Reading the sun across a Phoenix yard

Most people carry a rough model of the sun: it rises in the east, sits high in the south at noon, and sets in the west. That model is true in Phoenix, and it is not precise enough to plan a planting bed with.

The precise version has numbers, and they change more over the year than most people expect. The sun’s altitude at solar noon at Phoenix Sky Harbor runs from about 33 degrees above the horizon on 15 December to about 80 degrees on 15 June, a ratio of roughly 2.4. A wall that is in shade at midday in December is in sun at midday in June, and the same yard needs two different answers in the same year.

The sun is never north of overhead

The first fact, and the one that fixes everything else: Phoenix is at 33.4278 degrees north, which is outside the tropics. The sun therefore never gets north of directly overhead, so at solar noon it is due south every day of the year.

Computed from the NOAA equations, the noon bearing is about 179 degrees on 21 June (a fraction south of due south) and about 180 degrees on 21 December. For planning purposes it is due south all year, all twelve months, without exception.

Two consequences follow and both are as firm as the fact.

A north-facing wall receives no direct midday sun in any month. It is the coolest exposure on the property, and it stays that way through summer and winter alike, which is why it is the one place a shade-loving plant can be planted without second-guessing the season.

And every shadow falls to the north. A shadow is always cast away from the sun, so a tree on the south side of a yard shades the ground to its north, and a tree on the north side of a house shades nothing of the house at all. That is the whole reason the south side of a building collects heat: it is the side that faces the sun’s highest, longest, most direct passage.

Solar noon is not twelve o’clock

Clock time and solar time are not the same thing, and in Phoenix the gap runs to nearly three-quarters of an hour.

Solar noon (the moment the sun is at its highest for that day) lands at 12:13 pm in late October and November, 12:23 pm in September and December, 12:37 pm in January, and as late as 12:42 pm in mid-February. The swing comes from a combination of the equation of time, which reflects the earth’s elliptical orbit and its tilted axis, and Phoenix’s longitude within the Mountain time zone.

The compensation for living in Arizona is that none of this ever moves. Arizona stays on MST all year with no daylight saving, so 12:42 pm is 12:42 pm in February of every year. Any shade diagram you draw for your own yard stays valid indefinitely, which is not true in a state that shifts its clocks twice a year.

The shadow is the number you actually want

Altitude is useful to know and hard to picture. Shadow length is the same information in a shape you can see, and the conversion is one division: the shadow of an object is its height divided by the tangent of the sun’s altitude.

Which gives a multiple you can carry around. At solar noon at Phoenix:

Read the last column as a design rule. In June, a six-foot wall shades a strip about a foot wide on its north side at midday. In December, the same wall shades a strip about nine feet wide. If you want a north bed to be shaded at midday, June is the month that will disappoint you, and December is the month that will overdeliver.

Morning and afternoon are not the same problem

The shape of the day matters as much as the height of the sun, and this is where the site’s naming of one wall as “the afternoon wall” comes from.

On 15 July, the sun is 55.7 degrees above the horizon at 3 pm and its bearing is 259 degrees, west-southwest. At 4 pm it is 43.3 degrees and 269 degrees, which is due west. At 6 pm it is 18.5 degrees and 284 degrees, west-northwest. So on the hottest days of the year, the sun’s last three hours are spent on the west face of anything on the property, at an angle low enough that the shadow stretches a long way across the ground.

The morning is a mirror image in geometry and not in effect. The sun’s altitude at 9 am on 15 July is the same 55 degrees it will have at 3 pm, because the geometry is symmetric around solar noon. What is not symmetric is the temperature. The normal low for July is 84.5 °F, which is the neighbourhood of the temperature at dawn, and the normal high is 106.5 °F. A shadow at 9 am is falling on air that is twenty degrees cooler than the air a shadow at 3 pm is falling into.

That asymmetry is the practical argument for weighting a shade plan towards the west. An eastern tree works, and it works in the coolest part of the day. A western tree intercepts the sun when the air is already at its hottest and the heat has had all day to build in the walls and the paving.

What the arithmetic leaves out

The equations give the sun’s position; they describe geometry rather than weather. Three things they do not include, and each is worth a walk around your own property before you plant anything.

They do not know about your neighbours. A wall, a mature tree or a two-storey house next door removes sun that a latitude calculation has no way of seeing.

They do not know about reflected light. A west-facing block wall in July is hot to the touch in the late afternoon, and it re-radiates that heat back into the yard after the sun has left it. Paving and gravel do the same. A bed that is technically in shade at 5 pm can sit beside a surface that is still radiating.

And they do not account for monsoon debris. From mid-June to the end of September the dust that settles on leaves and screens does real work on the amount of light reaching the ground, and a shade plan designed in June can look a shade lighter by August.

There is one more thing worth doing, and it costs an hour a year. Walk the yard at 9 am, at solar noon, at 4 pm and at 6 pm on a day near the June solstice, and again on a day near the December one, and note where the shade actually falls. The arithmetic above will tell you where it should fall; the walk tells you what the walls, the neighbour’s tree and the paving are doing to it. A plan built from both is the one that still works in August.

The Sun Path & Shade Finder runs the same arithmetic for any date you choose, with the sunrise and sunset bearings and the afternoon wall for the day. Its limits are stated on the tool page and they are the three above: it knows where the sun is, and it does not know what is standing between the sun and your yard. Once you have the sun’s path, the trees that will actually shade it are the next decision, and the Desert Plant Picker will filter those on sun, water and mature size, so that the tree you plant matches the exposure the arithmetic has revealed.