblical day under each card is reckoned at Jerusalem. That is deliberate: the months open at the new moon as it falls there, exactly as the strip under this site’s header and the calendar page reckon it. Were the count shifted per reader, a Dutchman and a Spaniard would see different biblical dates for the same day of the same month.

Where the site takes your position from is stated under the ribbon. By default it is your time zone, which is good to within a few minutes for almost everyone. If you want it exact there is one button, and only when you press it does your browser ask you where you are.

What the marks mean

Under each day in the ribbon a small word appears where there is something to report. new, 1st, full and 3rd point out the four phases: the day on which the new moon, first quarter, full moon or last quarter actually falls — not the day it most looks like it. The exact instant is in the panel above.

near and far mark perigee and apogee, the closest and furthest points of the orbit. The moon does not move in a circle but in an ellipse, and the difference is large enough to see: about 356,400 kilometres at perigee against better than 406,700 at apogee. A full moon that falls at perigee is what the papers call a supermoon — the disc is then some fourteen per cent wider than a full moon at apogee. A small brass lozenge in the corner of a day means an appointed time falls on it.

Where the figures come from

Everything on this page is worked out in your own browser, from chapters 47, 48 and 49 of Jean Meeus, Astronomical Algorithms. Chapter 47 gives the position of the moon — sixty periodic terms for the longitude and the distance, sixty more for the latitude. Chapter 48 gives the illuminated fraction, computed from the true elongation and the two distances rather than from the moon’s age: near the quarters the two differ by a percent, and that would show. Chapter 49 gives the four phases, and that is the same source the calendar opens its months with.

The results are checked. The position agrees to the last printed digit with the worked example Meeus gives himself, and the times of rising and setting differ by under three seconds from a wholly independent ephemeris. No server is involved and nothing is fetched: cut the connection and this keeps working.

The photograph of the moon is “Full Moon Moon Png Clipart” by Andrea Stöckel, via publicdomainpictures.net, released under CC0 into the public domain. I have cropped it to the disc and reduced it to 384 pixels; the shadow over it is computed, not supplied.

The calendar, for the whole year →  ·  Texts and datasets used →