Sun and Moon alignment planner

Say where you will stand and which way you are pointing — a bearing, or simply the landmark at the far end — and the tool returns the days over the next twelve months when the Sun or the Moon crosses that line, low enough to sit in the shot. Free, no account, nothing stored.

What it does, and why it is here

Every city has one: the evening the sunset drops exactly down the axis of a street, the morning the full moon sits inside the arch of a bridge. New York calls it Manhattanhenge; everywhere else it happens too, and nobody publishes the date.

It cannot be looked up because it depends on three things at once — your position, the bearing of the axis you are shooting along, and the fact that the Sun’s setting point walks up and down the horizon across the year, crossing any given bearing on two dates (once going one way, once coming back). The Moon does the same, but over a month rather than a year, and its phase has to cooperate.

The other reason to compute rather than guess: the alignment is not a moment, it is a window. The body takes minutes to cross the axis, and its bearing shifts a little each day, so what you actually want is the run of days when it is close enough — which is what the answer gives.

What you fill in

• City — where you will be standing. The whole answer is computed from there.
• Bearing you are aiming at — in degrees clockwise from north: 0 north, 90 east, 180 south, 270 west. Read it off a map along the axis you want the body to sit in.
• Or the place the axis points at — the easier route. Name the spire, the hill or the far bank, and the bearing from here to there is worked out for you; the field above can then stay empty.
• Body — the Sun or the Moon.
• How low — the maximum height the body may have at the moment it crosses. This is the field that decides whether you get a photograph: low is where the warm light is, and where the body actually sits on whatever is at the end of the axis rather than floating above it.
• Full moon only — for the Moon, keeps the passes at least 90% lit. A crescent lined up with a bridge is a geometric fact and not a picture.

What you get back

• When it lines up — the windows, grouped, over the twelve months ahead. Sun alignments generally come in two runs a year; lunar ones recur monthly, filtered by phase.
• Every pass kept — each one with its date, the exact time, the body’s height at that moment, and for the Moon its phase.
• A card to share for a pass, when you are organising other people around it.
• An honest empty answer, which is the part worth having. If the body crosses your bearing but never as low as you asked, it says so and gives the range it does cross in — between such and such a degree — so you can decide whether to accept a higher one. And if it never crosses that bearing at all from your latitude, it says that instead: from here, it stays on the other side of the sky.

What it does not do

It does not know your horizon. The calculation assumes a flat one, so a hill or a tower behind the target hides a body the tool says is at 2° — check the skyline yourself, or ask for a slightly higher elevation to compensate.

No weather, as ever. And it plans the Sun and the Moon only: for the Milky Way core, which is the other thing people align with a landscape, there is a different calculation.

Reading a bearing off a map

The one input people get wrong, and it is worth thirty seconds.

Two consequences. First, the Sun rises and sets due east and due west only at the equinoxes; the rest of the year it swings north or south of that, which is exactly what makes the two crossing dates. Second, an axis pointing due south will never produce a low Sun from the northern hemisphere — that is when the answer comes back saying so.

Questions

Is it free? Do I need an account?

Free, no account, no limit.

How do I find the bearing of my street?

Either read it off a map along the axis, or skip it entirely: name the landmark the axis points at and the tool computes the bearing from your position to it.

How far ahead does it look?

Twelve months.

Are the times local?

Yes, in the time zone of the place you named.

What if I only know the direction roughly?

That is usually fine: the tool returns a window of days, not a single instant, so a few degrees of error shifts the window rather than losing it.

Why are there two windows a year for the Sun?

Because its setting point walks along the horizon and back again over the year, crossing any given bearing once on the way out and once on the way home.

Can I use it for the Milky Way, or for a planet?

No — the Sun and the Moon only. The Milky Way core has its own planner.

Does it work in the southern hemisphere?

Yes. The geometry is the same; the seasons and the side of the sky are mirrored.

Sources and further reading

• Azimuth — the bearing the form asks for.
• Manhattanhenge — the best-known example of exactly this alignment.
• Position of the Sun — why the setting point moves across the year.