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.

Why the moon matters

This page does not draw the moon because it is beautiful. In the text the moon has work to do: it opens the month. And since every feast in the law is a day within a month, none of those days is fixed for as long as the month is not fixed. It can be read in four steps, and every step says so in as many words.

What the moon was made for. One verse says it outright:

He appointed the moon for seasons: the sun knoweth his going down. Psalm 104:19

Seasons here translates the Hebrew mo’adim. That is not simply a word for times of the year: it is the word with which Leviticus announces the feasts — the feasts of the Lord, even holy convocations (Leviticus 23:4). The same word stands on the fourth day of creation:

And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years: Genesis 1:14

A caution belongs here at once. English puts seasons in Genesis 1:14, and then it sounds like summer and winter. Whether Genesis is already thinking of the feasts is a question of reading, and you may settle it differently than I do. That it is the same Hebrew word as in Leviticus 23 is not a question of reading: you can check it.

Where the month begins. Just before the exodus Israel is given a calendar, and it is the first thing it is given:

This month shall be unto you the beginning of months: it shall be the first month of the year to you. Exodus 12:2

The word for month here is chodesh, and in Hebrew that is the same word as for new moon. Psalm 81 uses it in that second sense:

Blow up the trumpet in the new moon, in the time appointed, on our solemn feast day. Psalm 81:3

One word, then, for two things that in the text are one thing: the month is the moon’s month, and it begins where the moon begins again.

The new moon was itself an appointed time. An offering belonged to it:

And in the beginnings of your months ye shall offer a burnt offering unto the Lord; two young bullocks, and one ram, seven lambs of the first year without spot; Numbers 28:11

And it was blown upon. Numbers 10:10 sets the two side by side in one sentence — the appointed times and the first days of the months:

Also in the day of your gladness, and in your solemn days, and in the beginnings of your months, ye shall blow with the trumpets over your burnt offerings, and over the sacrifices of your peace offerings; that they may be to you for a memorial before your God: I am the Lord your God. Numbers 10:10

And so the convocations hang on it. Leviticus 23 lists the feasts and dates every one of them the same way: a day within a month. Passover on the fourteenth of the first month (Leviticus 23:5). The Day of the Trumpet on the first of the seventh (Leviticus 23:24). The Feast of Tabernacles on the fifteenth of that same month (Leviticus 23:34). Not one of those days is hung on a day of the week or on a position of the sun; every one is counted from the beginning of a month.

That is the hinge. Put the first day of the month one day wrong, and every day in that chapter goes one day wrong with it. Determining the new moon is therefore not a side matter beside the feasts. It is the first step of them, and it is why this page exists.

What new moon precisely means — the moment the moon passes the sun, or the evening on which the first crescent can be seen again — is the one question this site gives no settled answer to, because the text does not answer it. What that choice does to every date is set out on the calendar page.

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. There are two turns in the drawing, and they do different work. The first decides how the shadow falls across the face of the moon: that is the angle the sunlight makes with the moon’s own north. The second tips that whole face in your view, and depends on your latitude and on the hour. Add the two together and draw one turn, and Mare Crisium ends up 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; two lozenges mean two fall on the same day. That happens with the Day of Firstfruits, which is the day after the sabbath of the week of unleavened bread and so in some years coincides with the first or the last day of that feast — in 2027 with the last, in 2028 with the first. Which appointed times they are is named in the panel above.

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 →